Question

// Concrete implementation built atop skeletal implementation
  static List<Integer> intArrayAsList(final int[] a) {
    if (a == null)
      throw new NullPointerException();
    return new AbstractList<Integer>() {
      public Integer get(int i) {
        return a[i]; // Autoboxing 
      }

      @Override
      public Integer set(int i, Integer val) {
        int oldVal = a[i];
        a[i] = val; // Auto-unboxing
        return oldVal; // Autoboxing
      }

      public int size() {
        return a.length;
      }
    };
  }

So far I knew we can not instantiate an abstract class at all . But what aren’t we doing the same thing here with return new AbstractList<Integer>() ? I am confused .

Answer 1

No, you are creating an anonymous class. You are subclassing your abstract class and you provide an implementation and instantiate it at the same time.

If you try this:

return new AbstractList<Integer>();

you will get an error since you won’t be providing a concrete implementation.

If you are confused you can always check out the official tutorials. Here it is:
Java Inner Classes