Inner ClassesInner classesFour kinds of inner classesMember classesMember classes IIAnonymous inner classesExample anonymous inner classStatic member classesLocal classesSummaryThe EndJan 14, 2019Inner Classes2Inner classesAll the classes so far have been “top level”It is possible (and useful) to define a class inside another classThe usual access modifiers (public, protected, private) can be usedInner classes were not in Java 1.0They had to be added in laterAs a result, inner classes are not as well done as some other aspects of the language3Four 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 names4Member 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( ); }}5Member 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 method6Anonymous inner classesAnonymous inner classes are convenient for short code (typically a single method) b.addActionListener(anonymous inner class);The anonymous inner class can be either: new Superclass(args) { body } or new Interface() { body }Notice that no class name is given--only the name of the superclass or interfaceIf it had a name, it wouldn’t be anonymous, now would it?The args are arguments to the superclass’s constructor (interfaces don’t have constructors)7Example 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 () { 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 class8Static member classesstatic class StaticMember { … }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 useful9Local classesA local class is a class defined inside a methodLike any other local declarations, the class declaration is available only within that methodHowever, objects created from that local class can “escape” the class by being assigned to nonlocal variablesBecause its instances may exist after the method exits, code in the 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 classes10Summary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 class11The