How do I define a method which takes a lambda as a parameter in Java 8?

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Lambdas are purely a call-site construct: the recipient of the lambda does not need to know that a Lambda is involved, instead it accepts an Interface with the appropriate method.

In other words, you define or use a functional interface (i.e. an interface with a single method) that accepts and returns exactly what you want.

Since Java 8 there is a set of commonly-used interface types in java.util.function.

For this specific use case there’s java.util.function.IntBinaryOperator with a single int applyAsInt(int left, int right) method, so you could write your method like this:

static int method(IntBinaryOperator op){
    return op.applyAsInt(5, 10);
}

But you can just as well define your own interface and use it like this:

public interface TwoArgIntOperator {
    public int op(int a, int b);
}

//elsewhere:
static int method(TwoArgIntOperator operator) {
    return operator.op(5, 10);
}

Then call the method with a lambda as parameter:

public static void main(String[] args) {
    TwoArgIntOperator addTwoInts = (a, b) -> a + b;
    int result = method(addTwoInts);
    System.out.println("Result: " + result);
}

Using your own interface has the advantage that you can have names that more clearly indicate the intent.

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