What's the nearest substitute for a function pointer in Java?

Question

I have a method that\'s about ten lines of code. I want to create more methods that do exactly the same thing, except for a small calculation that\'s going to change one li

Answer 1

Method references using the :: operator

You can use method references in method arguments where the method accepts a functional interface. A functional interface is any interface that contains only one abstract method. (A functional interface may contain one or more default methods or static methods.)

IntBinaryOperator is a functional interface. Its abstract method, applyAsInt, accepts two ints as its parameters and returns an int. Math.max also accepts two ints and returns an int. In this example, A.method(Math::max); makes parameter.applyAsInt send its two input values to Math.max and return the result of that Math.max.

import java.util.function.IntBinaryOperator;

class A {
    static void method(IntBinaryOperator parameter) {
        int i = parameter.applyAsInt(7315, 89163);
        System.out.println(i);
    }
}

import java.lang.Math;

class B {
    public static void main(String[] args) {
        A.method(Math::max);
    }
}

In general, you can use:

method1(Class1::method2);

instead of:

method1((arg1, arg2) -> Class1.method2(arg1, arg2));

which is short for:

method1(new Interface1() {
    int method1(int arg1, int arg2) {
        return Class1.method2(arg1, agr2);
    }
});

For more information, see :: (double colon) operator in Java 8 and Java Language Specification §15.13.