Inner Classes In Java – Static, Non Static, Method Inner and Anonymous Classes
What are inner classes?
A class which is defined within another class is the inner class.
What is the use of inner class?
The inner class is mainly used as logically grouping classes at one place.If you have a class and that class is used only by one other class, then we can group the first class within another class.
There are two types of Inner class,
- Static Nested Class
- Non Static Nested Class
- Method Local
- Anonymous Inner Class
Static Nested vs Non-Static Nested Class:
Static Nested Class cannot access outer class variables and methods, where as Non Static nested class have full access to outer class variables and methods.
Static Nested Class:
Static class declared within the outer class.
To access the static inner class,
outerclass.innerclass innerclass = new outerclass.innerclass()
public class StaticNested {
public void display() {
System.out.println("Outer Class");
}
static class Inner {
public void display() {
System.out.println("Static Inner");
}
}
public static void main(String arfs[]) {
StaticNested staticNested = new StaticNested();
StaticNested.Inner innerClass = new StaticNested.Inner();
staticNested.display();
innerClass.display();
}
}
Output:
Non Static Nested Class:
Non Static class declared within the outer class.
outerclass.innerclass innerclass = new outerclass.innerclass()
public class NonStaticInner {
public void display() {
System.out.println("Outer Class");
}
class Inner {
public void display() {
System.out.println("Non Static Inner");
}
}
public static void main(String args[]) {
NonStaticInner nonStaticInner = new NonStaticInner();
NonStaticInner.Inner innerClass = nonStaticInner.new Inner();
nonStaticInner.display();
innerClass.display();
}
}
Output:
Method Local Inner Class:
A class declared within a method and create object and call method within the same method.
public class LocalMethod {
public void display() {
System.out.println("Outer method");
class Inner {
public void display() {
System.out.println("Method Inner");
}
}
Inner inner = new Inner();
inner.display();
}
public static void main(String arfs[]) {
LocalMethod localMethod = new LocalMethod();
localMethod.display();
}
}
Output:
Anonymous Nested Class:
In Anonymous inner class, we use the interface as nested class.
Interface cannot be initialized, so how are we using new keyword?
If we look through the code, we use new keyword for interface but we are not actually creating an object. We use new and implement the interface method and close the curly braces with colon.
interface Anony{
void display();
}
public class AnonymousInner {
public void display() {
Anony anonymous = new Anony() {
public void display() {
System.out.println("Printing from Inner side");
}
};
anonymous.display();
}
public static void main(String arfs[]) {
AnonymousInner anony = new AnonymousInner();
anony.display();
}
}
Output:
