Java Inner Classes
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This document discusses different types of inner classes in Java, including regular inner classes, private members, instantiating inner classes, creating inner classes from outside the outer class, referencing inner/outer classes from inner classes, method-local inner classes, anonymous inner classes, argument-defined anonymous inner classes, and static nested classes. Regular inner classes cannot be accessed directly or contain static content. Inner classes can access private members of the outer class. To instantiate an inner class, an instance of the outer class is needed. Anonymous and static nested classes are also described.
Java Inner Classes
- 1. Inner Classes Jussi Pohjolainen 1
- 2. Regular Inner Class class Outer{! class Inner{ }! }! javac Outer.java! =>! Outer.class! Outer$Inner.class! 2
- 3. Running the inner-class • You can't access the Inner class in a usual way – java Outer$Inner – because regular Inner-class cannot contain main-method. It cannot contain any static content! 3
- 4. Private members • Inner class can have access to Outer classes private members class Outer {! private int x = 7;! class Inner {! public void seeOuter() {! System.out.println(x);! }! }! }! 4
- 5. Instantiating an Inner class • To instantiate: You must have an instance of the outer class! • Instantiating an Inner Class from within Outer classes code: class Outer{! private int x = 7;! public void makeInner(){! Inner x = new Inner();! x.seeOuter();! }! class Inner{! public void seeOuter(){! System.out.println(x);! }! }! 5 }!
- 6. Creating an Inner Class from Outside of the Outer class • You have to have instance of the Outer-class – Outer outer = new Outer(); • After that, you create the Inner object – Outer.Inner inner = outer.new Inner() Class Class Object new Object • One Liner: – Outer.Inner inner = (new Outer()).new Inner() 6
- 7. Referencing Inner / Outer class from Inner class class Outer{ class Inner{ public void seeOuter(){ System.out.println(this); System.out.println(Outer.this); } } } 7
- 8. Method-local Inner Classes • Class inside a method • Can be instantiated only within the method (below the class) • Can use Outer classes private members • Cannot use methods variables!! – Unless the variable is final... 8
- 9. Example class Outer{! private int x = 7;! public void method(){! final String y = "hi!";! String z = "hi!";! class Inner{! public void seeOuter(){! System.out.println(x); // works!! System.out.println(y); // works! //System.out.println(z); // doesn't work! }! }! Inner object = new Inner();! object.seeOuter();! }! }! 9
- 10. Anonymous Inner Classes class Popcorn {! public void pop() {! System.out.println("popcorn");! }! }! Subclass ! of class Food {! Popcorn! Popcorn p = new Popcorn() {! public void pop() {! superclass! System.out.println("subclass Popcorn!");! }! };! Notice the semicolon! ! }! 10
- 11. Anonymous Inner classes (Interface) interface Cookable {! public void cook();! }! ! class Food {! Implementation of the Interface Cookable p = new Cookable() {! public void cook() {! System.out.println("Cook implementer");! }! };! }! 11
- 12. Argument-Defined Anonymous Inner Class class MyClass{! void go(){! Bar b = new Bar();! b.doStuff(new Foo() {! public void foof() {! System.out.println("foofy");! }! });! }! }! ! 12
- 13. Static Nested Classes • Static member of the enclosing class. • class BigOuter{ – static class Nested { } • } • Does not have access to the instance variables! • BigOuter.Nested n = new BigOuter.Nested() 13