java：传递对象并返回相同对象的类型

Question

I have an abstract class A

public astract class A{
   public abstract A f(A a) throws IllegalArgumentException;
};

but I want object a passed to f to have the same type of the object itself and return an object of same type, otherwise throw an IllegalArgumentException. How can I implement that one time for all subclasses? There is a way to do that at the class A level?

Answer 1

Java does not provide a way to state this requirement declaratively. However, you can make a run-time check at the level of your abstract class by following the Template Method design pattern:

public abstract class A {
    public final A f(A a) {
        if (a.getClass() != this.getClass()) {
            throw new IllegalArgumentException();
        }
        A res = fImpl(a);
        if (res.getClass() != this.getClass()) {
            throw new InvalidOperationException();
        }
        return res;
    }
    protected abstract A fImpl(A a); // Implementing classes put functionality here
}

The above code changes f(A) from abstract method to template method by providing shared functionality for all subclasses of A . In this case the shared functionality is argument checking, and also the return type checking.

This is the method that the clients of your class are going to use. The implementors of the abstract class, on the other hand, will be implementing fImpl method, which can assume that its argument A is a subclass that matches their own class.

Answer 2

public abstract class A
{
    public A f(A a) throws IllegalArgumentException
    {
        if (!this.getClass().equals(a.getClass()))
        {
            throw new IllegalArgumentException();
        }
        return a;
    }
}