问“实例化”和“子类”的区别

EN

AbstractClass a = new AbstractClass(); //illegal

class ConcreteClass extends AbstractClass { ... }
ConcreteClass c = new ConcreteClass(); //legal

class ConcreteClass extends AbstractClass { ... }
AbstractClass a = new ConcreteClass(); //legal

abstract class A{
  //Class A is abstract hence, can not be instantiated. The reason being abstract class provides the layout of how the concrete sub-class should behave. 

   pubic abstract void doSomething();
   //This abstract method doSomething is required to be implemented via all the sub-classes. The sub-class B and C implement this method as required.

}

class B extends A{
   //Class B is subclass of A
  public void doSomething(){ System.out.println("I am class B"); }
}
class C extends A{
   //Class C is subclass of A
  public void doSomething(){ System.out.println("I am class C"); }
}

if you try to do this, it would generate an exception

A a = new A();

But this would work fine.
B b = new B();
or 
A a = new B(); //Note you are not instantiating A, here class A variable is referencing the instance of class B