java中多态的理解--摘自StackOverflow

The clearest way to express polymorphism is via an abstract base class (or interface)

public abstract class Human{
...

4000
public abstract void goPee();
}

This class is abstract because the 

goPee()

 method
is not definable for Humans. It is only definable for the subclasses Male and Female. Also, Human is an abstract concept — You cannot create a human that is neither Male nor Female. It’s got to be one or the other.

So we defer the implementation by using the abstract class.

public class Male extends Human{
...
@Override
public void goPee(){
System.out.println("Stand Up");
}
}

and

public class Female extends Human{
...
@Override
public void goPee(){
System.out.println("Sit Down");
}
}

Now we can tell an entire room full of Humans to go pee.

public static void main(String[] args){
ArrayList<Human> group = new ArrayList<Human>();
group.add(new Male());
group.add(new Female());
// ... add more...

// tell the class to take a pee break
for (Human person : group) person.goPee();
}

Running this would yield:

Stand Up
Sit Down
...