英文:
Is an object accessible from a previous reference after it has been upcast (in Java)?
问题
A是一个父类。让B和C成为它的子类。
A a;
B b = new B(args);
a = b;
类型为B的对象仍然可以通过b访问，还是已经永久性地向上转型为A类型？
英文:
Say A is a parent class. Let B and C be its child classes.
A a;
B b = new B(args);
a = b;
Is the type B object still accessible via b, or has it been permanently upcast to type A?
答案1
得分: 1
强制类型转换不会改变对象本身。它只是为您提供了一种“以不同方式看待对象”的方式（将其视为“A”而不是“B”）。但它们是同一对象的不同视角。通过a进行的任何更改将通过b可见，依此类推。
没有进行类型转换时也是如此。例如，如果存在一对具有明显行为的setValue/getValue方法，您将会看到：
B b1 = new B(args);
B b2 = b1;
b1.setValue(10);
System.out.println(b2.getValue()); // 10
赋值不会复制对象 - 它会从b1到b2复制对对象的引用。
您可能会发现我在解释变量、引用和对象之间区别的答案也很有帮助。
英文:
The cast doesn't change the object at all. It just gives you "a different way of seeing the object" (viewing it as an "A" instead of a "B"). They're different views on the same object though. Any changes you make via a will be visible via b etc.
The same is true without the cast. For example, if there's a setValue/getValue pair of methods with the obvious behavior, you'd see:
B b1 = new B(args);
B b2 = b1;
b1.setValue(10);
System.out.println(b2.getValue()); // 10
The assignment doesn't copy the object - it copies a reference to the object from b1 to b2.
You may find my answer explaining the difference between a variables, references and objects helpful too.
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