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