OOP in Java: Class inheritance with method chaining

Question

I have a parent class, which defines a collection of chainer methods (methods that return "this"). I want to define multiple child classes that contain their own chainer methods but that also "override" the parent methods so that an instance of the child class is returned instead of the parent class.

I don't want to have to repeat the same methods in each child class, which is why I have a parent class that contains the methods that all the child classes share. Thanks.

class Chain {
  public Chain foo(String s){
    ...
    return this;
  }
}

class ChainChild extends Chain {
  //I don't want to add a "foo" method to each child class
  /*
  public ChildChain foo(String s){
    ...
    return this;
  }
  */

  public ChainChild bar(boolean b){
    ...
    return this;
  }
}

ChainChild child = new ChainChild();
child.foo().bar(); //compile error: foo() returns a "Chain" object which does not define the bar() method.

possible duplicate of [Can I have an abstract builder class in java with method chaining without doing unsafe operations?](http://stackoverflow.com/questions/5818504/can-i-have-an-abstract-builder-class-in-java-with-method-chaining-without-doing) — artbristol, Feb 24 '13 at 17:19
i am curious, in what situations are you making use of such (`child.foo().bar();`) calls? — vijay, Feb 24 '13 at 17:23
@vijay There are certain settings that all child classes make use of. For example, one method takes a boolean parameter that enables/disables a certain setting. — Michael, Feb 24 '13 at 17:30
@vijay If you are curious, the class can be found here: http://code.google.com/p/ez-vcard/source/browse/trunk/src/main/java/ezvcard/Ezvcard.java I have already found a certain solution, but was wondering if there is a better way of doing it. :) — Michael, Feb 24 '13 at 17:31

OldCurmudgeon · Accepted Answer · 2013-02-25T09:56:43.303

15

A method in the parent class that returns this will still return a reference to the object of the child class. You will only be able to treat it as an object of the parent class (unless you cast it) but it will actually be of its original type.

You could consider using generics like this:

// This seems a bit too contrived for my liking. Perhaps someone else will have a better idea.
public class Parent<T extends Parent<T>> {
    T foo () {
        return (T) this;
    }
}

public class Child extends Parent<Child> {
    public void bar () {
        Child c = foo();
    }
}

edited Feb 25 '13 at 09:56

answered Feb 24 '13 at 17:17

OldCurmudgeon

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I realize that. But the problem is that, even though it's the same object, I cannot call the methods in the child class because the object is type-casted as the parent class. – Michael Feb 24 '13 at 17:18
3

I think @luiggi's technique might be tidier. – OldCurmudgeon Feb 24 '13 at 17:27

score 13 · Answer 2 · answered Feb 24 '13 at 17:20

13

I've written this sample using generics based on your needs.

class Parent {
    public <T extends Parent> T foo() {
        return (T)this;
    }
}

class Child extends Parent {

}

class AnotherChild extends Parent {

}

public class Test {
    public static void main(String[] args) {

        Parent p = new Child();
        System.out.println(p);
        Child c = p.foo();
        System.out.println(c);
        //throws ClassCastException here since Child is not AnotherChild
        AnotherChild ac = p.foo();
        System.out.println(ac);
    }
}

answered Feb 24 '13 at 17:20

Luiggi Mendoza

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Hmm nice, although technically it's just moving where the cast occurs. – Philip Whitehouse Feb 24 '13 at 20:52
1

@PhilipWhitehouse that's what OP was looking for. – Luiggi Mendoza Feb 24 '13 at 20:56
The problem here is that T is inferred from the variable declaration, not from the class itself. – d4n3 May 14 '13 at 10:25

d4n3 · Answer 3 · 2013-05-14T10:44:58.360

Here's an improvement on OldCurmudgeon's method:

public class Parent<This extends Parent<This>> {

    @SuppressWarnings("unchecked")
    private final This THIS = (This)this;

    public This foo() {
        //...
        return THIS;
    }

    public This bar() {
        //...
        return THIS;
    }    
}

public class Child extends Parent<Child> {

    // you can override super with covariant return type
    @Override
    public Child bar() {
        super.bar();

        return this;
    }
}


Child child = 
new Child()
.foo()
.bar();

The improvement is making an unchecked cast only once and save it to a constant THIS, which you can reuse as the return value for method chaining.

This removes unchecked casts in every method and makes the code clearer.

You can override Parent's methods by returning Child instead of This (covariant return type).

score 0 · Answer 4 · answered Feb 24 '13 at 17:18

0

You can just cast it.

ChainChild child = new ChainChild();
((ChainChild) child.foo()).bar();

You could try:

public ? extends Chain foo() {
    return this;
}

But I'm not sure that will help, even if it compiles.

answered Feb 24 '13 at 17:18

Philip Whitehouse

4,293
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3

Ewww ugly. I do not want to cast it :) – Michael Feb 24 '13 at 17:19
The `? extends` might work, but I'm not sure - essentially you are trying to assume information which is hidden from you as part of the fundamentals of encapsulation. – Philip Whitehouse Feb 24 '13 at 17:21
3

second case will not compile – pavko_a Feb 24 '13 at 17:22

OOP in Java: Class inheritance with method chaining

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