Java: Method signature

Question

I want to make an interface Mergeable that declares a method merge(). A class that implements this will have to implement the method like follows:

SomeObject.java

public class SomeObject implements Mergeable @Override public SomeObject merge(SomeObject other) {...}

See that the return and parameter type are set to the implementing class. Is there a way to enforce this and how would I set the method signature in the interface?

why do you want to enforce a method which returns only one type of object and declare it in interface? — Prasad Kharkar
– Prasad Kharkar, Commented Apr 9, 2015 at 18:28
@PrasadKharkar the method should return the type of whichever class implements it. So if I have another class Y it should return Y. — user2066880
– user2066880, Commented Apr 9, 2015 at 18:42

Louis Wasserman · Accepted Answer · 2015-04-09 18:28:44Z

7

Look at the Comparable interface for an example. You'll probably have to write something like

interface Mergeable<T> { T merge(T other); }

...and then anytime you want to accept something Mergeable, write

<T extends Mergeable<T>> void somethingWithMergeables(T merge1) { ... }

answered Apr 9, 2015 at 18:28

Louis Wasserman

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2 Comments

Paul Boddington Over a year ago

Why not interface Mergeable<T extends Mergeable<T>>?

Louis Wasserman Over a year ago

That'd work, too, but you'd need to use the same constraint anyway. It doesn't get you much, really.

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