30. Explain Outer and Inner classes (or Nested classes) in Java? When will you use an Inner Class?

In Java not all classes have to be defined separate from each other. You can put the definition of one class inside the definition of another class. The inside class is called an inner class and the enclosing class is called an outer class. So when you define an inner class, it is a member of the outer class in much the same way as other members like attributes, methods and constructors.

Where should you use inner classes? Code without inner classes is more maintainable and readable. When you access private data members of the outer class, the JDK compiler creates package-access member functions in the outer class for the inner class to access the private members. This leaves a security hole. In general we should avoid using inner classes. Use inner class only when an inner class is only relevant in the context of the outer class and/or inner class can be made private so that only outer class can access it. Inner classes are used primarily to implement helper classes like Iterators, Comparators etc which are used in the context of an outer class.

Member inner class

Anonymous inner class

public class MyStack {

private Object[] items = null;

public Iterator iterator() {

return new StackIterator();

}

//inner class

class StackIterator implements Iterator{

public boolean hasNext(){…}

}

}


public class MyStack {

private Object[] items = null;

public Iterator iterator() {

return new Iterator {

public boolean hasNext() {…}

}

}

}




Explain outer and inner classes?


Class

Type

Description

Example

Outer Class

Package member class

or interface


Top level class. Only type JVM can recognize.


class Outside{}

Outside.class


Inner Class

static nested class or

interface


Defined within the context of the top-level class. Must be static &

can access static members of its containing class. No relationship between the instances of outside and Inside classes.


//package scope

class Outside {

static class Inside{ }

}

Outside.class ,Outside$Inside.class


Inner Class

Member class


Defined within the context of outer class, but non-static. Until

an object of Outside class has been created you can’t create Inside.


class Outside{

class Inside(){}

}

Outside.class , Outside$Inside.class


Inner Class

Local class


Defined within a block of code. Can use final local variables and

final method parameters. Only

visible within the block of code that defines it.


class Outside {

void first() {

final int i = 5;

class Inside{}

}

}

Outside.class , Outside$1$Inside.class


Inner Class

Anonymous

class


Just like local class, but no name is used. Useful when only

one instance is used in a method. Most commonly used in AWT event model.


class Outside{

void first() {

button.addActionListener ( new ActionListener()

{

public void actionPerformed(ActionEvent e) {

System.out.println(“The button was pressed!”);

}

});

}

}

Outside.class , Outside$1.class




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