2024-08-01

The private access modifier in Java

What does the private access modifier do in Java? I used to believe that a member or constructor declared private is only accessible in the same class that defines it. However, this simple definition fails in the case of nested classes.

As per java language specification, any member or constructor that is declared as private, is accessible within the body of the top level class or interface.

Let's try to understand this with an example. 

public class TopLevel {

  static class NestedClass {
    private String secretMessage = "Secret message from the NestedClass";
  }

  public static void main(String[] args) {
    NestedClass nc = new NestedClass();
    System.out.println(nc.secretMessage); // Secret message from the NestedClass
  }
}

The above program when compiled and run, prints "Secret message from the NestedClass". Even if the secretMessage field is declared as private inside the NestedClass, it is still accessible from the main method.

Let's look at another example, where another nested class (NestedClass2) is accessing the private field of NestedClass. 

public class TopLevel {

  static class NestedClass {
    private String secretMessage = "Secret message from the NestedClass";
  }

  static class NestedClass2 {
    // even NestestedClass2 can access the private field of NestedClass
    public void displaySecretMessage() {
      NestedClass nc = new NestedClass();
      System.out.println(nc.secretMessage);
    }
  }

  public static void main(String[] args) {
    NestedClass2 nc2 = new NestedClass2();
    nc2.displaySecretMessage();  // Secret message from the NestedClass
  }
}

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