Inner classes ,annoumous and outer classes in java

Inner classes ,annoumous and outer classes in java

• 1.
  Compose by Adil Mehmood Adilm717@gmail.com
• 2.
  What is Innerclasses Top-Level (or Outer) Class You can put a class inside an another class.  A class that contains other classes is a TLC. Classe that you can write inside another class. Common applications include iterators and GUIs Syantax: class Outer{ class Inner{ } } note: there is no top level static class in java unlike C# there is only inner static class
• 3.
  Nested Class Nested class: •Class declared inside another class. Two kinds of nested classes: • Member class: class declared at the member-level of a TLC. • Local class: class declared inside a method, constructor, or initializer block
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  Inner Class Inner class(IC)refers to two special kinds of nested class: • Non-static member class (member class with no staticmodifier). • Local class inside a non-static member of a TLC. Why called inner class? • Because an object made from the class will containa reference to the TLC. • Use TLC.this.memberfrom inside inner class to access member of TLC. Restrictions: • Inner class fields can be static, but then must also be final. • No staticmethods or other inner classes (same for other members?) • See language references for even more details
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  Handy way tothink of inner classes inside a TLC At the member level: - just like a variable or method. - called member class. At the statement level: - just like a statement in a method - called local class At the expression level: - just like an expression - called anonymous class
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  Member Class (MemberLevel) Rules Structure: public class OuterClass{ tlc_members public class InnerClass{ mc_members } } When to use? • The inner class generates objects used specifically by TLC. • The inner class is associated with, or “connected to,” the TLC
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  How does visibilitywork? • The inner class can be public, private, protected, or package. • Instances of the inner class type have access to all members of the outer class (including private and static members). Some restrictions: • Cannot have same name as TLC or package (not that you would want to!). • Cannot contain static members; can have static finalfields (constants).
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  How do youuse a member class? OuterClass oref = new OuterClass(); OuterClass.InnerClass iref = oref.new InnerClass() iref.doSomething(); new OuterClass().new InnerClass(); • Not valid: InnerClass iref = new InnerClass(); iref.doSomething();
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  Internal references withthis: • Inside inner class, the this refers to current instance of the inner class. • To get to current instance of TLC, save the TLC’s this as field in the TLC or simply use TLC.this.
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  public class MemberClass{ public static void main(String[] args) { // one way: OC a = new OC(); OC.IC b = a.new IC(); b.print(); // outputs 3 // another way: new OC().new IC().print(); // outputs 3 } } class OC { private int x = 1; public class IC { private int y = 2; public void print() {System.out.println(x+y);} } }
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  Local Classes (StatementLevel) Local class location: • Statement level declaration. • Usually written in methods. See also constructors and initializers. Scope: • Local to block. • Can access all members of the TLC. • Actually, things can get confusing here! - An object of local class might persist after method ends. - Java does have rules for dealing with the matter
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  More restrictions: • Cannotbe used outside of block. • No modifiers. • Enclosing block’s variables must be finalfor local class to access. • No static, but can have static final(constants). • Terminate with a semicolon! The class is effectively an expression statement. • Cannot have same name of TLC
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  public class LocalClass{ public static void main(String[] args) { new OC().print(); } } class OC { public void print() { final String s = "test: "; class Point { private int x; private int y; public Point(int x,int y) { this.x=x; this.y=y; } public String toString() { return s+"("+x+","+y+")"; } }; System.out.println(new Point(1,2)); } // method print } //
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  Anonymous Class Location andstructure: • Defined and created at expressionlevel. • So, has no name and no modifiers. Syntax: new classname( argumentlist) { classbody} new interfacename( argumentlist) { classbody} Adapter class: • Adapter class defines code that another object invokes. • Common in GUIs and iterators. Some restrictions: • No modifiers. • No static, but can have static final(constants). • No constructors, but can use initializers for samepurpose! (See Section 1.2.) When to use? • Class has very short body. • Only one instance of class needed. • Class used right after defined; no need to create new class
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  In example below,we print a Pointagain. But, we cannot say new Point, because we have not defined a Pointclass. Instead, I use a placeholder, class Object. You will often find yourself using interface names instead. public class AnonymousClass { public static void main(String[] args) { new OC().print(); } } class OC { public void print() { final String s = "test: "; System.out.println(new Object() { private int x=1; private int y=2; public String toString() { return s+"("+x+","+y+")"; } } ); }
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  Instantiating an Innerclass 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
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  Creating an InnerClass 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() ; • One Liner: –  Outer.Inner inner = (new Outer()).new Inner() ;
• 18.
  Method-local Inner Classes Classinside 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...
• 19.
  class Outer{! private intx = 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();! }! }!