Topic 6: Inner Classes

Transcription

1 Topic 6: Inner Classes What's an inner class? A class defined inside another class Three kinds: inner classes static nested classes anonymous classes this lecture: Java mechanisms later: motivation & typical uses Readings: from online Java Tutorial (link on web page) 1

2 Inner Classes First understanding of Java: all classes are "top level", usually one per file But... you can also declare a class inside another class Why would you want to? Common reason #1: sub-objects within objects Common reason #2: group related classes together 2

3 Example: Linked List doubly-linked list: two classes ListCell: contains data, pointers to next & previous cells LinkedList: pointers to head & tail ListCell is helper class Only used for LinkedList Picture cells as being "inside" list: LinkedList head ListCell ListCell ListCell tail 3

4 ListCell as Inner Class public class LinkedList { // inner class private class ListCell { int value; ListCell next; ListCell previous; } // end ListCell private ListCell head; private ListCell tail; LinkedList methods may create and use ListCell objects. Outside users of LinkedList can't see ListCell class implementation detail....plus lots of methods... } // end LinkedList 4

5 More Capabilities of Inner Classes public class SlideDemo { private int a; public InnerClass subobject; public class InnerClass { public int b; public int sum() { return a + b; } // end sum } // end class InnerClass reference to a in enclosing object // constructor public SlideDemo(int aval, int bval) { a = aval; subobject = new InnerClass(); subobject.b = bval; } // end constructor public static void main(string args[]) { SlideDemo tester = new SlideDemo(7, 5); System.out.println(tester.subObject.sum()); tester.a = 4; tester.subobject.b = 3; System.out.println(tester.subObject.sum()); } // end main } // end class SlideDemo 5

6 Limits on Inner Class Use public class OuterClass {... public InnerClass subobject; public class InnerClass { public int b;... } // end class InnerClass } // end class OuterClass an outside user can reference the inner object: OuterClass x = new OuterClass(); int y = x.subobject.b; an outside user may not create an instance of InnerClass: InnerClass y = new OuterClass.InnerClass(); // illegal InnerClass objects may only exist inside OuterClass objects 6

7 Implementation Details Our mental picture of the objects: a: 13 subobject b: 7 subobject is "inside" enclosing object How Java really does it: a: 13 subobject b: 7 pointer from inner object to enclosing object 7

8 Static Nested Classes public class OuterClass { private int a; public InnerClass subobject; public static class InnerClass { public int b; public int sum() { return a + b; } // end sum class is illegal } // end class InnerClass reference to members of enclosing inner class with "static" called a "nested class" pointer from "inner" object to // main for testing "outer" object is gone public static void main(string args[]) { OuterClass tester = new OuterClass(); tester.a = 7; tester.subobject.b = 5; System.out.println(tester.subObject.sum()); } // end main } // end class OuterClass 8

9 Why Use Static Nested Classes? Define one class inside another to reflect relationship between classes. One common use is exceptions (next topic) public class MyClass { // data and methods for MyClass // MyClass may raise exceptions public static class MyException {.. } } // end MyClass A MyException object is not a part of a MyClass object Will be created by MyClass methods (instance or static methods) 9

10 Anonymous Classes: Motivation Suppose we want a slight modification of another class. Example: a stack that prints debug output automatically. One way: extend a stack implementation class DebugStack extends ArrayStack { public void push(int i) { super.push(i); System.out.println("just pushed " + i); } public int pop() { int result = super.pop(); System.out.println("just popped " + result); return result; } } 10

11 Anonymous Class What if we only want to create one instance of DebugStack? Can use an anonymous class: Stack mystack = new ArrayStack() { public void push(int i) { super.push(i); System.out.println("just pushed " + i); } public int pop() { int result = super.pop(); System.out.println("just popped " + result); return result; } }; Note: anonymous classes are useful only for changing existing methods not adding new ones 11

12 Anonymous Classes & Interfaces You can use an anonymous class to create an object that implements an interface Stack mystack = new Stack() { public void push(int i) {...} public int pop() {...} boolean isempty() {...} }; 12

13 What's Important Here? understand the basic idea of an inner class understand differences between the three kinds don't sweat the syntax of anonymous classes you will always have examples to copy from! Examples of all three will come up during this course. 13