Scott Grissom, copyright 2004Ch 3: Java Features Slide 1 Why Java? It is object-oriented provides many ready to use classes platform independent modern features GUI exceptions multiple threads networking suitable for multiple courses

Scott Grissom, copyright 2004Ch 3: Java Features Slide 2 State (3.2.1) Instance Variables avoid public generally use private Class Variables values associated with the entire class static int count Symbolic Constants final int SIZE = 100; make them static?

Scott Grissom, copyright 2004Ch 3: Java Features Slide 3 Behavior (3.2.2) method signatures overloaded methods have unique signature but identical name Instance Methods invoked by an object generally public Class Methods invoked using class name public static int getCount( ) answer = Math.sqrt(16.0);

Scott Grissom, copyright 2004Ch 3: Java Features Slide 4 Main methods the first method called by the Java Runtime System specific signature public static void main(String args[]) Command Line Arguments System.out.println(args[I]) Sample execution java program arg1 arg2 Sample result arg1 arg2

Scott Grissom, copyright 2004Ch 3: Java Features Slide 5 Identity (3.2.3) Objects are instantiated and associated with a reference Memory is allocated Constructor is invoked Reference is returned Scenarios multiple references no references garbage collection Parameters passed by value copies are made

Scott Grissom, copyright 2004Ch 3: Java Features Slide 6 Inheritance (3.3) most state and behavior is available to the subclass not inherited private members constructors overloading vs. overriding keyword final prevents a class from being extended prevents a method from being overridden prevents a variable from being changed

Scott Grissom, copyright 2004Ch 3: Java Features Slide 7 abstract (3.3.2) abstract methods must be overridden and contain no implementation abstract classes contain one or more methods and cannot be instantiated interfaces allows specification form of inheritance all methods are abstract no instance variables constants are OK a class implements an interface can implement multiple interfaces similar to an abstract class?

Scott Grissom, copyright 2004Ch 3: Java Features Slide 8 Polymorphism (3.3.4) Polymorphic references are resolved at run-time, not during compilation called “late binding” References can point to classes and subclasses Mammal m = new Dog( ); Example with mammals Dog, Pig, Cat, Tiger speak method story: working for toy company Sample Code Mammals ark[SIZE]; for (index=0; index<ark.length; index++) ark[index].speak( );

Scott Grissom, copyright 2004Ch 3: Java Features Slide 9 Class Object Every class is a subclass of Object Several methods are provided It is useful to override them String toString( ) boolean equals(Object) Object clone( )

Scott Grissom, copyright 2004Ch 3: Java Features Slide 10 Compiling 3.4 Source code is converted to machine code for some languages such as C Instead, Java is interpreted Source code (java files) is compiled into bytecode (class files) JVM runs the program on multiple platforms

Scott Grissom, copyright 2004Ch 3: Java Features Slide 11 Exceptions ( ) handles rare and unexpected events we must detect and recover some must be handled (checked) others are optional (unchecked) Each exception has a name Arithmetic Exception Index Out of Bounds Null Pointer File Not Found Illegal Argument

Scott Grissom, copyright 2004Ch 3: Java Features Slide 12 Throwing Exceptions A simple way to handle error messages Sample Code if (value < 0) throw new IllegalArgumentException (“value must be positive”); method terminates It is often more appropriate to ‘catch’ the exception and handle it appropriately

Scott Grissom, copyright 2004Ch 3: Java Features Slide 13 Catching Exceptions Sample Code try{ // possibility of exception x = x / y; } catch (ArithmeticException e){ System.out.println (“error: “ + e.toString()); e.printStackTrace(); } multiple catch clauses can be provided but only one will be executed Flow of control continues after last catch clause

Scott Grissom, copyright 2004Ch 3: Java Features Slide 14 What can go wrong? European Space Agency Ariane rocket in 1996 embedded software written in Ada exception not caught rocket self destructed

Scott Grissom, copyright 2004Ch 3: Java Features Slide 15 Object Serialization (not in book) Java provides support to save and load objects from external files Information is stored in binary For each class you must: import java.io.*; implement Serializable No additional methods must be written!

Scott Grissom, copyright 2004Ch 3: Java Features Slide 16 Saving to a file This sample code is often embedded in a try/catch block Sample Code SomeObject obj; FileOutputStream outFile = new FileOutputStream (“filename”); ObjectOutputStream objectOut = new ObjectOutputStream (outFile); objectOut.writeObject (obj); outFile.flush(); outFile.close();

Scott Grissom, copyright 2004Ch 3: Java Features Slide 17 Loading from a file This sample code is often embedded in a try/catch block Sample Code SomeObject obj; FileInputStream inFile = new FileInStream (“filename”); ObjectInputStream objectIn = new ObjectInputStream (inFile); obj = (SomeObject) objectIn.readObject (); inFile.close();

Scott Grissom, copyright 2004Ch 3: Java Features Slide 18 Javadoc Documentation generates API documentation in HTML parses ‘special’ comments within source code /** */ javadoc *.java -author pulldown menu in BlueJ does this automatically

Scott Grissom, copyright 2004Ch 3: Java Features Slide 19 Doc comments Should precede class declarations variable declarations method headers The first sentence should be a concise summary optionally followed by additional information Followed by zero or more special tags

Scott Grissom, copyright 2004Ch 3: Java Features Slide 20 Special parameter-name description

Scott Grissom, copyright 2004Ch 3: Java Features Slide 21 Sample Refer to our online Style Guide /** Calculate the sqare Scott value to find square square root of value */ double squareRoot (double value){