Java - Class - Nested classes

In this tutorial we will learn about nested classes in Java programming language.

We have already discussed about how to create a class and how to instantiate an object of a class in the previous tutorials. Feel free to check them out.

What are nested classes in Java?

If we define a class inside another class then such classes are called nested classes.

Syntax of nested classes

class OuterClassName {

  // some code...
  
  class InnerClassName {
    //some code...
  }

}

Where, OuterClassName is the name of the outer class which encloses the InnerClassName class.

Points to note!

Following are the points to note about nested classes.

class A {
  class B {
    // some code...
  }
}

• If class B is defined inside class A then, B can't exists independently of A.
• Class B has direct access to all the member variables including the private variables and methods of class A.
• Class A has no direct access to the member variables and methods of class B.

Example

In the following example we have an outer class Company and an inner class Project.

class Company {

  private int totalProject;

  Company() {
    System.out.println("Inside Company class constructor.");
    this.totalProject = 0;
    System.out.println("Total Company projects: " + this.totalProject);
  }

  public void createProject() {
    // creating a new project object
    Project prj1 = new Project(10);
    prj1.displayDuration();

    // creating another project object
    Project prj2 = new Project(20);
    prj2.displayDuration();

    // the following lines will give error
    // because outer class can't access the inner class
    // members directly
    // so, commented out
    // 
    // System.out.println("DummyVariable of Project class : " + dummyVariable);
  }

  private void greetings() {
  	System.out.println("Greetings from private method of Company class.");
  }

  class Project {

    private int durationInDays;

    // don't try to access this directly from
    // outer class Company as it is a part of
    // the inner class Project
    public int dummyVariable = 10;

    Project(int duration) {
      System.out.println("Inside Project class constructor.");

      // set the project duration
      this.durationInDays = duration;

      // increment the total projects
      // note! totalProject is a private member of Company class
      // but it is accessible from the inner class Project
      totalProject++;
      System.out.println("Total Company projects: " + totalProject);

      // calling the private method of the outer class Company
      // from the inner class Project
      System.out.println("Calling private method of Company class from Project class.");
      greetings();
    }

    public void displayDuration() {
      System.out.println("Inside Project class displayDuration method.");
      System.out.println("Project duration: " + this.durationInDays);
    }

  }
}

public class Example {
  public static void main(String[] args) {
    // create an object
    Company obj = new Company();

    // now create a new project of the company
    obj.createProject();
  }
}

Output:

$ javac Example.java 
$ java Example
Inside Company class constructor.
Total Company projects: 0
Inside Project class constructor.
Total Company projects: 1
Calling private method of Company class from Project class.
Greetings from private method of Company class.
Inside Project class displayDuration method.
Project duration: 10
Inside Project class constructor.
Total Company projects: 2
Calling private method of Company class from Project class.
Greetings from private method of Company class.
Inside Project class displayDuration method.
Project duration: 20

In the above code we are creating an outer class Company and inside it we are creating an inner class Project.

From the output we can easily tell that we can directly access the methods and variables of the outer class Company from the inner class Project.