Java-fall011.opennet Technologies Introduction to the Java Language and Object-oriented Concepts Fall Semester 2001 MW 5:00 pm - 6:20 pm CENTRAL (not Indiana)

java-fall011.opennet Technologies Introduction to the Java Language and Object-oriented Concepts Fall Semester 2001 MW 5:00 pm - 6:20 pm CENTRAL (not Indiana) Time Geoffrey Fox and Bryan Carpenter PTLIU Laboratory for Community Grids Computer Science, Informatics, Physics Indiana University Bloomington IN 47404 gcf@indiana.edu

java-fall012 Overview

java-fall013 What is Java, in a Nutshell?  What is Java? –A simple, object oriented, distributed, interpreted, robust, safe, architecture neutral, portable, high performance, multithreaded, dynamic, programming language.  Java is interesting because –It is both a general purpose object-oriented language along the lines of C++, and –It is particularly designed to interface with Web pages (Java on client) and to enable distributed applications over the Internet with Java on Server. –It has good software engineering properties  The Web is becoming the dominant software development arena; this is driving Java as perhaps the best supported, most widely taught language. –Even outside the Web, e.g. in scientific computing, Java is as good and in some respects better than other languages.

java-fall014 Architecture of Java (Client) Applications  Java applications are compiled and run on a machine just like any other general programming language such as C/C++. No web server or network are required although Java applications may also use network connections for distributed computing.  One can download dynamically for applets or servlets  There are also native (conventional) Java compilers Java code is compiled to produce byte code run by Java Virtual Machine (JVM) to produce results

java-fall015 Java Applications in a Nutshell  Java programs written in a file with extension “.java”.  Applications are.java files with a main() method. This is called by the Java system.  Compile and run a Java application (using bytecodes): –Run the compiler on a.java file: javac MyProgram.java producing a file of Java byte code, MyProgram.class –Run the interpreter on a.class file: java MyProgram which executes the byte code  The tools javac and java are part of JDK.

java-fall016 The Simplest Java Application: Hello,World!  Since Java is object-oriented, programs are organized into modules called classes, which may have data in variables called fields, and subroutines called methods. class HelloWorld { public static void main (String[] args) { System.out.println(“Hello World!”); } Each program is enclosed in a class definition. main() is the first method that is run. The notation class.method or package.class.method is how to refer to a public method (with some exceptions). Syntax is similar to C - braces for blocks, semicolon after each statement.

java-fall017 Java vs. JavaScript  Despite the name, JavaScript is a different language from Java, albeit with some similarities.  A JavaScript program is written directly in the HTML page, and executed by the JavaScript interpreter, so also allows dynamic web page content in the browser window.  JavaScript is special purpose - it is an object-based language that deals directly with browser entities like windows, text fields, forms, frames and documents.  JavaScript can respond to browser events like mouse clicks and user-typed text.  JavaScript is fast to write, but not as powerful as Java.

java-fall018 Multi-tier Architecture  Distributed applications on the web naturally have a multi-tier architecture.  Java plays a role at all three levels: –Graphical User Interface and client side analysis systems, including visualization –Middle layer servers and software integration, including web servers, distributed object servers and other application servers. –Less important for back end client software, which may be legacy code. Middle level servers Client user interface running through browser Internet Internet or proprietary network Backend computing or databases

java-fall019 History of Java Language and Team  Starts in 1991 as Project Green—semi-autonomous task force in Sun focusing on operating software for consumer electronic devices, e.g. smart set-top boxes.  Gosling (creator of Sun NeWS) considers C++ too complex and initiates development of Oak, later renamed to Java. Similarities to UCSD P-system.  A PDA (Personal Digital Assistant), codename “*7”, based on Oak/Java ready in 1993. Green Team becomes FirstPerson, Inc.  *7 proposal to Time-Warner rejected in 1993. 3DO deal falls through in 1994. FirstPerson, Inc. dissolves.  Small group (~30 people, becomes the Java Team) continues and decides to adapt Oak as a Web technology.

java-fall0110 History of Java Language and Team to Dec. 95  Experimental Web browser written in Java, called WebRunner, later renamed HotJava, ready in 1994.  Alpha release of Java and HotJava April '95.  Netscape licenses Java in May '95 and builds Java into Netscape 2.0  Beta Java Development Kit (JDK) and first Java books Summer/Fall '95.  Dec 4, 1995 Business Week cover story: "Software Revolution—The Web Changes Everything" presents Java as a breakthrough force in the expanding Web/Internet.  In next week, SGI, IBM, Adobe, Macromedia and finally Microsoft adopt/license Java. Java is adopted by Web community.  The rest is as they say history …………………..

java-fall0111 Java Features—Simple and Familiar  Familiar as it looks like C++, but simpler to program. –omits several confusing features of C++ including operator overloading, multiple inheritance, pointers and automatic type coercions  Adds automatic garbage collection to make dynamic memory management much easier than in C or C++. –No more frees or deletes. No more memory leaks.  Adds Interface construct, similar to Objective C concept, to compensate for the lack of multiple inheritance.  Small kernel is suitable for Java ports to consumer electronic devices. –But need customization J2ME to be effective (remove unnecessary features and support special capabilities of PDA’s etc.)

java-fall0112 Java Features—Architecture-Neutral  C/C++ programming in a heterogeneous network environment demands compatibility across several vendor platforms and their compilers.  Solved in Java by designing platform-independent binary representation called Java bytecode— comparable to P-code in UCSD Pascal.  Java compiler reads Java source and generates Java bytecode, which is shipped to user.  Each client must have a Java Virtual Machine program, which interprets (“runs”) Java bytecodes.

java-fall0113 Java Features—Robust  Java enforces compile-time type checking and this eliminates some error prone constructs of C/C++.  Pointer arithmetic is eliminated which allows for, e.g., runtime checking of array subscripts, and enforces security of the Java model.  Explicit declarations are always required; argument types of methods are always checked (unlike C). This allows the Java complier to perform early error detection.  Java is most secure of popular languages because it is strict and security was built in

java-fall0114 The Java 2 Platform Sun are now offering 3 “editions”:  Java 2 platform, Standard Edition (J2SE) –Refines earlier JDKs –Available in version 1.4.  Java 2 platform, Enterprise Edition (J2EE) –Incorporates multiple technologies for server-side and multi-tier applications.  Java 2 platform, Micro Edition (J2ME) –Optimized run-time environment for consumer products.

java-fall0115 Installing JDK  Become administrator (for Windows) or root (for Linux, etc.) –This is recommended though probably not essential.  Go to http://java.sun.com/j2se/1.4  Select “Download Java 2 SDK, standard edition, v1.4.x,” for Windows or Linux, etc.  Download the software, then read http://java.sun.com/j2se/1.4/install-windows.html or http://java.sun.com/j2se/1.4/install-linux.html, etc. –Pay attention to instructions for setting PATH.  These give a command line interface for the Java compiler javac and the Java interpreter (JVM driver) java, etc. –If you are familiar with UNIX environments, but want to use these commands from Windows, consider installing Cygwin: www.cygwin.com.

