Inner Classes

Nested Classes  An nested class is a class that is defined inside another class.  To this point we have only studied top-level classes. –at most one public per file –arbitrarily many package-scope per file –either package or public ONLY  Nested classes introduced in jdk1.1

Why use nested classes?  Simplifies many coding tasks –can define small classes on the fly near the objects created from them + concise syntax –can access outer classes iv’s automatically – no need to pass a this pointer to the constructor of separate outer class –can be hidden from other classes in the same package  However, price to pay in terms of complexity, number of gotchas, etc.

Pre jdk1.1  In jdk1.0, the clean and simple class rules were ballyhooed as a major improvement over C++  Addition of inner classes complicates things significantly  However, they do make certain code much less awkard to write, particularly when writing GUIs  Still, you do not have to use them, but they can be quite cool and I do recommend it in moderation!

Types of nested classes  Inner classes –local anonymous or named –non-local named only  Static nested classes –non-local named only

Non-local inner classes  Simply a nested class that does not have the static attribute and is not defined within a class method.  Can be private, public, package, protected, abstract, etc. just like any class member.  Think of outer class as owning inner class – inner class can only be instantiated via outer class reference (including this)  Inner class has access to all outer class iv’s, private or otherwise!

Simple non-local inner class example class Outer{ private int x1; Outer(int x1){ this.x1 = x1; } public void foo(){ System.out.println(“fooing”);} public class Inner{ private int x1 = 0; void foo(){ System.out.println(“Outer value of x1: “ + Outer.this.x1); System.out.println(“Inner value of x1: “ + this.x1); }

Simple example, cont -- driver Rules for instantiation a little funny public class TestDrive{ public static void main(String[] args){ Outer outer = new Outer(); // can create in regular way Inner inner = outer.new Inner(); //must call new through //outer object handle inner.foo(); // note that this can only be done if inner is visible // according to the regular scoping rules }

Inner class rules  Note that inner class can access outer class instance variables (even private ones).  It does this using the object reference.this  Refer to public inner class as.

Another example public class TalkingClock{ private int interval; private boolean beep; public TalkingClock(int interval, boolean beep){ …} public void start(){ …} public class TimePrinter implements ActionListener //inner class { …} }

When to use non-local inner classes  Most typically used when inner class is instantiated from outer class.  If classes naturally “belong together”, it is cumbersome to pass a this pointer to a separate outer class just so second class can access first class’s properties/methods.  Note that inner class can access outer class’s private data, making them even more powerful than mechanism implied above!

Local inner classes  Inner classes may also be defined within class methods.  These are called local inner classes.  Principle advantage is scoping: such classes are completely inaccessible anywhere but the method itself where they are defined.  Thus, they have no visibility attribute (public, etc.)  Also, can NOT access local variables other than those declared with final attribute.

Example: define TimePrinter in start() method public void start(){ class TimePrinter implements ActionListener{ public void actionPerformed(ActionEvent event){ Date now = new Date(); System.out.println(“At the tone, the time is “ + now); if (beep) Toolkit.getDefaultToolkit().beep(); } ActionListener listener = new TimePrinter(); Timer t = new Timer(interval, listener) t.start(); }

Local anonymous inner classes  Local inner classes can be taken a step further – it is not required to give them an explicit name.  This is very convenient when you want to use a class only once and the code that it contains is succinct.  Very common example is defining Swing callback functions.

Anonymous inner class example public void start(int interval, final boolean beep){ ActionListener listener = new ActionListener(){ public void actionPerformed(ActionEvent event){ Date now = new Date(); System.out.println("At the tone, the time is " + now); if (beep) Toolkit.getDefaultToolkit().beep(); } }; Timer t = new Timer(interval, listener); t.start(); }

Anonymous class example but.addActionListener( new ActionListener(){ public void actionPerformed(actionEvent ae){ //do work here } );