Creating the instance of abstract class or anonymous class

Solution 1:

You create an anonymous class that extends your abstract class.

In the snipped below, you are extending AbstractDemo and provide implementations for its abstract methods.

new AbstractDemo() {
    @Override
    void showMessage() {
        // TODO Auto-generated method stub
    }

    @Override
    int add(int x, int y) {
        // TODO Auto-generated method stub
        return 0;
    }
};

Solution 2:

Here's what happened in this short and innocent piece of code:

AbstractDemo ad = new AbstractDemo() {

            @Override
            void showMessage() {
                // TODO Auto-generated method stub

            }

            @Override
            int add(int x, int y) {
                // TODO Auto-generated method stub
                return 0;
            }
        };

1. New class was defined (without a name, so called anonymous class)
2. This new class extends AbstractDemo class
3. Abstract methods of AbstractDemo were overriden in this new class
4. New instance of this new class was created and assigned to ad variable

Read more about anonymous classes in Java here.

Solution 3:

You can not create an instance of an abstract class.
You can create an instance of a class that extents your abstract class.

The whole point of an abstract class is that it's abstract -- you've defined an interface but not an implementation. Without an implementation, instantiating the class wouldn't produce a meaningful or useful result. If it does/would make sense to instantiate objects of that class, then you simply don't want to use an abstract class in the first place.

You can use anonymous class concept for an instance like the below:

AbstractDemo abstractDemo  = new AbstractDemo() {
            @Override
            void showMessage() {
                // TODO Auto-generated method stub
            }
            @Override
            int add(int x, int y) {
                // TODO Auto-generated method stub
                return 0;
            }
        }; 

Solution 4:

@Override // Here
void showMessage() {
    // TODO Auto-generated method stub
}

@Override // here
int add(int x, int y) {
    // TODO Auto-generated method stub
    return 0;
}

first of all you have to know that you can't create an instance for abstract class, here you are just creating a anonymous class which is act like a inner class that extends the abstract class so its belongs to your anonymous class.