使用Java注释将类替换为派生类

Question

Say I extend a Java class like java.util.List to create a custom class called MyList ... Is there any way if my peers write code using List , I can convert it into MyList during compilation/runtime?

ie If they have something like:

List groceryList = new List();

it should be compiled/run like:

MyList groceryList = new MyList();

I know annotations can do something of this sort. Is it possible to use them in this case? If yes, how?

Answer 1

This is known as downcasting .

The short answer is that if they created the object with the new command, such as in your example, then no, you will not be able to cast it.

What you can do, is define a constructor in MyList class that accepts the object-to-be-converted as a parameter and uses it to create a new object. The list is a bit special because you can create new lists from existing ones . For example:

public MyList (List oldList)
{
    super(oldList); //this will copy the objects of the list to your custom list
    //But you could do any other operations to create the new object from the parameter(s)
    ...
}

and use it as

MyList groceryList = new MyList(oldList);

Answer 2

尽管在Java中进行向上转换通常是安全的，但除非确实需要，否则在Java中进行向下转换不是您应该经常使用的方法，因为子类可能不支持超类可以支持的某些属性或方法。