java-fall0116 Java Books—I  Core Java, by Gary Cornell and Cay S. Horstmann, offers detailed coverage of the whole language and packages for advanced programmers, including the Swing Set. (We will not cover these user interface issues here) Also Volume 2 gives good coverage of advanced topics such as JDBC, RMI, JavaBeans and security.  Java, How to Program, by Deitel and Deitel, Prentice-Hall, starts with beginning programming concepts and progresses rapidly through Java language. It has the most programming exercises and also has companion teaching multimedia books. The third edition has Swing Set and also the advanced API’s.

java-fall0117 Java Books—II  The Java Programming Language, by Ken Arnold and James Gosling, David Holmes, 3 rd Edition, Addison-Wesley, 2000, is the classic on the language basics for intermediate and advanced programmers. It covers threads and I/O packages, but not applets or windowing packages.  Java in a Nutshell, by David Flanagan, is the language reference book in the familiar O'Reilly series. One of the better references. Also Java Examples in a Nutshell.  The Java Language Specification, second edition. James Gosling, Bill Joy, Guy Steele, Gilad Bracha, April 2000. The ultimate reference for hardened computer scientists/compiler writers.

java-fall0118 Resources for the Java Programming Language  http://java.sun.com web site has plenty of references including –Tutorial: http://web2.java.sun.com/docs/books/tutorial –Books: http://web2.java.sun.com/docs/books  Collection of Java Resources: http://www.gamelan.com http://www.jroundup.com/

java-fall0119 Java Language Basics

java-fall0120 Obvious similarities to C, C++  Java syntax has many similarities to C, C++.  All variables must be declared  Syntax of expressions and control structures almost identical to C, C++  C or C++ style comments allowed.  For Fortran programmers, syntax is a bit different (; to end line { …} to delineate blocks ) for basic material. Fox thinks “structured programming syntax more natural than Fortran 90. –Arrays not as good for Science as those in Fortran –No Array syntax as in Fortran90

java-fall0121 Obvious differences from C, C++  No low-level pointers or pointer arithmetic. –Instead have variables and expressions of reference type.  No malloc() or free() to allocate more memory for dynamically created data structures —instead have a “new” operator for creating objects, plus automatic garbage collection.  Can declare variables almost anywhere (like C++).  No struct, union, enum, typedef—classes and objects are used uniformly instead.

java-fall0122 Primitive types rationalized  Java characters use 16-bit Unicode Worldwide Character Encoding instead of 8-bit ASCII. Supports all alphabets and languages.  Primitive types for integers and floats have machine independent semantics.  Boolean expressions in Java have value “true” or “false” (not 0, 1,...)

java-fall0123 Three kinds of comments in Java  /* ignore all between stars */ –As for C  // ignore all till the end of this line –As for C++  /** this is a documentation comment */ –Should appear immediately before, e.g., class or method definition, and describe intended use.

java-fall0124 Documentation Comments  Used by documentation-generating tools like javadoc to produce documentation, typically in HTML form.  Optionally include formatting tags like @param, which flags a description of a method parameter: /** This method does what it feels like. @param bar This is a pointless argument. */ void foo (int bar) {...}  Other formatting tags include @returns which flags a description of a method result value, or @see name, which creates a hypertext link to name.

java-fall0125 Java Keywords  Java reserves the following keywords: abstrac t boolean break byte case catch char class const continu e default do double else extends final finally float for goto if implemen ts import instanceo f int interface long native new package private protecte d public return short throw throws transie nt try void volatile while  goto is not allowed in Java, but it’s still reserved (to protect Fortran programmers)!  null, true, and false are literals with special meaning.

java-fall0126 Java Language—Program Structure  Source code of a Java program consists of one or more compilation units, each implemented as a file with extension “.java”.  Each compilation unit can contain: –a package statement –import statements –class declarations and/or interface declarations.  In typical Java development environments, exactly one of the class (or interface) declarations in each compilation should be marked public.  The file should be named after the public class. e.g. if the public class is Foo, the file name should be “Foo.java”.

java-fall0127 Java Types  Each Java variable or expression has a definite type, given by a declaration such as int i; double x, y, z; Color c;  There are two sorts of type: –Primitive types like ints or booleans are built into the language. –Reference types. These include class types like Color, and array types (and also interface types).

java-fall0128 Primitive Types  There are 4 integer types: byte short int long Sizes are 8, 16, 32 and 64 bits, respectively.  float is 32 bits, double is 64 bits. Floating point arithmetic and data formats are defined by IEEE 754 standard.  char format is defined by 16 bit Unicode character set.  boolean is either true or false.  One can use casts for arithmetic conversion, as in: int i ; float x ; i = (int) x ;

java-fall0129 Reference Types  These are the types associated with composite entities like objects and arrays.  They are called reference types because a variable or expression in a Java program with reference type represents a reference (or pointer) to a composite entity. –Any variable of reference type may take the value null.  Reference types can be divided into: –Class types –Interface types (discussed later) –Array types

java-fall0130 Strings—an Example of a Class Type  Java environments provide predefined classes for common data types. Every Java environment provides a String class.  Declaration of a String variable looks like: String s ; // variable declaration  The variable declaration itself doesn’t create any objects. We can create a new String object by, e.g.: s = new String(“This is the text”) ; // object creation  These may be combined on one line: String s = new String (“This is the text.”) ;

java-fall0131 A Constructor Function  In the object creation expression: new String (“This is the text.”) the term String (“This is the text.”) is a constructor invocation.  All classes have special “functions” called constructors. These functions have the same name as the class. They initialize the fields of the object.  Constructor functions are only used in object creation operations—nearly always directly after a new operator.  In this example the constructor has one argument: a string literal. –We will see later that in general constructors can have arbitrary argument lists.

java-fall0132 Some features of Strings.  Strings are Java objects, but Java provides some syntax peculiar to strings.  In fact literal string in double quotes itself refers to a pre-existing String object—so in practice we may drop new operation for string constants: String s = “This is the text.” ;  After creation, characters of a string object never change. –In other words: string objects are immutable.

java-fall0133 Operations on Strings.  Although a String object is immutable, String-valued variables can be reassigned to refer to new string objects: String str = “Chicken soup with rice” ; int n = str.indexOf( ‘w’ ) ; str = str.substring(0,n) + “is n” + str.substring(n+6) ; // Result: “Chicken soup is nice”.  The operator + is used for concatenation (special syntax for strings).  indexOf() and substring() are methods of the String class— and are not a special syntax! –They illustrate the general syntax of method invocation on an object.

