CSC 224 Java for Programmers Winter 2002 (2002/01/ /03/13) Monday/Wednesday 3: :40 pm Instructor: John Petlicki Office hours: Monday and Wednesday 1:30 – 3:00 pm (CST Building) Home page:

Review of Resources CD-ROM of "Java Software Solutions" contains: Java(TM) 2 SDK, Standard Edition (for Microsoft Windows) Forte for Java Community Edition (for Microsoft Windows) Slides in PowerPoint 97 format Keyboard materials Example programs

Characteristics of Java Simple Object-oriented Strongly typed Interpreted Architecture neutral Garbage collected Multithreaded Robust Secure Portable

Simple Many programmers are already familiar with Java syntax (borrowed from C and C++). Difficult aspects of C++ are removed. No header files No pointer arithmetic (pointer Syntax) No structures or unions No operator overloading No virtual base classes Etc.

Object-oriented A technique of developing programs by creating entities that contain logically related data and methods that interface to the data

Strongly typed Every variable must have a declared type. Java has eight primitive types  4 types of integers  2 types of floating numbers  1 character type (for Unicode characters)  1 boolean type for truth values

Interpreted The Java source code is compiled into Java bytecode. The Java interpreter can execute Java bytecodes on any machine to which the interpreter has been ported.

Architecture neutral Java bytecode can be run on many different processors. Bytecode  is not dependent on the machine code of any particular computer.  is designed to be easily translated into the native machine code of any machine.

Garbage Collected The programmer does not have to explicitly return allocated memory to the system. Java performs automatic garbage collection when object references are lost.

Multithreaded A program can be designed as multithreaded in order to perform multiple tasks at the same time Can take advantage of multiprocessor systems

Robust Emphasis on early checking of possible problems. Dynamic run-time checking. Eliminate situations that are error-prone Pointer model eliminates possibility of overwriting memory and corrupting data

Secure Security features Java will not corrupt memory outside its own process space. Web browser can prevent Java applets from reading or writing local files And more

Portable The size of primitive data types and the behavior of arithmetic on them is specified and constant. The libraries have portable interfaces – the abstract Window class has implementations for UNIX, Windows and Macintosh.

A Java Program /* * Example HelloWorld.java * Prints the message "Hello, World!“ */ public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, world.”); }

Java programming steps Create a source code program with a text editor (HelloWorld.java) Compile the source code into Java bytecode (javac HelloWorld.java) Run the bytecode with a Java interpreter (java HelloWorld) Java interpreters translate bytecode instruction to machine code and then carry out the instruction. A compiler and interpreter are in the Java 2 SDK (Software Development Kit )

Installing JDK 1.3 For Microsoft Windows platform, run the executable j2sdk1_3 file on the CD For other platforms other than Microsoft Windows, obtain the SDK directly from Sun, at

Java Development Environments DOS command line interface basic with SDK 2. (javac, java) Forte -- heavyweight IDE forte_ce_2.exe on the CD ROM Bluej – lightweight IDE Download from And others

Installing Bluej (optional) Be sure the JDK is installed Download from Download the Bluej tutorial Follow the installation instructions

Java Basics Program structure Naming conventions Package Identifiers Reserved words Data types Type conversions Arithmetic operators and expressions Variable declaration and initialization Assignments

Packages Java provides mechanisms to organize large-scale programs in a logical and maintainable fashion. Class --- highly cohesive functionalities File --- one class or more closely related classes Package --- a collection of related classes or packages

Java Class Library The Java class library is organized into a number of packages: java.lang --- general java.awt --- GUI java.io --- I/O java.util --- utilities java.applet --- applet java.net --- networking

Example using import // Example Time // Prints a greeting message with // the current time. import java.util.*; public class Time { public static void main(String[] args) { System.out.println("Hello!”); System.out.println("The time is " + new Date()); }

Fully qualified class name public class Time1 { public static void main(String[] args) { System.out.println("Hello!"); System.out.println("The time is " + new java.util.Date()); }

Java Reserved Words abstract boolean break byte byvalue case cast catch char class const continue default do double else extends false final finally float for future generic goto if implements import inner instanceof int interface long native new null operator outer package private protected public rest return short static super switch synchronized this throw throws transient true try var void volatile while

