Introduction to Java 2 Programming

Introduction to Java 2 Programming Lecture 3 Writing Java Applications, Java Development Tools

Overview • More Syntax • Constants, Arrays • Strings, the Main Method • The Object Lifecycle • Java Programming Tools • Practical Exercises

More Syntax…Constants • Unlike other languages, Java has no Const keyword • Must use a combination of modifiers to make a constant • static – indicates a class variable. It’s not owned by an individual object • final – to make a variable with a single value. Can be assigned to once • E.g. public final static int MY_CONSTANT = 0;

More Syntax…Arrays • Fairly straightforward: • int[] myArray; • int[] myArray = {1, 2, 3}; • myArray[1]; • myArray[2] = 4; • Arrays have a length property, which holds their size.

More Syntax…Strings • Strings are objects • The compiler automatically replaces any string literal with a String object • E.g. “my String” becomes new String(“my string”); • Strings are immutable • Can’t be changed once created… • ..but can be concatenated (using +) to create new strings

More Syntax..Strings, StringBuffers • Compiler automatically replaces String concatenation with a StringBuffer object • E.g. “my String” + “ other String” becomes… • new StringBuffer(“my String”).append(“other String”).toString(); • Take care with String concatenation • Explicitly using a StringBuffer is often more efficient • Can reuse buffer rather than discarding it

More Syntax…The main method • To turn a class into an application, give it a “main” method: • public static void main(String[] args) • Must be of this format • Can then be invoked from the command-line • Try to minimise the amount of code in the main method: • Create objects and invoke their behaviour

Overview • More Syntax • Constants, Strings, Arrays • The Object Lifecycle • Java Programming Tools • Practical Exercises

Object Life-Cycle -- Creation • Objects are created with the new operator • E.g. new String(“my String”); • Causes a constructor to be invoked • Constructor is a special method, used to initialise an object • Class often specifies several constructors (for flexibility) • new operator chooses right constructor based on parameters (overloading)

Object Life-Cycle -- Creation public class MyClass { private int x; public MyClass(int a) { x = a; } public MyClass(String number) { x = Integer.parseInt(number); } } Can then create an instance of MyClass as follows: MyClass object = new MyClass(5); //first constructor MyClass object2 = new MyClass(“5”); //second constructor

Object Life-Cycle -- Destruction • No way to explicitly destroy an object • So no equivalent to C++ destructor • Objects destroyed by the Garbage Collector • Once they go out of scope (I.e. no longer referenced by any variable) • No way to reclaim memory, entirely under control of JVM • There is a finalize method, but its not guaranteed to be called (so pretty useless!) • Can request that the Garbage Collector can run, buts its free to ignore you

Java Programming Tools • The CLASSPATH • Common source of frustration! • Must set this for any of the java tools to work correctly. • Similar to PATH, but finds class files rather than executables • Basically a list of directories and Jar files in which the JVM will look for referenced classes • set CLASSPATH=%CLASSPATH%;c:\intro2java\bin • Compiler and JVM executables • java, javac • Both found in %JAVA_HOME%\bin

Java Programming Tools -- Javadoc • Javadoc • Automatically generates HTML documentation from Java source code • Extremely flexible. Can be customised in a number of different ways, including adding special “tags” • Very efficient way to produce development documentation for your application.

Java Programming Tools -- Javadoc /** * A <i>simple</i> Calculator * * @author Leigh Dodds */ public class Calculator { /** * Adds two numbers together and returns the result */ public int plus(int x, int y) { return x + y; } }

Java Programming Tools - Jar • Java ARchive • Basically a zip file, used to package java classes • E.g. for delivery as an applet or application • Manifest • An index of the contents of a jar file • Major benefit is indicating which class holds the “main” method • Allows application to be launched automatically from a jar file.

The Calculator

Introduction to Java 2 Programming