Inner classes ,annoumous and outer classes in java

Compose by
Adil Mehmood
Adilm717@gmail.com

What is Inner classes
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

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

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

Handy way to think 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

Member Class (Member Level)
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

How does visibility work?
• 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).

How do you use 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();

Internal references with this:
• 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.

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);}
}
}

Local Classes (Statement Level)
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

More restrictions:
• Cannot be 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

public class LocalClass {
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
} //

Anonymous Class
Location and structure:
• 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

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 {
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+")"; }
} );
}

Instantiating an Inner 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

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() ;
• One Liner:
–  Outer.Inner inner = (new Outer()).new
Inner() ;

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...

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();!
}!
}!

Inner classes ,annoumous and outer classes in java

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Inner classes ,annoumous and outer classes in java