Java中创建子类的新对象的语法含义？

Question

下面是我在学习java教程时编写的java程序

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{}

如您所见，我创建了一个类Parent，一个接口Interface1，以及一个子类Child，它扩展了Parent并实现了Interface1

然而，我对以下声明有疑问：

 Parent obj1 = new Child();

在这种情况下，obj1是Parent或Child的实例？这句话到底是什么意思？在{}左边的{}是什么意思，在{}右边的{}是什么意思。那么Parent obj1 = new Child();作为一种陈述的含义是什么

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、Child和Interface1的实例。创建obj1时的Parent单词没有任何区别吗？在创建obj1时编写Parent的意义是什么

我发现很难用语言表达，但我希望你明白我的问题

谢谢

Answer 1

将对象指定给名称时，如中所示：

Parent p = new Child();

名称“p”指的是一个“子”对象，尽管名称表示它是父对象。这很常见，尽管示例中类型名称的选择可能会产生误导。一个更好的现实例子是：

List a = new ArrayList();

在本例中，您通过更通用的“列表”引用特定的“ArrayList”对象。这是您在实际代码中经常看到的情况，一般来说，您希望将其指定给最不特定的类型，以便使代码最灵活

Answer 2

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是子对象，正如您定义的，所有子对象的实例都是父对象的实例，因此obj1也是父对象的实例

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意味着obj1是一个局部变量，可以保存任何作为父对象实例的对象。构造子对象的事实意味着它实际上可以被赋值，因为所有Child对象都是Parents

Didnt the Parent word while creating obj1 make any difference ?

是的，如果你在Child中添加一个方法，而不是在Parent中添加，你将无法在obj1上调用它，因为obj1可以容纳任何父对象，而不仅仅是子对象

What was the significance of writing Parent while creating obj1 ?

它允许存储任何Parent，即使不是Child

也许下面的类命名（作为类比）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");}
}