java-fall0134 Array Types  As for objects, declaring an array variable is distinct from creating on the array: int states[] ; // variable declaration and: states = new int[128] ; // array creation  Again, these can be combined: int states[] = new int[128] ;  Alternative (better?) syntax for declaration: int[] states ;

java-fall0135 Subscripts  With states is declared as above: int states[] = new int[128] ; it can be subscripted by integers from 0 to 127.  Subscripts are checked at runtime: states[-1] or states[128] will immediately generate exceptions. –This subscript checking is an example of where Java trades robustness for performance  Array length is given by the length instance variable: int len = states.length ; // assigns len = 128  One cannot change the length of an Array – the Java vector class is designed for this

java-fall0136 Arrays of Objects  Arrays of arbitrary objects can be constructed, e.g.: Color manycolors[] = new Color[1024];  This creates an array of object references. It does not create actual objects for individual elements.  Before you use the array elements, you may need to use object constructors to allocate each object, e.g.: for (int i = 0 ; i < 1024 ; i++) manycolors [i] = new Color() ;

java-fall0137 Multidimensional Arrays  Multidimensional arrays are arrays of arrays. In general these arrays may be “ragged”: int graph[][] = new int[2][]; graph[0] = new int[4]; // Row 0 has length 4 graph[1] = new int[7]; // Row 1 has length 7... graph[1][1] = 9;  Shorthand syntax for creating a rectangular array: char icon[][] = new char [16][16]; // 16 by 16 array –Note icon is still logically an arrays of arrays, and nothing in Java forces it to stay rectangular. E.g. later someone might do icon [8] = new char [17] ; // Now ragged!  Does not violate rule that Array lengths constant as icon [8] points to a new array of length 17.

java-fall0138 Java Language—Expressions  Most Java expressions are similar to C or Fortran. Here are some examples: –arithmetic: 2 + 3 (2 + 3) * i –auto-increment and decrement: i++ // equivalent to i = i +1 –Boolean: ((i > 0) && (j > 0)) || (state == –1) –bit operations: i << 1 // Shift bit pattern 1 place left –conditional expression: (i > 0) ? expression1 : expression2 False True

java-fall0139 Java Language—More Expressions  Java has some expressions of its own: –string concatenation: “fred” + “jim” // Value is “fredjim” – object “instance of” test: (a instanceof B) // true iff object a has type (class) B

java-fall0140 Java Control Flow. I: if Statements  Conditional execution of statements: if (some Boolean expression) { statements to be executed if true }  Optional else clause: if (some Boolean expression) { statements to be executed if true } else { statements to be executed if false }  Nested example: if (some Boolean expression) {... } else if (another Boolean expression) {... } else {... }

java-fall0141 Control Flow II: while Loop Constructs  Normal while loop: while (any Boolean) { Stuff to do } Example: int i = 0 ; while(i < a.length) { a [i] = i * i ; i++ ; }  while loop with test at end: do { What to do } while (another Boolean) ;

java-fall0142 Control Flow III: The for Loop Construct  In Java, most often one uses the C++-like variant (cf. DO loops in Fortran): for (declaration1 ; booleanExpression ; expressionList2) { Statements to do } The declaration declaration1 is effected at start of loop, comma-separated expressionList2 is evaluated after every iteration, and the loop terminates when booleanExpression is false.  Typical example: for (int i = 0 ; i < a.length ; i++) a [i] = i * i ;  The original C-like form (no declaration) also available: for (expressionList1 ; booleanExpression ; expressionList2) { Statements to do }

java-fall0143 Control Flow IV: The switch Construct  Identical to C: switch (expression) { case Constant1: // Do following if expression==Constant1 Bunch of Stuff break; case Constant2: // Do following if expression==Constant2 Bunch of Stuff break; default: // Do the following otherwise Bunch of Stuff break; }

java-fall0144 Control Flow V: break and continue  Unlabeled break statement immediately exits the enclosing switch, while, do or for construct: while (true) if (++i == a.length || a[i] == v) break ;  Labeled break statement allows to exit an arbitrary enclosing statement, provided it is labeled: assign: { if (i >= a.length) break assign ; a[i] = v ; } (This is not the best way to do this!)  The continue statement skips to the end of the current iteration of the enclosing while, do or for.

java-fall0145 The Java Object Model: Classes, Instances and Methods

java-fall0146 The Java Object Model Overview  Programs are composed of a set of modules called classes. Each class is a template specifying a set of behaviors involving the data of the class.  Each class has variables, or fields, to hold the data, and methods—akin to functions or procedures in other languages—to define the behaviors.  Each object in a program is created as an instance of a class. Each class instance has its own copy of the instance variables defined for the class.  Classes can be used for data encapsulation, hiding the details of the data representation from the user of the class (e.g., by marking variables as private). Instance Variables Methods

java-fall0147 Defining a Class  A class declaration consists of: –a header giving the class name, modifiers, and possible superclass and interface structure. and a class body usually containing: –declarations of fields (possibly with initializations)—class variables and instance variables. –declarations of methods. –declarations of constructors. These “functions” look like methods, but have the same name as the class. They do initialization when objects—class instances—are created. –nested class and interface definitions. –class or (rarely) instance initialization statements.

java-fall0148 Example: a Predefined Class  A (small) part of the Java Date class: public class Date implements Serializable, Cloneable { public Date( ) {...} // Constructor public Date(long msSinceEpoch) {...} // Constructor public int getTime( ) {...} // Accessor public void setTime(long msSinceEpoch) {...} // Mutator public boolean after(Date when) {...} // Comparision public boolean equals(Object obj) {...} // Comparision... }  Note: all variables, methods and constructors visible from “outside” the class—parts of Date that programmers writing code in other classes are allowed to use—have the public modifier in their declaration.

java-fall0149 Creating a Class Instance  The Date class represents a particular date and time, with a resolution of milliseconds.  The first of the two Date constructors (“no-argument constructor”) constructs an instance of the Date class and sets its value to the current moment: new Date()  Constructors (like methods) can be overloaded. Constructors of same name are distinct if they have distinct argument types. If ms is a long, the object: new Date(ms) represents a moment ms milliseconds after January 1, 1970, 00:00:00 UTC (Coordinated Universal Time).  Java will become obsolete (2^63 – 1) / 1000 seconds after that (approximately 292 million years AD, UTC)...

java-fall0150 Example of Using a Class  An example application using a method of the Date class: import java.util.Date; public class DateTest { public static void main (String[ ] args) { Date early = new Date(1000) ; // very early seventies! Date today = new Date() ; // Now! if (today.after(early)) System.out.println( "Today is not early!") ; }

java-fall0151 Instance Variables  A very simple class: public class Complex { public double real ; public double imaginary ; }  Essentially like a C struct. (i.e. just a collection of variables) Every instance of Complex has its own real and imaginary variables. These fields are therefore called instance variables.  Use: Complex z = new Complex() ; // Default constructor z.real = 0.0 ; z.imaginary = 1.0 ;

java-fall0152 Class Variables  Besides instance variables, a class may contain “global variables” that are not associated with any instance.  A class variable (also called a static variable) is flagged by the static modifier in its declaration: class Potato { public String name; static public int num = 0 ; // Class variable—number of potatoes. } Potato p = new Potato(), q = new Potato() ; p.name = “one potato” ; q.name = “two potato” ; Potato.num += 2 ; // static field prefix is class name.

java-fall0153 Method Definitions  Subprograms in Java are called methods. In the abstract, the declaration format is: methodModifiers returnType methodName (parameter list) { declarations and statements }  The parameter list contains the types and names of all the parameters.  The declarations and statements are the body of the method. Parameter names, and variables declared in the body, are local to it.  Control returns from a method when the body finishes execution or a return statement is executed. return statements may return a result value.  Parameters are passed by value but note the value of an Object reference is a reference and not the Object.

