Java Nested Static Class

Java Nested Static Class

Java Nested Static Class: In this tutorial, you will find out about the nested static class with the help of examples. You will likewise find out about how static classes differs from inner classes.

As learned in the past tutorial, we can have a class inside another class in Java. Such classes are known as settled classes. In Java, nested classes are of two types:

• Nested non-static class (Inner class)

• Nested static class.

We have just examined internal classes in the past tutorial. Visit Java Nested Class if you need to find out about inner classes.

In this tutorial, we will find out about nested static classes.

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Java Nested Static Class

We use the keyword static to make our nested class static.

Note: In Java, only nested classes are allowed to be static.

Like regular classes, static nested classes can include both static and non-static fields and methods. For example,

Class Animal {
   static class Mammal {
      // static and non-static members of Mammal
   }
   // members of Animal
} 

Static nested classes are associated with the outer class.

To access the static nested class, we don’t need objects of the outer class.

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Example: Static Nested Class

class Animal {

// inner class
   class Reptile {
      public void displayInfo() {
        System.out.println("I am a reptile.");
      }
   }

// static class
   static class Mammal {
      public void displayInfo() {
        System.out.println("I am a mammal.");
      }
   }
}

class Main {
   public static void main(String[] args) {
      // object creation of the outer class
      Animal animal = new Animal();

      // object creation of the non-static class
      Animal.Reptile reptile = animal.new Reptile();
      reptile.displayInfo();

      // object creation of the static nested class
      Animal.Mammal mammal = new Animal.Mammal();
      mammal.displayInfo();

   }
}

Output

I am a reptile.
I am a mammal.

In the above program, we have two nested class Mammal and Reptile inside a class Animal.

To create an object of the non-static class Reptile, we have used

Animal.Reptile reptile = animal.new Reptile()

To create an object of the static class Mammal, we have used

Animal.Mammal mammal = new Animal.Mammal()

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Accessing Members of Outer Class

In Java, static nested classes are related with the external class. This is the reason static nested classes can just access the class members (static fields and methods) for the external class.

Let’s see what will happen if we try to access non-static fields and methods of the outer class.

Example: Accessing Non-static members

class Animal {
  static class Mammal {
   public void displayInfo() {
     System.out.println("I am a mammal.");
   }
 }

 class Reptile {
   public void displayInfo() {
     System.out.println("I am a reptile.");
   }
 }

 public void eat() {
   System.out.println("I eat food.");
 }
}

class Main {
 public static void main(String[] args) {
   Animal animal = new Animal();
   Animal.Reptile reptile = animal.new Reptile();
   reptile.displayInfo();

   Animal.Mammal mammal = new Animal.Mammal();
   mammal.displayInfo();
   mammal.eat();
 }
}

Output

Main.java:28: error: cannot find symbol
    mammal.eat();
          ^
  symbol:   method eat()
  location: variable mammal of type Mammal
1 error
compiler exit status 1

In the above example, we have created a non-static method eat() inside the class Animal.

Now, if we try to access eat() using the object mammal, the compiler shows an error.

It is because mammal is an object of a static class and we cannot access non-static methods from static classes.

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Static Top-level Class

As referenced above, just settled classes can be static. We can’t have static high level classes.

We should perceive what will occur if we attempt to make a high level class static.

static class Animal {
 public static void displayInfo() {
   System.out.println("I am an animal");
 }
}

class Main {
 public static void main(String[] args) {
   Animal.displayInfo();
 }
}

Output

Main.java:1: error: modifier static not allowed here
static class Animal {
       ^
1 error
compiler exit status 1

In the above example, we have attempted to make a static class Animal. Since Java doesn’t allow static top-level class, we will get an error.

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