Java method that returns a function?

Question

I'm using Guava collections' transform functions, and finding myself making a lot of anonymous functions like this pseudocode:

    Function<T, R> TransformFunction = new Function<T, R>() {
        public R apply(T obj) {
            // do what you need to get R out of T
            return R;
        }
    };

...but since I need to reuse some of them, I'd like to put the frequent ones into a class for easy access.

I'm embarrassed to say (since I don't use Java much), I can't figure out how to make a class method return a function like this. Can you?

I think it would help if you narrowed it down a little... give an example of a method you want to write that returns a `Function`, and how you'd use the method. At the moment, several responders have taken stabs at what they think you want, and it looks to me like they've all missed. — ajb, Apr 11 '15 at 23:15

satnam · Accepted Answer · 2015-04-11T23:23:39.083

11

I think what you want to do is make a public static function that you can re-use throughout your code.

For example:

  public static final Function<Integer, Integer> doubleFunction = new Function<Integer, Integer>() {
    @Override
    public Integer apply(Integer input) {
      return input * 2;
    }
  };

Or if you want to be cool and use lambdas

public static final Function<Integer, Integer> doubleFunction = input -> input * 2;

edited Apr 11 '15 at 23:23

answered Apr 11 '15 at 23:07

satnam

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Oh, I *want* to be cool and use lambdas, but stuck pre-Java 8 for this project. – ewall Apr 11 '15 at 23:17
(This violates Java conventions regarding naming of static fields) – Ingo Bürk Apr 11 '15 at 23:18
The non-lambda example is invalid, you should have the method there. – Bubletan Apr 11 '15 at 23:21

score 2 · Answer 2 · edited Apr 11 '15 at 23:24

2

Simply encapsulate it into a class:

public class MyFunction implements Function<T, R> {
    public R apply(T obj) {
        // do what you need to get R out of T
        return R;
    }
};

Then you can use the class in client code like this:

Function<T, R> TransformFunction = new MyFunction();

If your functions are related to each other, you could also encapsulate them into an enum, because enums can implement interfaces.

edited Apr 11 '15 at 23:24

Ingo Bürk

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answered Apr 11 '15 at 23:07

Mick Mnemonic

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1

Please use correct code markup (see the formatting help) rather than awkwardly formatting each line individually. – Ingo Bürk Apr 11 '15 at 23:18
3

Sorry, doing my best on crappy mobile (please do edit). – Mick Mnemonic Apr 11 '15 at 23:22
Interesting idea on using an `enum`, never would've thought of that on my own. – ewall Apr 11 '15 at 23:29

Java method that returns a function?

2 Answers2