java-fall0154 Local variables  Formal parameters of methods, and variables declared inside the bodies of methods, are local variables.  These are a third kind of variable in Java: they are neither instance variables or class variables.

java-fall0155 Static and Non-static Methods  Like fields, methods come in two varieties, which are properly called instance methods and class methods.  The terms non-static methods and static methods are also commonly used.  In all Java applications illustrated so far, the main() method had the modifier static—the main method of an application is required to be a static method.  All other examples of methods illustrated so far were instance methods.

java-fall0156 Instance Methods  Instance methods operate in the context of a particular class instance (i.e. a particular object).  The instance variables of the current object can be accessed without any prefix: public class Complex { // Adds z to the current object public void add(Complex z) { real += z.real ; imaginary += z.imaginary ; } public double real ; public double imaginary ; }

java-fall0157 Invoking an Instance method  This example initializes a and b, then increments the value of a by amount b: Complex a = new Complex(), b = new Complex() ; a.real = 0.707 ; a.imaginary = -0.707 ; b.real = -1.0 ; b.imaginary = 0.0 ; a.add(b) ; // Method invocation This is why one would like operator overloading to represent last as a = a + b;

java-fall0158 this  Within an instance method or constructor the keyword this refers to the current instance. –i.e. the object on which the method was invoked, or which the constructor is initializing.  Appropriate usage—passing self-reference to some other method: public class Complex {... Definition of add(), etc. public void addTo(Complex accumulator) { accumulator.add(this) ; } –The invocation a.addTo(b) adds the value of a to b, i.e. it is equivalent to b.add(a).

java-fall0159 this as a prefix  this refers to current instance  Some programmers will write the this prefix explicitly on every access to an instance variable, e.g.: public void negate() { this.real = – this.real ; this.imaginary = – this.imaginary ; }  This is legal, but ugly!  One time you must use this as a prefix to an instance variable is when the field is hidden by declaration of a local variable with the same name. –The only common example is in constructor declarations. A constructor parameter whose value is used to initialize a field is conventionally given the same name as the field it initializes. See examples later.

java-fall0160 Static Methods  A static method does not operate in the context of a particular instance.  Instance variables of the class cannot be accessed inside the body of a static method unless an explicit object prefix is given.  The keyword this cannot be used in the body of a static method.  To invoke a static method it should be prefixed by the name of the class (similar rule to accessing class variables). –This prefix can be omitted if the method is invoked from another method, etc, defined in the same class.

java-fall0161 Constructors  Constructors are “functions” (not, strictly speaking, methods) that have the same name as the class they belong to.  Any number of constructors can be defined for a class, provided they can be distinguished by the number and type of their parameters (overloading).  If no constructors are explicitly defined, the compiler generates a single default constructor with no arguments. –Note: the default constructor disappears once any explicitly-defined constructor is given!

java-fall0162 A Better Potato class Potato { public Potato(String name) { this.name = name ; // Idiomatic use of this num++ ; } public static int getNum() { // A static method return num ; } private String name ; // Note: now private private static int num = 0 ; // Also private } Potato p = new Potato(“one potato”), q = new Potato(“two potato”) ; System.out.println(“There are ” + Potato.getNum() + “ potatoes”) ; One should add as name private Public String getName(){ // Accessor method return name; }

java-fall0163 Remarks  In the constructor, the unqualified symbol name refers to the local variable declared in the parameter list. –Because this declaration hides the declaration of name as an instance variable, we must prefix with this to access the latter.  The data fields are now private. This means they can be accessed only from methods within the class, not from other classes.  The method getNum() returns a “global” property of the class—the total number of Potato objects that have been created. –Hence it is natural to declare it as a static method—it is not associated with any individual instance.

java-fall0164 Type Conversions  Java allows implicit type conversions in some contexts.  Generally speaking the conversions allowed implicitly (without a cast) are what are called widening conversions.  For primitive types, the widening conversions are from any integer type to any wider integer type, (int to long, etc) or from a float to a double.  Narrowing conversions, by contrast, would include conversion from long to int, or from a floating point type to an integer type.  Narrowing conversions usually have to be specified explicitly with a cast, e.g. float x ; int i = (int) x ;

java-fall0165 Overloading  A class can declare several methods with the same name, providing each declaration has a different number of arguments, or different argument types. –We refer to the combination of the method name and its list of argument types as the signature of the method.  Example: class Shape { setColor(Color c) {...} setColor(int rgb) {...} setColor(int r, int g, int b) {...}... } –The method setColor() is overloaded with three different signatures.

java-fall0166 Calling an Overloaded Method  If the types of the argument expressions in a method invocation exactly match the types of the parameters in one particular declaration of the method, the compiler naturally chooses to call that particular method implementation.  There is a complication, though: the Java language allows implicit type conversion of method arguments. –The allowed conversions are the widening conversions.  In general overload resolution chooses the most specific method signature matching the actual arguments. –If there are several applicable signatures, and no single one is more specific than all the others, a compile time error is flagged.

java-fall0167 Examples of overload resolution void foo(long p) {...} // Signature I void foo(int p) {...} // Signature II void foo(long p, int q) {...} // Signature III void foo(int p, long q) {...} // Signature IV long l ; short s ; int i ; foo(l) ; // Exact match—use Signature I. foo(s) ; // Do widening conversion of s to int, and use // Signature II —unique “most specific” case. foo(l, s) ; // Uses Signature III —only case applicable by // widening conversions. foo(i, i) ; // Compile time error! Signatures III and IV // are both applicable but neither is more specific // than the other!

java-fall0168 Header of Class Definition—Details  In the abstract, the definition format is: classModifiers class className [ extends superclass ] [ implements interfaceList ] { body of class }  The optional extends and implements clauses will be discussed in detail in later lectures. u Note classmodifiers can be absent – called default mode

java-fall0169 Modifiers of Classes  Possible classModifiers are: –default—the class may be used by code in current package. –public—the class may be used freely by code outside the package. –abstract—the class contains abstract methods without implementation (abstract classes will have subclasses that define implementation of methods—see later). –final—this class cannot have a subclass: see later. –strictfp—all intermediate results in all float or double expressions appearing in the class have strict IEEE 754 exponents. –private—only allowed for a nested class. Meaning as for other members. –protected—only allowed for a nested class. Meaning as for other members. –static—only allowed for a nested class. Meaning analogous to other members. members of a class are: member classes (see later) fields Methods Nested classes discussed briefly later

java-fall0170 Modifiers of Fields I  In the abstract, the declaration format is: – fieldModifiers type variableDeclaratorList ; – where a variableDeclarator has the format: – fieldName [ dimensionStuff ] [ = expression ]  Examples are: –Static int fred = 3; –int fred = 3; // Equivalent to –protected int fred = 3;  Note fieldmodifiers can be absent – called default mode

java-fall0171 Modifiers of Fields II  Possible fieldModifiers are: –public—this field is accessible from any code. –protected—accessible from code in a subclass (as well as code in the same package—default accessibility). –private—only accessible from code in the same class. –static—this is a class variable: see earlier. –final—this field cannot be modified after it is initialized. –transient—the value of this field will not be included in a serialized representation of an instance. –volatile—any cached copy of the field maintained by an individual thread will be reconciled with the master copy every time the field is accessed.

