class SuperEx {
String s = "父类成员变量";
public void method(){
System.out.println("SupMethod");
}
}
class SubEx extends SuperEx {
String s = "子类成员变量";
public void method(){
System.out.println("SubMethod");
}
}
class Demo {
public static void main (String[] args) {
SuperEx sup = getInstance();
System.out.println(sup.s);
function( sup );
}
public static SuperEx getInstance(){
return new SubEx ();  
}
public static void function (SuperEx sup) {
sup.method();
}
}

先理解概念,多写写,我觉得就理解了.
我看刘意老师的视频,里面的注意事项都会讲出来..