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Method chaining between child and parent classes

So imagine we have the following.

public class Animal {
    public Animal walk(){
        // walk
        return this;
    }
}

public class Dog extends Animal{
    public Dog bark(){
        // bark
        return this;
    }
    public Dog scratch(){
        // scratch
        return this;
    }
}

I’m trying to do this,

Dog dog = new Dog()
    .bark()
    .walk() // error: required Dog but provided Animal, and so it can't find the child method.
    .scratch();

What are the possible ways to achieve this? And what’s the best one (convention)?

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Answer

I’ve seen at least two approaches.

  1. Make Animal generic, and use the generic type as return type:
public class Animal<A extends Animal<A>> {
    public A walk(){
        // walk
        return (A) this;
    }
}

public class Dog extends Animal<Dog> {
    public Dog bark(){
        // bark
        return this;
    }
    public Dog scratch(){
        // scratch
        return this;
    }
}
  1. Override the method like Alex R said in his comment.

I prefer option 2, because a) it doesn’t require me to use generics when I want just Animal, and b) it allows me to extend Dog without having to make Dog generic too. With some unit tests using reflection I can check that each method is properly overridden.

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