java-fall0172 Modifiers of Methods I u In the abstract, recall, the declaration format is: methodModifiers returnType methodName (parameter list) [throws exceptionList ] { declarations and statements }  An example is:  Public String getName(){ // Accessor method return name; } u Note methodmodifiers can be absent – called default mode

java-fall0173 Modifiers of Methods II  Possible methodModifiers are: –public—this method is accessible from any code. –protected—accessible from code in the same package (default), or a subclass. –private—only accessible from code in the same class. –abstract—the method has no implementation here—declaration has a semicolon in place of a body. –static—this is a class method: see earlier. –final—this method cannot be overriden: see later. –synchronized—other synchronized methods are locked out while this method is executing: see later. –native—the implementation of this method is given in a platform- dependent language. Declaration has a semicolon in place of a body. –strictfp—intermediate results in all float or double expressions appearing in the body have strict IEEE 754 exponents. Synchronized refers to treatment of concurrency Which is implemented with threads in Java Although an interesting feature of Java, we will Not need it in this class Strictfp does not allow default Intel 80 bit internal precision

java-fall0174 The Java Object Model: Inheritance and the Class Hierarchy

java-fall0175 Some Dependencies between Classes  Use –A uses B: the most informal and general relation. A might, for example, call a method from class B, or have a method with argument type B or return type B.  Containment –A has a B: an important special case of use—class A has a field of type B.  Inheritance –B is an A: class B has all the properties of class A. The compiler treats B as a special case of A, and allows an instance of B to be used in any place where an instance of A could appear. In general the class B will extend A with some extra properties of its own.

java-fall0176 Inheritance  The inheritance relation is (unexpectedly?) powerful; it is built into all fully object-oriented languages.  In Java, if some class A has been defined, we can subsequently declare a new class, B, and specify that it extends A.  Class A is called the superclass of B. Class B is a subclass of A.  The class B is automatically given (inherits) all the fields and method definitions of A. Further fields and methods can be added that are specific to B.  In particular, for every method signature in class A, class B will have a method with identical signature. Crucially, though, the class B may define a different implementation for some of those methods.

java-fall0177 Trivial use of Inheritance class Shape { void setColor(Color color) {this.color = color ; } Color color ; int x, y ; // position of center, say } class Circle extends Shape { void drawCircle() {...} double radius ; } class Rectangle extends Shape { void drawRectangle() {...} double height, width ; }  Subclasses automatically inherit color, x, y fields of Shape, and setColor() method.

java-fall0178 A Limited Kind of Polymorphism void setAllColors(Shape [] shapes, Color color) { for(int i = 0 ; i < shapes.length ; i++) shapes [i].setColor(color) ; } Shape [] bag = new Shape [N] ; bag [0] = new Circle() ; bag [1] = new Rectangle() ;... setAllColors(bag, Color.red) ;...  The function setAllColors works on a collection of shapes, and works correctly independently of whether each shape is actually a Circle or a Rectangle.

java-fall0179 Class Hierarchies  Class hierarchy diagrams represent inheritance relations between classes: Class: Shape Class: Circle Class: Rectangle  These diagrams become more complex as subclasses are further extended. But they are always trees, because in Java each subclass has a single superclass.

java-fall0180 Inheritance with Overriding class Shape { void draw() {} Color color ; int x, y ; } class Circle extends Shape { void draw() {...} double radius ; } class Rectangle extends Shape { void draw() {...} double height, width ; }  Subclasses override the definition of draw() in the superclass.  Bodies of methods contain the actual code for drawing a circle or rectangle, respectively.

java-fall0181 True Polymorphism void drawAll(Shape [] shapes) { for(int i = 0 ; i < shapes.length ; i++) shapes [i].draw() ; } Shape [] bag = new Shape [N] ; bag [0] = new Circle() ; bag [1] = new Rectangle() ;... drawAll(bag) ;  The draw() method invoked is the method defined in the class of the referenced object (Circle or Rectangle). –not the implementation defined in the compile-time type of the variable, namely Shape.  drawAll() correctly draws a mixed bag of shapes whose details may be unknown when this method is written.

java-fall0182 Runtime Lookup of Methods class Shape { void draw() {...} } Square s = new Square() ; s.draw() ; class Square extends Rectangle { // No declaration of draw() } class Rectangle extends Shape { void draw() {...} } class Circle extends Shape { void draw() {...} } Search up the inheritance tree until find first class that defines method.

java-fall0183 Inherited Methods and Overriding  The method associated with the actual class of the instance is called, even if it invoked from code in the superclass.  Suppose we add a drawInColor() method to Shape: class Shape { void draw() {} void drawInColor(Color color) { this.color = color ; draw() ; } Color color ; int x, y ; }  The implementation of drawInColor() is inherited by the subclasses. But when it is invoked on one, their own draw() methods are called! More polymorphism.

java-fall0184 Abstract Methods and Classes  In our example, the draw() method in the Shape class did nothing. It may not be necessary to give a implementation of this method in the base class at all, because it may be that it is only ever invoked on instances of subclasses representing concrete shapes (as here).  In this situation, the superclass and unimplemented methods can be declared abstract: abstract class Shape { //abstract class abstract void draw() ; // abstract method Color color ; int x, y ; } –It is not possible to create instances of abstract classes. One must create a subclass that overrides all abstract methods of the base class, giving implementations.

java-fall0185 Final Methods and Classes  If a method is declared final, it may not be overridden in subclasses (opposite extreme to abstract, which must be overridden!)  If we declared draw() in Rectangle to be final, we could never give a more specialized draw() in a subclass: class Rectangle extends Shape { final void draw() {...} // final method double height, width ; } class Square extends Rectangle { void draw() {...} // Compile-time error!! } –In places where the compiler can tell that a final method will be called, it can produce optimized code to avoid overheads of “late binding”.  A final class cannot be extended.

java-fall0186 Protected Access  By default (no modifier) a field or method of a class can be accessed by any code appearing in the same package. –Packages are discussed later.  The access modifier protected on a field or method means that this member can also be accessed by any subclass of the class in which it is declared.  Note this modifier increases accessibility from the default... –... because a subclass may be declared outside the package that contains the superclass. –Least accessible members are private (visible in declaring class only), followed by default (declaring package only), followed by protected (package and subclasses), followed by public (visible everywhere).

java-fall0187 Inner Classes  Java 2 introduced a major addition to the original language definition---"nested types".  A nested class (or interface) is a class/interface that is defined “inside” some other class (or interface).  Nested classes and interfaces are sometimes loosely referred to as "inner classes".  Technically, an inner class is only one of several kinds of nested types in Java 2 (see next slide).

java-fall0188 Nested Types  field member class – includes: includes: –static variable static member class –instance variable inner class  local variable local class  includes:  expression anonymous class  Anonymous classes used in way reminiscent of lambda functions in functional languages, (or "blocks" in Smalltalk) to define pieces of code that are used *very* locally –typically in the context of a single expression. –Popular for defining simple event-handlers in GUI programming, though many other applications as well.

