A class can be made static only if it is a nested class.

1. Nested static class doesn’t need reference of Outer class
2. A static class cannot access non-static members and methods of the Outer class
   It can be accessed by outer class name. It can access static data members of outer class including private.

Java allows us to define a class within another class. Such a class is called a nested class. The class which enclosed nested class is known as Outer class. In java, we can’t make Top level class static. Only nested classes can be static.

What are the differences between static and non-static nested classes?
Following are major differences between static nested class and non-static nested class. Non-static nested class is also called Inner Class.

1. Nested static class doesn’t need reference of Outer class, but Non-static nested class or Inner class requires Outer class reference.

2. Inner class(or non-static nested class) can access both static and non-static members of Outer class. A static class cannot access non-static members of the Outer class. It can access only static members of Outer class.

3. An instance of Inner class cannot be created without an instance of outer class and an Inner class can reference data and methods defined in Outer class in which it nests, so we don’t need to pass reference of an object to the constructor of the Inner class. For this reason Inner classes can make program simple and concise.

/* Java program to demonstrate how to 
implement static and non-static classes in a java program. */
class OuterClass{
   private static String msg = "GeeksForGeeks";
    
   // Static nested class
   public static class NestedStaticClass{
      
       // Only static members of Outer class is directly accessible in nested 
       // static class 
       public void printMessage() {
 
         // Try making 'message' a non-static variable, there will be 
         // compiler error  
         System.out.println("Message from nested static class: " + msg); 
       }
    }
    
    // non-static nested class - also called Inner class
    public class InnerClass{
        
       // Both static and non-static members of Outer class are accessible in 
       // this Inner class
       public void display(){
          System.out.println("Message from non-static nested class: "+ msg);
       }
    }
} 
class Main
{
    // How to create instance of static and non static nested class?
    public static void main(String args[]){
        
       // create instance of nested Static class
       OuterClass.NestedStaticClass printer = new OuterClass.NestedStaticClass();
        
       // call non static method of nested static class
       printer.printMessage();   
  
       // In order to create instance of Inner class we need an Outer class 
       // instance. Let us create Outer class instance for creating 
       // non-static nested class
       OuterClass outer = new OuterClass();        
       OuterClass.InnerClass inner  = outer.new InnerClass();
        
       // calling non-static method of Inner class
       inner.display();
        
       // we can also combine above steps in one step to create instance of 
       // Inner class
       OuterClass.InnerClass innerObject = new OuterClass().new InnerClass();
        
       // similarly we can now call Inner class method
       innerObject.display();
    }
}