Inner ClassesInner classesFour kinds of inner classesMember classesMember classes IIAnonymous inner classesExample anonymous inner classStatic member classesLocal classesSummaryThe EndInner ClassesInner classes•All the classes so far have been “top level”•It is possible (and useful) to define a class inside another class•Inner classes were not in Java 1.0–As a result, they are not as well done as some other aspects of the languageFour kinds of inner classes•Member classes–Simple and useful•Anonymous classes–Useful, but syntax is ugly •Static member classes (not too useful)•Local classes (not too useful)•Every class compiles to a separate .class file•Inner classes compile to files with a $ in their namesMember classes•A member class is an “ordinary” inner class class Outer { int n; class Inner { int ten = 10; void setNToTen ( ) { n = ten; } } void setN ( ) { new Inner( ).setNToTen ( ); }}Member classes II•Member classes are often used to handle events: Button b = new Button ("Click Me");b.addActionListener (new Clicker ( ));…class Clicker implements ActionListener { … }•A member class can access the variables of the enclosing class–This is what makes them so useful!•Member classes are very easy–Declare them where you would declare a field or a methodAnonymous inner classes•Anonymous inner classes are convenient for short code b.addActionListener (anonymous inner class);•The anonymous inner class can be either: new Superclass (args) { body } or new Interface (args) { body }•Notice that no class name is given--only the name of the superclass or interfaceExample anonymous inner class•An ActionListener is a Java-supplied interface for listening to Buttons and some other things•The format (from the previous slide) is new Interface (args) { body } b.addActionListener (new ActionListener ( ) { public void actionPerformed (ActionEvent e) { System.out.println (“Ouch!”);} } ) ;•Like member classes, anonymous inner classes have full access to the fields and methods of the containing classStatic member classes•static class Inner { … }•A static member class can access only static variables of the outer class•A static member class isn't “really” an inner class, but a top-level class that happens to be written inside another class•Static member classes are not too usefulLocal classes•A local class is a class defined inside a method•A local class cannot access variables declared in the method unless they are declared final–This makes them practically useless•There are many other restrictions on local classesSummary•Member classes–An ordinary class, just defined within another–Has full access to the variables of the enclosing class•Anonymous classes–Useful for short Listeners used in only one place– Has full access to the variables of the enclosing class•Static member classes–Defined inside another class, but acts like an outer class•Local classes–Defined within a method–Can access final variables in the enclosing classThe
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