java-fall0189 Critique of Nested Types  Nested types are undoubtedly a powerful and useful addition to the language. –Most useful nested types arguably static member classes (for software engineering) and anonymous classes (for slick "one-liners").  But the introduction of the whole paraphernalia of nested types has significantly complicated the Java language definition. –Many places where, for example, choice between inherited and enclosing class behaviors could be ambiguous--needs complicated rules to disambiguate. –JVM definition was frozen before Java 2 introduced nested types, so compilers must preprocess nested types to equivalent outer types--ugly hack.

java-fall0190 The Universal Superclass—Object  The Java language provides a superclass for all other classes. If no extends clause is given in a class definition, the class implicitly extends Object.  Array types are also considered to extend Object.  A variable of type Object can hold a reference to any object or array. –This is useful for generic capabilities which apply to all Object classes  Strictly speaking, Object is the root of every inheritance diagram.

java-fall0191 Methods on the Object class Public class Object { public final Class getClass() {... } // Basis for reflection. public String toString() {... } //A String representation public boolean equals(Object obj) {... } // Equality test public int hashcode() {... } // For use by hash tables protected Object clone() throws... {... } // Bit by bit copy public final void wait() throws... {... } // Deschedule this thread public final void wait(long millis) throws... {... } public final void wait(long millis, int nanos) throws... {... } public final void notify() throws... {... } // Reschedule any... public final void notifyAll() throws... {... } //... or all threads. protected void finalize() throws... {... } // invoked by GC. } Reflection is a powerful capability to allows you to find out about an Object at run-time toString labels a class equals test if Objects are identical which includes the case if two objects are identical references

java-fall0192 Reference Conversions  Conceptually, we saw, an instance of a subclass “is an” instance of the superclass.  Hence one can assign a reference to a subclass object to a variable of a superclass type. –Shape fred; Circle jim; fred = jim; //allowed  Concretely, this implies a conversion from a subclass type to a superclass type is regarded as a kind of widening conversion. –Recall widening conversions are allowed implicitly in various contexts.  Narrowing conversions on reference types go the other way—from a superclass down to some subclass. –Narrowing conversions require an explicit cast. –jim = (Circle) fred;  Good programming practice minimizes use of narrowing conversions, but sometimes they are necessary.

java-fall0193 An Aside on Coding Conventions  http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.ht ml is a good set of “best practice” conventions http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.ht ml  Code conventions are important to programmers for a number of reasons:  80% of the lifetime cost of a piece of software goes to maintenance.  Hardly any software is maintained for its whole life by the original author.  Code conventions improve the readability of the software, allowing engineers to understand new code more quickly and thoroughly.  If you ship your source code as a product, you need to make sure it is as well packaged and clean as any other product you create.  There are another set at http://g.oswego.edu/dl/html/javaCodingStd.html http://g.oswego.edu/dl/html/javaCodingStd.html  Bryan’s favorite Rule: Do not require 100% conformance to rules of thumb such as the ones listed above

java-fall0194 Simple Collections  The package java.util contains a family of collection classes.  Here we will only mention two of the most widely used: –Vector, and –Hashmap.  Note Vector is supposed eventually to be superceded by ArrayList. –Consider using ArrayList in your future programs, but Vector is so widespread we describe it here.

java-fall0195 A Vector is Like an Array  A Vector can be used essentially like an ordinary array.  It has a well-defined current size, returned by the size() inquiry.  This can be set with setSize(), but usually a Vector is grown dynamically using methods on next slide.  Vector stores all elements as if the have type Object  If 0 < idx < size(), the methods: void set(int idx, Object obj) Object get(int idx) respectively assign and retrieve value of element idx.

java-fall0196 A Vector can Grow and Shrink  Typically one grows a vector by adding a new element at the end with: void addElement(Object obj) Causes size() to be incremented by 1.  An arbitrary element can be removed by Object remove(int idx) This method causes higher elements to be shifted down one place, and size() to be decremented by 1.  An element can be inserted in an arbitrary place by insertElementAt(Object obj, int idx) Element at idx and higher are shifted up one place, and size() is incremented by 1.

java-fall0197 Using Vector void drawAll(Vector shapes) { for(int i = 0 ; i < shapes.size() ; i++) ( (Shape) shapes.get(i) ).draw() ; // Narrowing conversion } Vector bag = new Vector() ; bag.addElement(new Circle()) ; // Widening conversion bag.addElement(new Rectangle()) ;... drawAll(bag) ;  For polymorphism, Vector stores items in Object references. Hence, get() returns an Object, which usually needs to be cast back to a more specific type.  If the referenced object is not an instance of the type in the cast, a run-time ClassCastException occurs.

java-fall0198 A HashMap is an Associative Array  For future reference, we also discuss HashMap here  A HashMap is similar to a vector, but the “index” is an arbitrary object—very commonly a string. –Perl has excellent support of this using {} syntax  This index is now called a “key”.  In simple cases you create a HashMap with the no-argument constructor, then put key-value pairs in it using Object put(Object key, Object obj) (returns old value if key was already in the table).  Retrieve the element currently indexed by key by: Object get(Object key)  Remove the element currently indexed by key by: Object remove(Object key)

java-fall0199 Using HashMap  HashMap table = new HashMap() ; // Create  table.put(“red”, “stop”) ; // Insert hashes  table.put(“green”, “go”) ;  String s = (String) table.get(“red”) ; // returns “stop”  String t = (String) table.remove(“green”) ; // returns “go”  String u = (String) table.get(“green”) ; // returns null

java-fall01100 Widening Conversions on Arrays  There is a widening conversion between two array types if there is a widening reference conversion between their component types.  This is useful, but can lead to anomalies if used carelessly: Circle [] bag = new Circle [N] ; setAll(bag) ; // Widening: Circle [] to Shape []. // OK at compile-time. void setAll(Shape [] shapes) { shapes [0] = new Circle() ; shapes [1] = new Rectangle() ; // Widening: Rectangle to Shape. // But throws ArrayStoreException... // if invoked as above! }  Effect would be to assign Rectangle to array of Circles. Requires the compiler to add a new kind of run-time check.

java-fall01101 Overloading with Inheritance void foo(Object p) {...} // Signature I void foo(Shape p) {...} // Signature II void foo(Object p, Shape q) {...} // Signature III void foo(Shape p, Object q) {...} // Signature IV Object o ; Shape s ; Circle c ; foo(o) ; // Exact match—use Signature I. foo(c) ; // Do widening conversion of c to Shape, and use // Signature II—unique “most specific” case. foo(o, c) ; // Uses Signature III—only case applicable by // widening conversions. foo(s, s) ; // Compile time error! Signatures III and IV // are both applicable but neither is more specific // than the other!

java-fall01102 Overload Resolution across Classes class Shape { void foo(Circle q) {...} // Signature I } class Circle extends Shape { void foo(Shape q) {...} // Signature II } Shape s ; Circle c ; s.foo(c) ; // Uses Signature I—exact match. c.foo(c) ; // Compile time error! Signatures I and II // are both applicable but neither is more specific // than the other!  In compile-time overload resolution (choice of signature), the prefix object expression is treated on the same footing as an extra argument.

