How are Anonymous inner classes used in Java?

What is the use of anonymous classes in Java? Can we say that usage of anonymous class is one of the advantages of Java?

By an "anonymous class", I take it you mean anonymous inner class.

An anonymous inner class can come useful when making an instance of an object with certain "extras" such as overriding methods, without having to actually subclass a class.

I tend to use it as a shortcut for attaching an event listener:

button.addActionListener(new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
        // do something
    }
});

Using this method makes coding a little bit quicker, as I don't need to make an extra class that implements ActionListener -- I can just instantiate an anonymous inner class without actually making a separate class.

I only use this technique for "quick and dirty" tasks where making an entire class feels unnecessary. Having multiple anonymous inner classes that do exactly the same thing should be refactored to an actual class, be it an inner class or a separate class.

Anonymous inner classes are effectively closures, so they can be used to emulate lambda expressions or "delegates". For example, take this interface:

public interface F<A, B> {
   B f(A a);
}

You can use this anonymously to create a first-class function in Java. Let's say you have the following method that returns the first number larger than i in the given list, or i if no number is larger:

public static int larger(final List<Integer> ns, final int i) {
  for (Integer n : ns)
     if (n > i)
        return n;
  return i;
}

And then you have another method that returns the first number smaller than i in the given list, or i if no number is smaller:

public static int smaller(final List<Integer> ns, final int i) {
   for (Integer n : ns)
      if (n < i)
         return n;
   return i;
}

These methods are almost identical. Using the first-class function type F, we can rewrite these into one method as follows:

public static <T> T firstMatch(final List<T> ts, final F<T, Boolean> f, T z) {
   for (T t : ts)
      if (f.f(t))
         return t;
   return z;
}

You can use an anonymous class to use the firstMatch method:

F<Integer, Boolean> greaterThanTen = new F<Integer, Boolean> {
   Boolean f(final Integer n) {
      return n > 10;
   }
};
int moreThanMyFingersCanCount = firstMatch(xs, greaterThanTen, x);

This is a really contrived example, but its easy to see that being able to pass functions around as if they were values is a pretty useful feature. See "Can Your Programming Language Do This" by Joel himself.

A nice library for programming Java in this style: Functional Java.

Anonymous inner class is used in following scenario:

1.) For Overriding(subclassing), when class definition is not usable except current case:

class A{
    public void methodA() {
        System.out.println("methodA");
    }
}

class B{
    A a = new A() {
        public void methodA() {
            System.out.println("anonymous methodA");
        }
    };
}

2.) For implementing an interface, when implementation of interface is required only for current case:

interface InterfaceA{
    public void methodA();
}

class B{
    InterfaceA a = new InterfaceA() {
        public void methodA() {
            System.out.println("anonymous methodA implementer");
        }
    };
}

3.) Argument Defined Anonymous inner class:

interface Foo {
    void methodFoo();
}

class B{
    void do(Foo f) { }
}

class A{
    void methodA() {
        B b = new B();
        b.do(new Foo() {
            public void methodFoo() {
                System.out.println("methodFoo");
            } 
        });
    } 
} 

I use them sometimes as a syntax hack for Map instantiation:

Map map = new HashMap() {{
   put("key", "value");
}};

vs

Map map = new HashMap();
map.put("key", "value");

It saves some redundancy when doing a lot of put statements. However, I have also run into problems doing this when the outer class needs to be serialized via remoting.

They're commonly used as a verbose form of callback.

I suppose you could say they're an advantage compared to not having them, and having to create a named class every time, but similar concepts are implemented much better in other languages (as closures or blocks)

Here's a swing example

myButton.addActionListener(new ActionListener(){
    public void actionPerformed(ActionEvent e) {
        // do stuff here...
    }
});

Although it's still messily verbose, it's a lot better than forcing you to define a named class for every throw away listener like this (although depending on the situation and reuse, that may still be the better approach)

You use it in situations where you need to create a class for a specific purpose inside another function, e.g., as a listener, as a runnable (to spawn a thread), etc.

The idea is that you call them from inside the code of a function so you never refer to them elsewhere, so you don't need to name them. The compiler just enumerates them.

They are essentially syntactic sugar, and should generally be moved elsewhere as they grow bigger.

I'm not sure if it is one of the advantages of Java, though if you do use them (and we all frequently use them, unfortunately), then you could argue that they are one.

GuideLines for Anonymous Class.

1. Anonymous class is declared and initialized simultaneously.

2. Anonymous class must extend or implement to one and only one class or interface resp.

3. As anonymouse class has no name, it can be used only once.

eg:

button.addActionListener(new ActionListener(){

            public void actionPerformed(ActionEvent arg0) {
		// TODO Auto-generated method stub
		
	}
});

Yes, anonymous inner classes is definitely one of the advantages of Java.

With an anonymous inner class you have access to final and member variables of the surrounding class, and that comes in handy in listeners etc.

But a major advantage is that the inner class code, which is (at least should be) tightly coupled to the surrounding class/method/block, has a specific context (the surrounding class, method, and block).

new Thread() {
        public void run() {
            try {
                Thread.sleep(300);
            } catch (InterruptedException e) {
                System.out.println("Exception message: " + e.getMessage());
                System.out.println("Exception cause: " + e.getCause());
            }
        }
    }.start();

This is also one of the example for anonymous inner type using thread

An inner class is associated with an instance of the outer class and there are two special kinds: Local class and Anonymous class. An anonymous class enables us to declare and instantiate a class at same time, hence makes the code concise. We use them when we need a local class only once as they don't have a name.

