链接在Java中调用对象和继承

Question

When declaring a method for chained calls usually it returns this at the end of the method.

So I declare:

public class Foo {

    public Foo setTitle(String title){
        ...
        return this;
    }

}

And:

public class Bar extends Foo{

      /* OTHER STUFF */
}

If you call new Bar().setTitle("Test") it returns a Foo 's reference.

Is possible to declare the method in order to return automatically a Bar 's reference without override the method in Bar for clarity, brevity and maintainability?

Thanks

Answer 1

Is possible to declare the method in order to return automatically a Bar's reference without override the method in Bar for clarity, brevity and maintainability?

No. You could hook up some weird generics - Foo<T extends Foo> or the like - but it wouldn't be very satisfactory.

Basically there's need to be some language support for "this type", where the only valid expressions of that type were null and this . That doesn't exist, so you're left with overriding:

public Bar setTitle(String title) {
    super.setTitle(title);
    return this;
}

Or:

public Bar setTitle(String title) {
    return (Bar) super.setTitle(title);
}

It's just one of those cases where inheritance ends up being a pain :(

Answer 2

I did try with generics...

public class Foo<T extends Foo> {

    public T setTitle(String title){
        .....
        return (T) this;
    }
}

public class Bar extends Foo<Bar>{

    /* OTHER STUFF */
}

Seems to work.

Answer 3

I think its actullay returning Bar Only. Just do the type casting to reference the object as Bar as below:

    Bar bar = (Bar)(new Bar().setTitle("Foo Title"));
    System.out.println(bar.getTitle());//prints Foo Title

Assuming getTitle() method present in Foo returning the title.

If you add another method eg getBarTitle in Bar class:

      public String getBarTitle(){
          return getTitle()+" of Bar";
      }

then

  System.out.println(bar.getBarTitle());//prints Foo Title of Bar

Explanation: When you are calling setTitle on new Bar() , the method is called on Bar Object and this is representing Bar object not Foo , hence it returns Bar onject only with class type as Foo , which is super class. In this process, it doesn't change the original class type, which is bar at all, hence the type casting should serve the purpose for you.

Answer 4

You can use generics with upper bounds:

public class Foo<T extends Foo<T>> {
    public T setTitle() {
        ...
        return (T) this;
    }
}

public class Bar extends Foo<Bar> {
    ....
}

Answer 5

To be precide, it does not return a Foo reference, it returns a Bar reference, you can check by using System.out.println(reference.getClass().getName()).

In the same way when you write,

List l=new ArrayList(); 

(forget about LIst being an interface, it is beside the point)

l is actually a reference to ArrayList even if this information is "hidden", but calling l.someMathod() calls ArrayList methods, not List methods - that's how Object Orientation works!