The java.util.function catalogue

Key points

• Java 8 shipped ~43 interfaces in java.util.function so libraries would stop inventing incompatible ones.
• Four carry almost all the weight: Function, Predicate, Consumer, Supplier.
• Choose by shape: what goes in, what comes out. That is the only question.
• Primitive variants like IntPredicate exist purely to avoid boxing in hot loops.
• @FunctionalInterface is optional but makes the compiler reject a second abstract method.

Example

import java.util.function.*;

public class Main {
    public static void main(String[] args) {
        Function<String, Integer> length   = s -> s.length();
        Predicate<String>         isLong   = s -> s.length() > 4;
        Consumer<String>          print    = s -> System.out.println("  consumed: " + s);
        Supplier<String>          make     = () -> "generated";

        System.out.println("Function  : " + length.apply("streams"));
        System.out.println("Predicate : " + isLong.test("hi"));
        print.accept("a value");
        System.out.println("Supplier  : " + make.get());

        // BiFunction takes two arguments
        BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;
        System.out.println("BiFunction: " + add.apply(2, 3));

        // UnaryOperator is a Function whose input and output types match
        UnaryOperator<String> shout = s -> s.toUpperCase();
        System.out.println("UnaryOp   : " + shout.apply("quiet"));
    }
}

Ask what goes in and what comes out - the answer names the interface.