对象类型声明

问题描述

好的，所以，
当你有类的层次结构如

  public class A {。 ..} 
  

和

  public class B extends A {...} 
  

...当你创建对象时，有什么区别：

  A object = new A（）; 
 A object = new B（）; 
 B object = new B（）;感谢您的时间。
 
 >解决方案
  A object = new A（）; 
  
您正在创建一个 A实例引用类型A.您只能访问A方法/属性和父类方法/属性。
  A object = new B ）; 
  
您正在参考中创建 B实例这样， object 可以以多态方式工作，例如，如果你使 object.method（）和方法在B中被覆盖，那么它将调用此覆盖方法。您必须注意不要打破。您可以只访问A方法/属性和父项方法/属性。这是当你只需要超类型契约时的首选方法。
  B object = new B 
  
您正在创建一个 B实例类型 B 的引用变量。您可以只访问B方法/属性和父项方法/属性。
 
Ok.. So,When you have a hierarchy of classes such as 
public class A {...}
and,
public class B extends A {...}
...When you create objects, what is the difference between:
A object = new A();
A object = new B();
B object = new B();
Thank you for your time.
 解决方案 
A object = new A();
You are creating an A instance in a reference of type A. You may can access only A methods/properties and parents methods/properties.
A object = new B();
You are creating  B instance in a reference of type A. In this way object could behave in a polymorphic way, for example if you make object.method() and method is overriden in B then it will call this override method. You have to take care in not to break the Liskov Substitution Principle. You may can access only A methods/properties and parents methods/properties. This is the preferred way when you only need supertype contract.
B object = new B();
You are creating a B instance in a reference variable of type B. You may can access only B methods/properties and parents methods/properties.
                        
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