java - "ParentClass objectName1= new SubClass() "和 "SubClass objectName2 = new SubClass()"有什么区别？

Question

我想知道在从另一个扩展的类中创建对象的这两种方法之间是否存在任何功能差异。

我只是在处理基本的继承，除了 Cat extends Mammal 之类的东西之外没有什么复杂的。

Answer 1

ParentClass objectName = new SubClass() - 父类引用持有子类对象

class ParentClass{
 void m1(){
   System.out.println("parent m1");
  }
 void m2(){
   System.out.println("parent m2");
  }
}

class SubClass extends ParentClass{
//subclass overriding parent class m1 method
 @Override
 void m1(){
   System.out.println("child m1");
  }
 void m3(){
   System.out.println("child m3");
  }
}

class Test{
public static void main(String[] args){
  ParentClass p = new SubClass();
  p.m1();// out puts child m1 as child class is overriding m1 method
  p.m2();//out puts parent m2 as child class is not overriding m2 method
  p.m3();//gives compile time error as parent class reference doesn't know about m3 method

}
}