Why can't we instantiate an abstract class in Java?

Question

I understand:

1. Since an abstract class is nothing on its own, e.g. vehicle, we want to create an object of an concrete implementation, like Car, Bike, etc.

Answer 1

Well actually you can - but only if you implement any methods that have been declared abstract or left out.

/**
 * A classic adaptor pattern.
 *
 * @param 

 * @param 
 */
public static interface Adapter {

  public Q adapt(P p);

}

/**
 * An I walks an iterator of one type but delivers items of a different type.
 *
 * Please fill in the `next()` method. Use an Adaptor for convenience.
 *
 * @param 
 * @param 
 */
public abstract static class I implements Iterator {

  protected final Iterator it;

  public I(Iterator it) {
    this.it = it;
  }

  @Override
  public boolean hasNext() {
    return it.hasNext();
  }

  @Override
  public void remove() {
    it.remove();
  }

}

/**
 * Use an adaptor to transform one type into another.
 *
 * @param 
 * @param 
 */
public static class IA extends I {

  private final Adapter adaptor;

  public IA(Iterator it, Adapter adaptor) {
    super(it);
    this.adaptor = adaptor;
  }

  @Override
  public T next() {
    // Implement the abstract method on-the-fly.
    return adaptor.adapt(it.next());
  }

}

Added

The IA class instantiates an object of the I abstract class and implements the next method that was missing from the I class. You are actually creating an object of an anonymous that implements the implied abstract method.