Identifiers are the words a programmer creates in a program made up of letters, digits, underscore character (_), and the dollar sign cannot begin with a digit Java is case sensitive, therefore Total and total are different identifiers

White Space Spaces, blank lines, and tabs are collectively called white space White space is used to separate words and symbols in a program Extra white space is ignored A valid Java program can be formatted many different ways Programs should be formatted to enhance readability, using consistent indentation

Variables A variable is a name for a location in memory A variable must be declared, specifying the variable's name and the type of information that will be held in it data type variable name int total; int count, temp, result; Multiple variables can be created in one declaration

Variables A variable can be given an initial value in the declaration int sum = 0; int base = 32, max = 149; b When a variable is referenced in a program, its current value is used

Data Types Type byte short int long float Double Storage 8 bits 16 bits 32 bits 64 bits 32 bits 64 bits Min Value ,768 -2,147,483,648 < -9 x /- 3.4 x with 7 significant digits +/- 1.7 x with 15 significant digits Max Value ,767 2,147,483,647 > 9 x 10 18

Boolean Type boolean boolean constants: true false

Character Type char 16-bit Unicode character. ASCII is a subset of Unicode --- ISO-8859 (Latin-1) Examples of character constants: 'A' 'y' '8' '*' ' ' (space) '\n' (new line).

Escape Sequences \b backspace \f form feed \n new line (line feed) \r carriage return \t tab \" double quote \' single quote \\ backslash \uhhhh: hex-decimal code, e.g. \u000A \ddd: octal code, e.g. \040

Data Conversions Sometimes it is convenient to convert data from one type to another For example, we may want to treat an integer as a floating point value during a computation Conversions must be handled carefully to avoid losing information Widening conversions are safest because they tend to go from a small data type to a larger one (such as a short to an int ) Narrowing conversions can lose information because they tend to go from a large data type to a smaller one (such as an int to a short )

Data Conversions In Java, data conversions can occur in three ways: assignment conversion arithmetic promotion casting Assignment conversion occurs when a value of one type is assigned to a variable of another Only widening conversions can happen via assignment Arithmetic promotion happens automatically when operators in expressions convert their operands

Data Conversions Casting is the most powerful, and dangerous, technique for conversion Both widening and narrowing conversions can be accomplished by explicitly casting a value To cast, the type is put in parentheses in front of the value being converted For example, if total and count are integers, but we want a floating point result when dividing them, we can cast total : result = (float) total / count;

Legal data conversion (No Information Loss)

String -- a class type Examples of string literals: "watermelon" "fig" "$%&*^%!!" "354" " " (space) "" (empty string) Constructor String name = new String(“John”); Concatenation operator + methods: (page 75) charAt(int i) length()

Arithmetic Operators and Expressions + addition - subtraction * multiplication / division % remainder precedence and association

Example Time2 import java.util.*; public class Time2 { public static void main(String[] args) { Calendar calendar = Calendar.getInstance(); int hour = calendar.get(Calendar.HOUR_OF_DAY); int minute = calendar.get(Calendar.MINUTE); int second = calendar.get(Calendar.SECOND); System.out.println("The time is " + hour + ":" + minute + ":" + second); System.out.println("The time is " + hour / 10 + hour % 10 + ":" + minute / 10 + minute % 10 + ":" + second / 10 + second % 10); }

Variable declaration / initialization int x, y, z; long count; char a, b; boolean flag; float massInKilos; short timeInSeconds = 245; char ch1 = 'K', ch2 = '$'; boolean isNew = true; double maxVal = ;

Assignments total = quantity * unitPrice; count = count + 1; count += 1; count++;

Constants an identifier that is similar to a variable except that it holds one value for its entire existence The compiler will issue an error if you try to change a constant In Java, we use the final modifier to declare a constant final int MIN_HEIGHT = 69; Constants: give names to otherwise unclear literal values facilitate changes to the code prevent inadvertent errors

Operator Precedence What is the order of evaluation in the following expressions? a + b + c + d + e 1432 a + b * c - d / e 3241 a / (b + c) - d % e 2341 a / (b * (c + (d - e))) 4123