An inner class is a class declared within another class. There are several types of inner classes including local inner classes and anonymous inner classes. Local inner classes cannot be invoked from outside the method they are declared in and can only access final parameters of the enclosing block. Anonymous inner classes are used when a local class is only needed once and help make code more concise by allowing declaration and instantiation at the same time. The .this operator refers to the current instance of the enclosing class from within an inner class. The .new operator is used to create an object of the inner class type by specifying the enclosing class instance. Inner classes increase encapsulation and can lead to more readable code by placing related classes closer together.

1. Nested Classes in Java

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Agenda
❖ Nested Classes
❖ Inner Classes
❖ Types of Inner Classes
➢ Local inner class
➢ Anonymous inner class
❖ .new operator
❖ .this operator

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Nested Classes
➔Nested class is a class which is declared inside another class or
interface
class OuterClass {
...
static class StaticNestedClass {
...
}
class InnerClass {
...
}
}

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Advantages of Nested Classes
➔ It is a way of logically grouping classes that are only used in one place
➔ Can access all the members of outer class including private data members and
methods.
➔ Can be declared private, public,protected, or package private, unlike regular
classes that can only be declared public or package private.
➔ It increases encapsulation as itself can be declared private and still access Outer
class’ private members
➔ It can lead to more readable and maintainable code as it places the code closer to
where it is used.

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Inner Classes
➔Inner class or non-static nested class is a class i.e. declared inside
the class or interface
➔An inner class is associated with an instance of its enclosing class
and has direct access to that
object's methods and fields
class Outer_class{
...
class Inner_class{
...
}
}

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Types of Inner Classes
➔Local inner class
◆Classes that are defined in a block, typically in a method.
◆Local inner class cannot be invoked from outside the method.
◆Can access only final parameters of the enclosing block, as it
captures that variable or parameter.
class Outer {
void hello(final int param) {
class Inner {
public void inner() {
System.out.println("Value of ’param’ : "
+ param);
}
}
}
}

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Types of Inner Classes
➔Local inner class
◆Can’t have static data members(unless declared final) and
static methods.
public class HelloWorld{
public static void main(String []args){
class abc {
static int b=6;
public static void inner(){
System.out.println("Value of ’b’
: "+b);
}
}
}
}
public class HelloWorld{
public static void main(String []args){
class abc {
static final int b=6;
public void inner(){
System.out.println("Value of
’b’ : "+b);
}
}
}
}

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Types of Inner Classes
➔Anonymous inner class
◆ Local Classes that have no name are known as anonymous inner class
◆ Help make code more concise by allowing to declare and instantiate a
class at the same time
◆ Used when a local class is to be used only once
◆ Used to override method of class or interface
Button btn = new Button();
btn.setText("Say 'Hello World'");
btn.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
System.out.println("Hello World!");
}
});

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.this Operator
➔ Refers to the current instance of the class. Example: Home.this
➔ In a simple class, saying Home.this and this will be equivalent. This
expression is only used in cases where there is an inner class, and one
needs to refer to the enclosing class.
class Hello {
class World {
public void doSomething() {
Hello.this.doAnotherThing();
// Here, "this" alone would refer to the instance of
// the World class, so one needs to specify that the
// instance of the Hello class is what is being referred to.
}
}
public void doAnotherThing() {
}
}

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.new Operator
➔ Creates an object of the inner class type.
➔ An Inner class has a reference to a specific instance of its outer class
public class Outer {
private final String message;
Outer(final String message) {
this.message = message;
}
public class Inner {
private final String message;
public Inner(final String message) {
this.message = message;
}
public void foo() {
System.out.println(Outer.this.message + message);
}
}
public static void main(final String args[]) {
new Outer("Hello").new Inner("World").foo();
}
}

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1. How to decide if a class should be made an inner class or not?
2. When shouldn’t Inner classes be used?
3. Advantages of Inner classes over Local Classes?
4. Is it possible to create an anonymous class which isn’t an inner class? If yes, how?
5. What will be the names of the class files created for Outer.java having a nested class, named ‘Inner’?
6. Which modifiers can be applied to the inner class?
7. Which modifiers can be applied to method local inner class?
8. Create a class Library, that takes details(Name & Author) of
Books from the user to add the Book to the Library and
displays the details of all the books present in the Library.
Book is an inner class.
Display a menu to the user :
i. Add a book to the Library.
ii. Display current books in the Library.
iii. Exit
Exercises
9. Explain the last line :
class OuterClass {
static class Inner1 {
static class Inner2 {
class Inner3 {
class Inner4 { }
}}}
}
class Test {
OuterClass.Inner1.Inner2.Innner3.Inner4 outerClass = new
OuterClass.Inner1.Inner2().new Inner3().new Inner4();
}