How to pass and use an arbitrary lambda functions as parameter

Question

I have a good understanding about lambda functions in Lisp. Java seems not to have the same flexibility as Lisp. How do I have to think about lambdas in Java? Given the code below, how can I do that?

public class Draw {
    GraphicsContext gc;

    static void draw(double x, double y, double w, double h, boolean drawRect) {
        if (drawRect) {
            gc.fillRect(x, y, w, h);
        } else {
            gc.strokeRect(x, y, w, h);
        }
    }

    // How do I do that?
    static void drawWithLambda(double x, double y, double w, double h /**, lambda */) {
        lambda(x, y, w, h);
    }

    public static void main(String[] args) {
        draw(0, 0, 50, 50, false); // OK
        drawWithLambda(0, 0, 50, 50, GraphicsContext::fillRect); // Ok?
    }
}

Please read this article: http://stackoverflow.com/questions/13604703/how-do-i-define-a-method-which-takes-a-lambda-as-a-parameter-in-java-8 — psliwa, Oct 07 '15 at 08:04

Tunaki · Answer 1 · 2015-10-07T08:23:12.253

Lambda in Java work in combination with the notion of functional interface.

The typical example is Function. Function is an functional interface whose functional method, apply, is a method that takes a single argument and return a result.

You can create your own functional interface, who would define a functional method taking 4 parameters and having no return type, like this:

@FunctionalInterface
interface RectangleDrawer {
    void draw(double x, double y, double w, double h);
}

(The FunctionalInterface annotation is not strictly necessary but it gives a clear intent).

You can then create a lambda that comply with the contract of this functional interface. The typical lambda syntax is (method arguments) -> (lambda body). In this example, it would be: (x, y, w, h) -> gc.fillRect(x, y, w, h). This compiles because the lambda declares 4 arguments and has no return type, so it can be represented as the functional method of RectangleDrawer defined before.

You example would become:

static GraphicsContext gc;

public static void main(String[] args) {
    draw(0, 0, 50, 50, (x, y, w, h) -> gc.fillRect(x, y, w, h));
    draw(0, 0, 50, 50, (x, y, w, h) -> gc.strokeRect(x, y, w, h));
}

static void draw(double x, double y, double w, double h, RectangleDrawer drawer) {
    drawer.draw(x, y, w, h);
}

In this particular case, it is possible to use a method reference to create the lambda, using the :: operator, which allows to write simpler code:

static GraphicsContext gc;

public static void main(String[] args) {
    draw(0, 0, 50, 50, gc::fillRect);
    draw(0, 0, 50, 50, gc::strokeRect);
}

static void draw(double x, double y, double w, double h, RectangleDrawer drawer) {
    drawer.draw(x, y, w, h);
}

Thank you very much. I think I got it.For every function I want to pass as parameter, I habe to define am interface containig only one function definition with the same signature as the function I'd like to use. — user2407434, Oct 07 '15 at 08:48
@user2407434 You don't have to always define an interace, you can use the pre-existing ones in the [`java.util.function`](https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html) package. — Tunaki, Oct 07 '15 at 08:49

Eran · Answer 2 · 2015-10-07T08:09:43.170

3

You must specify the type of a functional interface that the lambda method argument implements :

drawWithLambda(double x, double y, double w, double h, SomeFuncInterface lambda) {
    lambda.someMethod (x, y, w, h); // someMethod is the single method that
                                    // SomeFuncInterface implements
}

In order for this line to work

drawWithLambda(0, 0, 50, 50, GraphicsContext::fillRect);

the method fillRect of GraphicsContext must be compatible with the signature of the method of the SomeFuncInterface functional interface.

BTW, GraphicsContext::fillRect is a method reference, not a lambda expression. You can pass either lambda expressions, method references or any other implementation of the functional interface (regular class instance, anonymous class instance, etc...).

edited Oct 07 '15 at 08:09

answered Oct 07 '15 at 08:04

Eran

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`GraphicsContext::fillRect` won't compile because you can't make a static reference to the non-static method `fillRect`. You need to use `gc::fillRect` where `gc` is an instance of `GraphicsContext`. – Tunaki Oct 07 '15 at 08:27
1

@Tunaki My answer was general. I'm not familiar with `GraphicsContext` class, so I didn't know whether `fillRect` was static or not. – Eran Oct 07 '15 at 08:29

How to pass and use an arbitrary lambda functions as parameter

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