A predicate is a function with a single argument and it returns a boolean value.

Predicate Interface (functional interface)

It is a functional interface with only

one abstract method that can take in any type of argument but it always should return a boolean true

or false.

interface Predicate<T> {
    public boolean test(T t);
}

Because it is a functional interface, it can use lambda expression.

public class GreaterThanTwenty {
    public static void main(String[] args) {

        Predicate<Integer> p = i -> (i>20);

        System.out.println(p.test(15));
    }
}

Passing Predicate to a method

public class PredicateJoins {

    public static void main(String[] args) {
        Predicate<Integer> p = i -> i > 20;
        int[] x = {10, 20, 30};
        method1(p, x);
    }

    static void method1(Predicate<Integer> p, int[] x) {
        for (int i : x) {
            if(p.test(i)) {
                System.out.println(i);
            }
        }
    }
}

Predicate Joining

negate()

and(p)

or(p)

public class PredicateJoins {

    public static void main(String[] args) {
        Predicate<Integer> p1 = i -> i > 20;
        Predicate<Integer> p2 = i -> i % 2 == 0;
        int[] x = {10, 20, 30};
        method1(p1, x);// > 20
        method1(p2, x);// even number

        method1(p1.negate(), x);// <= 20

        method1(p1.and(p2), x);

        method1(p1.or(p2), x);
    }

    static void method1(Predicate<Integer> p, int[] x) {
        for (int i : x) {
            if(p.test(i)) {
                System.out.println(i);
            }
        }
    }
}