Anonymous Inner Class in Java

Nested Classes in Java is prerequisite required before adhering forward to grasp about anonymous Inner class. It is an inner class without a name and for which only a single object is created. An anonymous inner class can be useful when making an instance of an object with certain "extras" such as overriding methods of a class or interface, without having to actually subclass a class.

Tip: Anonymous inner classes are useful in writing implementation classes for listener interfaces in graphics programming. 

The syntax of an anonymous class expression is like the invocation of a constructor, except that there is a class definition contained in a block of code. 

Syntax:

// Test can be interface,abstract/concrete class
Test t = new Test() 
{
   // data members and methods
   public void test_method() 
   {
      ........
      ........
    }   
};

Now let us do discuss the difference between regular class(normal classes) and Anonymous Inner class

• A normal class can implement any number of interfaces but the anonymous inner class can implement only one interface at a time.
• A regular class can extend a class and implement any number of interfaces simultaneously. But anonymous Inner class can extend a class or can implement an interface but not both at a time.
• For regular/normal class, we can write any number of constructors but we can't write any constructor for anonymous Inner class because the anonymous class does not have any name and while defining constructor class name and constructor name must be same.

Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class 

Like local classes, anonymous classes can capture variables; they have the same access to local variables of the enclosing scope:  

• An anonymous class has access to the members of its enclosing class.
• An anonymous class cannot access local variables in its enclosing scope that are not declared as final or effectively final.
• Like a nested class, a declaration of a type (such as a variable) in anonymous class shadows any other declarations in the enclosing scope that have the same name.

Anonymous classes also have the same restrictions as local classes with respect to their members: 

• We cannot declare static initializers or member interfaces in an anonymous class.
• An anonymous class can have static members provided that they are constant variables.

Note: We can declare the following in anonymous classes as follows:

• Fields
• Extra methods (even if they do not implement any methods of the supertype)
• Instance initializers
• Local classes

Ways:

Anonymous inner classes are generic created via below listed two ways as follows: 

1. Class (may be abstract or concrete)
2. Interface

Now let us take an example with which we will understand anonymous inner class, let us take a simple program

Example

// Java program to demonstrate Need for
// Anonymous Inner class

// Interface
interface Age {

    // Defining variables and methods
    int x = 21;
    void getAge();
}

// Class 1
// Helper class implementing methods of Age Interface
class MyClass implements Age {

    // Overriding getAge() method
    @Override public void getAge()
    {
        // Print statement
        System.out.print("Age is " + x);
    }
}

// Class 2
// Main class
// AnonymousDemo
class GFG {
    // Main driver method
    public static void main(String[] args)
    {
        // Class 1 is implementation class of Age interface
        MyClass obj = new MyClass();

        // calling getage() method implemented at Class1
        // inside main() method
        obj.getAge();
    }
}

Output:

Output explanation:

In the above program, interface Age is created with getAge() method and x=21.  Myclass is written as an implementation class of Age interface. As done in Program, there is no need to write a  separate class Myclass. Instead,   directly copy the code of Myclass into this parameter, as shown here: 

Age oj1 = new Age() 
{
    @Override
    public void getAge() 
    {
        System.out.print("Age is " + x);
    }
};

Here, an object to Age is not created but an object of Myclass is created and copied in the entire class code as shown above. This is possible only with anonymous inner class. Such a class is called ‘anonymous inner class’, so here we call ‘Myclass’ as anonymous inner class.

Example:

// Java Program to Demonstrate Anonymous inner class

// Interface
interface Age {
    int x = 21;
    void getAge();
}

// Main class
class AnonymousDemo {
  
    // Main driver method
    public static void main(String[] args)
    {

        // A hidden inner class of Age interface is created
        // whose name is not written but an object to it
        // is created.
        Age oj1 = new Age() {
          
            @Override public void getAge()
            {
                // printing  age
                System.out.print("Age is " + x);
            }
        };
      
        oj1.getAge();
    }
}

Age is 21

Types of Anonymous Inner Class

Based on declaration and behavior, there are 3 types of anonymous Inner classes: 

1. Anonymous Inner class that extends a class
2. Anonymous Inner class that implements an interface
3. Anonymous Inner class that defines inside method/constructor argument

Type 1: Anonymous Inner class that extends a class

We can have an anonymous inner class that extends a class. For example, we know that we can create a thread by extending a Thread class. Suppose we need an immediate thread but we don't want to create a class that extends Thread class all the time. With the help of this type of Anonymous Inner class, we can define a ready thread.

Example

// Java program to illustrate creating an immediate thread
// Using Anonymous Inner class that extends a Class

// Main class
class MyThread {
  
    // Main driver method
    public static void main(String[] args)
    {
        // Using Anonymous Inner class that extends a class
        // Here a Thread class
        Thread t = new Thread() {
          
            // run() method for the thread
            public void run()
            {
                // Print statement for child thread
                // execution
                System.out.println("Child Thread");
            }
        };

        // Starting the thread
        t.start();

        // Displaying main thread only for readability
        System.out.println("Main Thread");
    }
}

Main Thread
Child Thread

Type 2: Anonymous Inner class that implements an interface

We can also have an anonymous inner class that implements an interface. For example, we also know that by implementing Runnable interface we can create a Thread. Here we use an anonymous Inner class that implements an interface.

Example

// Java program to illustrate defining a thread
// Using Anonymous Inner class that implements an interface

// Main class
class MyThread {
  
    // Main driver method
    public static void main(String[] args)
    {
        // Here we are using Anonymous Inner class
        // that implements a interface i.e. Here Runnable
        // interface
        Runnable r = new Runnable() {
          
            // run() method for the thread
            public void run()
            {
                // Print statement when run() is invoked
                System.out.println("Child Thread");
            }
        };

        // Creating thread in main() using Thread class
        Thread t = new Thread(r);

        // Starting the thread using start() method
        // which invokes run() method automatically
        t.start();

        // Print statement only
        System.out.println("Main Thread");
    }
}

Main Thread
Child Thread

Type 3: Anonymous Inner class that defines inside method/constructor argument

Anonymous inner classes in method/constructor arguments are often used in graphical user interface (GUI) applications. To get you familiar with syntax lets have a look at the following program that creates a thread using this type of Anonymous Inner class

Example

// Java program to illustrate defining a thread
// Using Anonymous Inner class that define inside argument

// Main class
class MyThread {
    // Main driver method
    public static void main(String[] args)
    {
        // Using Anonymous Inner class that define inside
        // argument
        // Here constructor argument
        Thread t = new Thread(new Runnable() {
          
            public void run()
            {
                System.out.println("Child Thread");
            }
        });

        t.start();

        System.out.println("Main Thread");
    }
}

Main Thread
Child Thread

However, constructors can not be declared in an anonymous class.