java-fall01103 Summary: Overloading vs. Overriding  Resolution of overloading occurs at compile time. The compiler chooses a unique method signature out of several different signatures available (or flags a compile time error if it cannot).  Overriding occurs in the context of a single signature. In general, if the class hierarchy contains several definitions with identical method signatures, the appropriate definition is chosen at run time.  Within the body of a class that overrides a method, the method from the superclass can be invoked instead by using the super prefix.

java-fall01104 Constructors and Inheritance  Constructors of subclasses must invoke a constructor of their superclass, to initialize the fields there.  If a superclass constructor is not explicitly invoked, the no-argument constructor of the superclass is called, implicitly, by the compiler. –A compile-time error is flagged if no such constructor exists.  If any superclass constructor other than the no- argument constructor is required, it must be invoked explicitly.  In this case the first statement of a subclass constructor is an explicit constructor invocation using the name super.

java-fall01105 Superclass Constructor Invocation class Shape { public Shape(Color color, int x, int y) { this.color = color this.x = x ; this.y = y ; } Color color ; int x, y ; } class Circle extends Shape { public Circle(Color color, int x, int y, double radius) { super(color, x, y) ; // superclass constructor invocation this.radius = radius ; } double radius ; }

java-fall01106 Exceptions

java-fall01107 Exceptions are Pervasive  Java has a concept of exceptions similar to C++.  Unlike C++, Java exceptions are strictly checked.  Most classes in the standard Java library throw some exceptions. We will see, these must be caught or thrown.  This means that it is almost impossible to write useful Java code without some knowledge of the exception mechanism!

java-fall01108 Exception Objects, and throw  Any kind of exception that can be thrown by Java code is described by an exception object. It’s class must be a subclass of Throwable.  If e is a Throwable object, the statement throw e ; behaves something like a break statement; it causes the enclosing block of code to end abruptly.  If the throw statement appears inside a try statement who’s catch clause matches the class of e, control is passed to the catch clause.  Otherwise the whole method (or constructor) ends abruptly. The exception e is thrown again at the point of invocation (in the calling code).

java-fall01109 throw compared with break try {... throw new MyException() ;... } catch (MyException e) {... }...  Control jumps to start of matching catch clause myBlock : {... break myBlock ;... }...  Control jumps to end of matching block

java-fall01110 Methods that throw exceptions  In general, any exception that might be thrown in the body of a method or constructor, in a place where it is not enclosed by a matching try-catch construct, must be declared in a throws clause in the header of the method: void foo() throws MyException { // throws clause... throw new MyException() ; // No enclosing // try-catch(MyException...)... }  The compiler will insist invocations of foo() are treated with the same care as actual throw statements—either enclosed in matching try-catch constructs, or declared in turn in the header of the calling method.

java-fall01111 Exception Handling in Nested Calls void method1() { try { method2() ; } catch (Exception3 e) { doErrorProcessing(e); } void method2() throws Exception3 { method3() ; // method2 just passes exception through } void method3 throws Exception3 { throw new Exception3() ; // create exception }

java-fall01112 Example using java.io import java.io.* ; PrintWriter out ; try { out = new PrintWriter(new FileOutputString(“filename”)) ; // create and open file out.write(“stuff put out”) ;... out.close() ; } catch (IOException e) { // Catches all I/O errors, including read and write stuff, System.err.println(“IO error: ” + e.getMessage()) ; System.exit(1) ; }

java-fall01113 How (not) to Ignore an Exception  Sometimes you can’t think of a good way to recover from an exception—e.g. an exception thrown by a library method. But the compiler forces you to do something.  Probably the worst thing you can do is to wrap the method invocation in a try-catch with an empty catch clause— –the useless try-catch constructs make the code unreadable, and –meanwhile, ignoring an error condition and silently carrying on the program may produce code even less reliable than, say, a typical C program, where the library error probably at least aborts the whole program!  Usually it is safer to have your methods throw the exceptions—all the way up to the main method, if necessary. Then at least the program will stop.  If you are really lazy you can just declare every method you ever write with throws Exception...

java-fall01114 Part of the Exception Hierarchy  catch(FileNotFoundException e) {... } would catch specific exception whereas  catch(IOException e) {... } would catch all IOexceptions Throwable... ErrorException RuntimeExceptionIOException EOFException FileNotFoundException InterruptedIOException

java-fall01115 Unchecked Exceptions  There are two exceptions (!) to the rule that all exceptions must be explicitly caught or thrown.  Error classes usually represent problems that might occur unpredictably in the JVM. For example OutOfMemoryError (although unusual in practice) might occur at almost any time.  RuntimeException classes usually represent errors “built into” the language—not thrown by a throw statement. There are about 20, including: –ArithmeticException, ArrayIndexOutOfBoundsException, NullPointerException, ClassCastException, etc.  Note that exceptions that are thrown but not caught appear as error message on stderr. For applets they appear in the “Java console” of the browser.

java-fall01116 Defining you own Exceptions  The Exception class has fields and methods to give information how the exception occurred. There are two constructors; one includes a custom message  Can throw an exception of type Exception with a unique message, or create a subclass: class MyException extends Exception { public MyException () { super ("This is my exception message.") ; } public MyException (String gripe) { super (gripe); } } public static void MyMethod() throws MyException {... throw new MyException() ;... }  Methods e.getMessage() and e.printStackTrace() can be used on exceptions.

java-fall01117 Interfaces

java-fall01118 Abstract Classes Revisited  Recall an abstract class is a class that contains some abstract method declarations, with no implementation.  An abstract class can only be instantiated indirectly, as a superclass of a class that overrides all the abstract methods, and gives them an implementation. You cannot directly create an instance of an abstract class. –Constructors, static methods, private methods cannot be abstract. –A subclass that does not override all abstract methods is still abstract. –A method that overrides a superclass method cannot be abstract  But an abstract class will generally also contain “non- abstract” members—method implementations, instance variables, etc—and constructors.

java-fall01119 Interfaces  An interface is something like an abstract class where every method is required to be abstract.  An interface specifies a collection of instance methods (behaviors) without giving the implementation of their bodies— akin to giving an API: public interface Storable { public abstract void store(Stream s) ; public abstract void retrieve(Stream s) ; }  Interfaces cannot include instance variables, constructors, or static methods.  They can include class variables, but only if they are declared final—essentially constant definitions.

java-fall01120 Implementing an interface  As for an abstract class, one cannot directly create an instance of an interface.  Unlike an abstract class, one cannot even extend an interface to create a class. An interface is not a class, and it cannot have subclasses.  Instead, a class must implement an interface: public class Picture implements Storable { public void store(Stream s) { // JPEG compress image before storing... } public void retrieve(Stream s) { // JPEG decompress image after retrieving... }

java-fall01121 An Interface is a Contract  Any class that implements an interface is guaranteeing a set of behaviors. The body of the class will give concrete bodies to the methods in the interface. –If any methods in the interface are not implemented, the class must be declared abstract.  Example: a class that defines the behaviour of a new thread must implement the Runnable interface: public interface Runnable { public void run() ; }  Any interface defines a type, similar to a class type. An instance of any class that implements a particular interface can be assigned to a variable with the associated interface type.

