Chapter 13 Interfaces and Inner Classes Slides prepared

Chapter 13 Interfaces and Inner Classes Slides prepared by Rose Williams, Binghamton University

Static Inner Classes • A normal inner class has a connection between its objects and the outer class object that created the inner class object – This allows an inner class definition to reference an instance variable, or invoke a method of the outer class • There are certain situations, however, when an inner class must be static – If an object of the inner class is created within a static method of the outer class – If the inner class must have static members © 2006 Pearson Addison-Wesley. All rights reserved 2

Static Inner Classes • Since a static inner class has no connection to an object of the outer class, within an inner class method – Instance variables of the outer class cannot be referenced – Nonstatic methods of the outer class cannot be invoked • To invoke a static method or to name a static variable of a static inner class within the outer class, preface each with the name of the inner class and a dot © 2006 Pearson Addison-Wesley. All rights reserved 3

Public Inner Classes • If an inner class is marked public, then it can be used outside of the outer class • In the case of a nonstatic inner class, it must be created using an object of the outer class Bank. Account account = new Bank. Account(); Bank. Account. Money amount = account. new Money("41. 99"); – Note that the prefix account. must come before new – The new object amount can now invoke methods from the inner class, but only from the inner class © 2006 Pearson Addison-Wesley. All rights reserved 4

Public Inner Classes • In the case of a static inner class, the procedure is similar to, but simpler than, that for nonstatic inner classes Outer. Class. Inner. Class inner. Object = new Outer. Class. Inner. Class(); – Note that all of the following are acceptable inner. Object. nonstatic. Method(); inner. Object. static. Method(); Outer. Class. Inner. Class. static. Method(); © 2006 Pearson Addison-Wesley. All rights reserved 5

Tip: Referring to a Method of the Outer Class • If a method is invoked in an inner class – If the inner class has no such method, then it is assumed to be an invocation of the method of that name in the outer class – If both the inner and outer class have a method with the same name, then it is assumed to be an invocation of the method in the inner class – If both the inner and outer class have a method with the same name, and the intent is to invoke the method in the outer class, then the following invocation must be used: Outer. Class. Name. this. method. Name() – using this inside an inner class refers to the object of the inner class © 2006 Pearson Addison-Wesley. All rights reserved 6

Nesting Inner Classes • It is legal to nest inner classes within inner classes – The rules are the same as before, but the names get longer – Given class A, which has public inner class B, which has public inner class C, then the following is valid: A a. Object = new A(); A. B b. Object = a. Object. new B(); A. B. C c. Object = b. Object. new C(); © 2006 Pearson Addison-Wesley. All rights reserved 7

Inner Classes and Inheritance • Given an Outer. Class that has an Inner. Class – Any Derived. Class of Outer. Class will automatically have Inner. Class as an inner class – In this case, the Derived. Class cannot override the Inner. Class • An outer class can be a derived class • An inner class can be a derived class also © 2006 Pearson Addison-Wesley. All rights reserved 8

Local Classes • A local class is defined within a block of Java code. • Local classes are completely hidden in their containing block. • When a class name is used only within a block it can be defined locally. • A local class can access instance variables of the outer class and only the final local K O O B e variables of the enclosing block. h nt © 2006 Pearson Addison-Wesley. All rights reserved ti o N 9

Local Classes: Example class Local. Class. Example{ private String name = "KFUPM"; public void method ( ) { int j = 20; final int k = 30; class Local { public void test ( ) { //System. out. println(j); //Error as j is not final System. out. println(k); //OK k is final //Like an inner class, instance variables of //the enclosing object can be accessed. System. out. println ( name ) ; } } Local loc = new Local ( ) ; loc. test ( ) ; } public static void main ( String [ ] args ) { Local. Class. Example obj = new Local. Class. Example ( ); obj. method ( ) ; } } © 2006 Pearson Addison-Wesley. All rights reserved K O O t in o N B e h t 10

Anonymous Classes • It is a local class without a name • If only one object has to be created from a class, and there is no need to name the class, then an anonymous class definition can be used – The class definition is embedded inside the expression with the new operator • Anonymous class has no constructors • It is either derived from a class, or implements an interface. Like: – An. Interface i = new An. Interface ( ) { // methods defs. … } – ASuperclass c = new ASuperclass(…) { // methods defs. … } © 2006 Pearson Addison-Wesley. All rights reserved 11

Anonymous Classes © 2006 Pearson Addison-Wesley. All rights reserved 12

Anonymous Classes © 2006 Pearson Addison-Wesley. All rights reserved 13

Anonymous Classes © 2006 Pearson Addison-Wesley. All rights reserved 14