Consider the example from doc where we have a Person class:

public class Person {

    public enum Sex {
        MALE, FEMALE
    }

    String name;
    LocalDate birthday;
    Sex gender;
    String emailAddress;

    public int getAge() {
        // ...
    }

    public void printPerson() {
        // ...
    }
}

and we have a method to print members that match search criteria as:

public static void printPersons(
    List<Person> roster, CheckPerson tester) {
    for (Person p : roster) {
        if (tester.test(p)) {
            p.printPerson();
        }
    }
}

where CheckPerson is an interface like:

interface CheckPerson {
    boolean test(Person p);
}

Now we can make use of anonymous class which implements this interface to specify search criteria as:

printPersons(
    roster,
    new CheckPerson() {
        public boolean test(Person p) {
            return p.getGender() == Person.Sex.MALE
                && p.getAge() >= 18
                && p.getAge() <= 25;
        }
    }
);

Here the interface is very simple and the syntax of anonymous class seems unwieldy and unclear.

Java 8 has introduced a term Functional Interface which is an interface with only one abstract method, hence we can say CheckPerson is a functional interface. We can make use of Lambda Expression which allows us to pass the function as method argument as:

printPersons(
                roster,
                (Person p) -> p.getGender() == Person.Sex.MALE
                        && p.getAge() >= 18
                        && p.getAge() <= 25
        );

We can use a standard functional interface Predicate in place of the interface CheckPerson, which will further reduce the amount of code required.

i use anonymous objects for calling new Threads..

new Thread(new Runnable() {
    public void run() {
        // you code
    }
}).start();

Anonymous inner class can be beneficial while giving different implementations for different objects. But should be used very sparingly as it creates problem for program readability.

One of the major usage of anonymous classes in class-finalization which called finalizer guardian. In Java world using the finalize methods should be avoided until you really need them. You have to remember, when you override the finalize method for sub-classes, you should always invoke super.finalize() as well, because the finalize method of super class won't invoke automatically and you can have trouble with memory leaks.

so considering the fact mentioned above, you can just use the anonymous classes like:

public class HeavyClass{
    private final Object finalizerGuardian = new Object() {
        @Override
        protected void finalize() throws Throwable{
            //Finalize outer HeavyClass object
        }
    };
}

Using this technique you relieved yourself and your other developers to call super.finalize() on each sub-class of the HeavyClass which needs finalize method.

You can use anonymous class this way

TreeSet treeSetObj = new TreeSet(new Comparator()
{
    public int compare(String i1,String i2)
    {
        return i2.compareTo(i1);
    }
});

Seems nobody mentioned here but you can also use anonymous class to hold generic type argument (which normally lost due to type erasure):

public abstract class TypeHolder<T> {
    private final Type type;

    public TypeReference() {
        // you may do do additional sanity checks here
        final Type superClass = getClass().getGenericSuperclass();
        this.type = ((ParameterizedType) superClass).getActualTypeArguments()[0];
    }

    public final Type getType() {
        return this.type;
    }
}

If you'll instantiate this class in anonymous way

TypeHolder<List<String>, Map<Ineger, Long>> holder = 
    new TypeHolder<List<String>, Map<Ineger, Long>>() {};

then such holder instance will contain non-erasured definition of passed type.

Usage

This is very handy for building validators/deserializators. Also you can instantiate generic type with reflection (so if you ever wanted to do new T() in parametrized type - you are welcome!).

Drawbacks/Limitations

1. You should pass generic parameter explicitly. Failing to do so will lead to type parameter loss
2. Each instantiation will cost you additional class to be generated by compiler which leads to classpath pollution/jar bloating

An Anonymous Inner Class is used to create an object that will never be referenced again. It has no name and is declared and created in the same statement. This is used where you would normally use an object's variable. You replace the variable with the new keyword, a call to a constructor and the class definition inside { and }.

When writing a Threaded Program in Java, it would usually look like this

ThreadClass task = new ThreadClass();
Thread runner = new Thread(task);
runner.start();

The ThreadClass used here would be user defined. This class will implement the Runnable interface which is required for creating threads. In the ThreadClass the run() method (only method in Runnable) needs to be implemented as well. It is clear that getting rid of ThreadClass would be more efficient and that's exactly why Anonymous Inner Classes exist.

Look at the following code

Thread runner = new Thread(new Runnable() {
    public void run() {
        //Thread does it's work here
    }
});
runner.start();

This code replaces the reference made to task in the top most example. Rather than having a separate class, the Anonymous Inner Class inside the Thread() constructor returns an unnamed object that implements the Runnable interface and overrides the run() method. The method run() would include statements inside that do the work required by the thread.

Answering the question on whether Anonymous Inner Classes is one of the advantages of Java, I would have to say that I'm not quite sure as I am not familiar with many programming languages at the moment. But what I can say is it is definitely a quicker and easier method of coding.

References: Sams Teach Yourself Java in 21 Days Seventh Edition

The best way to optimize code. also, We can use for an overriding method of a class or interface.

import java.util.Scanner;
abstract class AnonymousInner {
	abstract void sum();
}

class AnonymousInnerMain {
	public static void main(String []k){
		Scanner sn = new Scanner(System.in);
		System.out.println("Enter two vlaues");
			int a= Integer.parseInt(sn.nextLine());
			int b= Integer.parseInt(sn.nextLine());	
		AnonymousInner ac = new AnonymousInner(){
			void sum(){
				int c= a+b;
				System.out.println("Sum of two number is: "+c);
			}
		};
		ac.sum();
	}

}

One more advantage:
As you know that Java doesn't support multiple inheritance, so if you use "Thread" kinda class as anonymous class then the class still has one space left for any other class to extend.