java-fall01122 An Interface Defines a Type  Assume the classes Picture and StudentRecord both implement the Storable interface: public class StudentBody { Stream s;... public void register(Picture id_photo, StudentRecord id_card) { save(id_photo); save(id_card); } public void save(Storable o) { // o has type Storable o.store(s); // as Storable must have method store } Widening Conversion

java-fall01123 Classes can Implement Several Interfaces  Interfaces address some of the same requirements as multiple inheritance in C++ (for example), but avoid various complexities and ambiguities that come from inheriting implementations and instance variables from multiple superclasses.  A class can extend its superclass and implement several interfaces: class Picture implements Storable, Paintable { // Body must now include any methods in Paintable, // as well as store() and retrieve().... }  Instances of the class acquire all the implemented interface types, in addition to inheriting their superclass type.

java-fall01124 Interfaces can Extend other Interfaces  An interface can extend one or more other interfaces: interface Material extends Storable, Paintable { // Additional methods if necessary...... }  If non-trivial “lattices” of types are really needed, eg: NoColor GreenBlueRed AnyColor CyanYellowMagenta they can be implemented using interface types. e.g. Magenta extends Red but implements cyan

java-fall01125 Interfaces can hold Constant Definitions  Interfaces can hold fields, provided they are static and final.  An interface can be a natural place to define a collection of related constants, perhaps simulating a C-like enumeration type: public interface Direction { public final static int NORTH = 0 ; public final static int EAST = 1 ; public final static int SOUTH = 2 ; public final static int WEST = 4 ; }  Use constants by, eg, Direction.NORTH.  Sometimes a class will implement such an interface, just so it can access the included constants without using the Direction prefix.

java-fall01126 Interfaces can be used as Markers  The Java environment includes several examples of empty interfaces that are used only as markers.  By implementing such an interface, the programmer is typically telling the compiler or runtime system to treat the class in some special way: –Cloneable—the Object.clone() method will throw an exception if invoked on an object from a subclass that does not implement the empty Cloneable interface. –Serializable—the ObjectOutputStream.writeObject() method will not write an object that does not implement the empty Serializable interface. –Remote—any class whose methods may be invoked remotely using the RMI mechanism, must implement the empty Remote interface.

java-fall01127 Summary  Interfaces play a crucial role in structuring programs that need to declare multiple sets of behaviors such as applets and threads.

java-fall01128 Packages

java-fall01129 Packages  One file can contain several related classes, but only one of them can be public. If the public class is called Wheat, then the file must be called Wheat.java.  A set of classes in different files can be grouped together in a package. Each file must start with a package declaration, eg: package mill;

java-fall01130 Packages and Directory Structure (JDK)  In JDK, each of the files in one package must be in the same directory (which may be in an jar archive file).  For simple package names, the name of the directory should be the same as the package: Directory name: mill File: wheat.java: Stone.java: Package mill ; Public class Wheat { …} … Package mill ; Public class Stone { …} …

java-fall01131 Hierarchical Package Names  Packages can be grouped hierarchically. For example, the mill package could be nested in a package called agriculture. Then the name of the package would be changed to agriculture.mill (full name required).  In JDK, the classes of agriculture.mill should appear in a directory called: agriculture/mill (UNIX) agriculture\mill (Windows) (relative to some directory, which must appear on the user’s CLASSPATH).  Standard Java libraries are in packages with names like java.lang, java.util, java.io, etc.  If you need to construct a globally unique name, can use your Internet domain name, inverted, as a prefix, eg: edu.fsu.csit.mpiJava

java-fall01132 Fully Qualified Class Names  A class can always be referred to in Java code by its fully qualified name which includes the package name as a prefix, eg: public class VectorTest { public static void main (String [] args) { java.util.Vector bag = new java.util.Vector() ; bag.addElement(new java.lang.String(“item”)) ; }  Using fully qualified names is tedious in general.

java-fall01133 Import statements  The import declaration allows you to avoid giving fully qualified names, eg: import java.util.Vector ; // import declaration public class VectorTest { public static void main (String [] args) { Vector bag = new Vector() ; bag.addElement(new String(“item”)) ; }  Can also import all classes in, eg, java.util by import java.util.* ; (but note wildcard can only appear in last position).  Note classes (like String) in java.lang are automatically imported.

java-fall01134 CLASSPATH  The import declaration only controls conventions on naming within a source file. It doesn’t address basic accessibility of the class files. You can use a class without importing it.  In JDK (except for classes provided with the Java language) jar files or root directories of any package used (or class files for any classes not in any package) must be in the current directory, or in a directory in the CLASSPATH environment variable.  This variable is used by both the compiler javac and the JVM command, java.

java-fall01135 Java System Packages, I  java.lang contains essential Java classes and is by default imported into every Java file. So import java.lang.* is unnecessary. For example Thread, Math, Object and wrapper classes are here.  java.io contains classes to do I/O.  java.util contains various utility classes that didn't make it to java.lang. Date is here as are Vector, hashtables, etc.  java.net contains classes to do network applications. Sockets, Internet addresses, URLs etc.  java.applet has the classes needed to support applets  java.awt has the original classes to support windowing— The Abstract Windows Toolkit.  java.awt.image has image processing classes.

java-fall01136 Java System Packages, II  java.awt.datatransfer contains classes to transfer data from a Java program to the system clipboard (drag-and-drop).  java.beans contains classes to write reusable software components.  java.lang.reflect enables a program to discover the accessible variables and methods of a class at run-time.  java.rmi—classes for Remote Method Invocation.  java.security enables a Java program to encrypt data and control the access privileges provided.  java.sql—Java Database Connectivity (JDBC) enables Java programs to interact with a database using the SQL language.  java.text are classes that provide internationalization capabilities for numbers, dates, characters and strings.  java.util.jar combines java.class files and other files into one compressed file called a Java archive (JAR) file.

java-fall01137 Additional Java 1.2 System Packages  javax.accessibility—contracts between user interface components and assistive technology.  javax.swing—additional user interface components as well as providing standard “look and feel” for old ones. –border, colorchooser, event, filechooser, plaf, table, text, tree, undo  org.omg.CORBA—Provides the mapping of the Object Management Group CORBA APIs to the Java programming language, including the class ORB, which is implemented so that a programmer can use it as a fully-functional Object Request Broker (ORB).

java-fall01138 Further information  The Java 2 API specification: http://java.sun.com/products/j2se/1.4/docs /api documentation in javadoc format.  The Java Class Libraries, 2 nd Edition, Volumes 1 and 2, plus supplements for the Java 2 platform.

Java-fall011.opennet Technologies Introduction to the Java Language and Object-oriented Concepts Fall Semester 2001 MW 5:00 pm - 6:20 pm CENTRAL (not Indiana)

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Java-fall011.opennet Technologies Introduction to the Java Language and Object-oriented Concepts Fall Semester 2001 MW 5:00 pm - 6:20 pm CENTRAL (not Indiana)

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Presentation on theme: "Java-fall011.opennet Technologies Introduction to the Java Language and Object-oriented Concepts Fall Semester 2001 MW 5:00 pm - 6:20 pm CENTRAL (not Indiana)"— Presentation transcript:

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