What are the features of using nested classes, including static and inner classes? What is the difference between them?

Nested classes allow logical grouping of classes, increasing encapsulation and code readability.

Features:

• Encapsulation: Nested classes have access to the private members of the outer class.
• Logical grouping: Related classes are placed together.
• Readability: The code becomes more structured.

Differences between static and inner nested classes:

Characteristic	Static nested class (static nested class)	Inner class (inner class)
Access to outer class members	Only to static members	To all members (static and non-static)
Instance creation	Does not require an instance of the outer class	Requires an instance of the outer class
Use of this	No access to this of the outer class	Has access to this of the outer class (via OuterClass.this)
Syntax of definition	With the static keyword	Without the static keyword

Examples:

• Static nested class:

  public class OuterClass {
      private static int staticVar = 10;
      private int instanceVar = 20;
  
      static class StaticNestedClass {
          void display() {
              // Access only staticVar
              System.out.println("Static var: " + staticVar);
              // Does not have access to instanceVar
              // System.out.println("Instance var: " + instanceVar); // Error
          }
      }
  }
  

• Inner class:

  public class OuterClass {
      private static int staticVar = 10;
      private int instanceVar = 20;
  
      class InnerClass {
          void display() {
              // Access to all members
              System.out.println("Static var: " + staticVar);
              System.out.println("Instance var: " + instanceVar);
          }
      }
  }