Java Nested Static Class

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

• Nested non-static class (Inner class)
• Nested static class.

We have already discussed inner classes in the previous tutorial. Visit Java Nested Class if you want to learn about inner classes.

In this tutorial, we will learn about nested static classes.

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.

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()

Accessing Members of Outer Class

In Java, static nested classes are associated with the outer class. This is why static nested classes can only access the class members (static fields and methods) of the outer 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.

Static Top-level Class

As mentioned above, only nested classes can be static. We cannot have static top-level classes.

Let’s see what will happen if we try to make a top-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 tried to create a static class Animal. Since Java doesn’t allow static top-level class, we will get an error.