java - Java中子类创建新对象的语法含义是什么？

Question

我有下面的 java 程序，是我在学习 java 教程时编写的。

public class Primitive_prac {

   public static void main(String[] args) {
    // TODO code application logic here

    Parent obj1 = new Child();
    Child obj2 = new Child();

    System.out.println("ojb1 is instance of Parent: " + (obj1 instanceof Parent));
    System.out.println("ojb1 is instance of Parent: " + (obj1 instanceof Child));
    System.out.println("ojb1 is instance of Parent: " + (obj1 instanceof Interface1));

    System.out.println();

    System.out.println("ojb2 is instance of Parent: " + (obj2 instanceof Parent));
    System.out.println("ojb2 is instance of Parent: " + (obj2 instanceof Child));
    System.out.println("ojb2 is instance of Parent: " + (obj2 instanceof Interface1));

    }
}

class Parent{}
interface Interface1{}
class Child extends Parent implements Interface1{}

如你所见，我创建了一个类 Parent ，一个接口(interface)Interface1 ，以及一个子类 Child延伸Parent并实现Interface1 .

但是，我对下面的说法有疑问:

 Parent obj1 = new Child();

在本例中，obj1 是 Parent 的实例或Child ？上述说法的真正含义是什么？ Parent obj1是什么意思位于 = 的左侧new Child() 是什么意思？位于 = 的右侧。 Parent obj1 = new Child();是什么意思？作为一个声明？

下面是程序的输出。

run:
ojb1 is instance of Parent: true
ojb1 is instance of Parent: true
ojb1 is instance of Parent: true

ojb2 is instance of Parent: true
ojb2 is instance of Parent: true
ojb2 is instance of Parent: true
BUILD SUCCESSFUL (total time: 0 seconds)

我有点困惑，因为输出显示 obj1 和 obj2 都是 Parent 的实例, Child和Interface1 。难道是Parent创建 obj1 时的 word 有什么区别吗？写作的意义是什么Parent创建 obj1 时？

我发现很难用语言表达，但我希望你能明白我的问题。

谢谢。

Answer 1

In this case obj1 is an instance of Parent or Child ?

它是 Child 的一个实例，也是 Parent 的一个实例。

What is the meaning of the above statement really ?

obj1 是一个子对象，正如您所定义的那样，Child 的所有实例也是 Parent 的实例，因此 obj1 也是 Parent 的实例。

What is the meaning of Parent obj1 which is on the left side of = and what is the meaning of new Child()which is on the right side of =. And what is the meaning of Parent obj1 = new Child(); as a statement ?

Parent obj1 表示 obj1 是一个局部变量，可以保存作为 Parent 实例的任何对象。构造一个 Child 的事实意味着它实际上可以被赋值，因为所有 Child 对象都是 Parent 。

Didnt the Parent word while creating obj1 make any difference ?

是的，如果您向 Child 添加方法，但不向 Parent 添加方法，您将无法在 obj1 上调用该方法，因为 obj1 可以容纳任何父级，而不仅仅是一个子级。

What was the significance of writing Parent while creating obj1 ?

它允许存储任何父级，甚至不是子级。

也许以下类的命名(作为类比)可能更有意义:

public class Primitive_prac {

   public static void main(String[] args) {
    // TODO code application logic here

    Animal obj1 = new Bird();
    Bird obj2 = new Bird();
    obj1.move(); //valid, since obj1 is declared `Animal`
    obj1.fly(); // invalid, since obj1 could be any animal and not all animals implement fly();
    obj2.move(); // Valid, all birds are animals and all animals (as defined here) move, therefore birds can move
    obj2.fly(); //valid, obj2 can only be a bird or a subclass, and all such objects can fly()

    System.out.println("ojb1 is instance of Animal: " + (obj1 instanceof Animal));
    System.out.println("ojb1 is instance of Bird: " + (obj1 instanceof Bird));
    System.out.println("ojb1 is instance of Flying: " + (obj1 instanceof Flying));

    System.out.println();

    System.out.println("ojb2 is instance of Bird: " + (obj2 instanceof Animal));
    System.out.println("ojb2 is instance of Animal: " + (obj2 instanceof Bird));
    System.out.println("ojb2 is instance of Flying: " + (obj2 instanceof Flying));

    }
}

class Animal{ void move("Moving...") {} }
interface Flying{ void fly(); }
class Bird extends Animal implements Flying{
    void fly() {System.out.println("flap flap");}
}