静态和动态类型

Question

我正准备参加java考试。请看看以下两个练习。（我有解决方案，但解决方案没有解释）所以如果有人可以查看我的解释，那将非常感激。

1）

public interface Sale{}
public abstract class Clothing{}
public class Jacket extends Clothing implements Sale{}
public class LeatherJacket extends Jacket{}

以下哪项是可能的：

Sale var1 = new LeatherJacket();

是可能的，因为LeatherJacket是Jacket和Jacket的子类实现销售？ （我只是在这里猜测。）

Sale var2 = new Sale();

不可能。您无法创建接口类型的对象。

Clothing var3 = new Clothing();

不可能。您无法创建抽象类的对象。

Clothing var4 = new LeatherJacket();

可能，但为什么？

Jacket var5 = new LeatherJacket();

可能，但为什么呢？

LeatherJacket var6  = new Object();

不可能，但为什么不呢？

Answer 1

Clothing var4 = new LeatherJacket();

     

可能，但为什么？

这是允许的，因为LeatherJacket是Clothing的派生类。它是不通过抽象类Clothing实例化。它写了new LeatherJacket()而不是new Clothing()。

Jacket var5 = new LeatherJacket();

     

可能，但为什么呢？

这是允许的，因为所有的皮夹克都是夹克。但反过来却是不真实的。就像所有狮子都是动物一样，你可以把它写成：

Animal lion = new Lion();  //where Lion extends Animal

但你不能把它写成：

Lion lion = new Animal();  //not all animals are lions

LeatherJacket var6  = new Object();

     

不可能，但为什么不呢？

原因与前面的解释相同。 Object位于Java的最高层次结构中，每个类都是类Object的子类。因此，处于层次结构的顶部，这是不允许的。但是，这将是允许的：

Object var6 = new LeatherJacket();  //allowed (all leather jackets are objects)
LeatherJacket var 6 = new Object(); //not allowed (not all objects are leather jackets)

Answer 2

Clothing var4 = new LeatherJacket();

LeatherJacket扩展了延伸服装的夹克。

Jacket var5 = new LeatherJacket();

LeatherJacket扩展了夹克

LeatherJacket var6  = new Object();

Object是所有的超类。你不能这样做但反之亦然对象o = new LeatherJacket（）;

Answer 3

当你有这样的层次结构时，你可以通过维恩图来感知它：

以下内容应该为您提供一个非常好的线索：允许和不允许：

Object o = new Clothing();        //not allowed (Clothing is abstract!)
Object o = new Jacket();          //allowed (jacket is a subset of object)
OBject o = new LaatherJacket();   //allowed (leather jacket is a subset of object)

Clothing c = new Object();        //not allowed (Object is not a subset of clothing, cannot assume object is definitely a Clothing)
Clothing c = new Jacket();        //allowed (jacket is a subset of Clothing)
Clothing c = new LaatherJacket(); //allowed (leather jacket is a subset of Clothing)

Jacket j = new Object();          //not allowed (Object is not a subset of jacket, cannot assume object is definitely a jacket)
Jacket j = new Clothing();        //not allowed (Clothing is abstract!)
Jacket j = new LeatherJacket();   //allowed (leather jacket is a subset of jacket)

LeatherJacket j = new Object;     //not allowed (Object is not a subset of jacket, cannot assume object is definitely a leather jacket)
LeatherJacket j = new Clothing(); //not allowed (Clothing is abstract!)
LeatherJacket j = new Jacket();   //not allowed (Jacket is not a subset of jacket, cannot assume jacket is definitely a leatherjacket)