Difference Between Predicate and Function in Java 8

Both Predicate and Function are functional interfaces from the Java 8 java.util.function package and are extensively used with Lambdas and Stream API.

In this article, we take a look at how they differ.

Predicate in Java 8:

Predicate is a boolean-valued functional interface with a test() method that takes one argument and returns either true or false.

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


Example: To know if a number is even or odd:

    public static void main(String[] args) {

        Predicate<Integer> evenOrOdd = num -> num % 2 == 0;
        int testNumberForEvenOdd = 5;
        boolean isEven = evenOrOdd.test(testNumberForEvenOdd);
        System.out.println(testNumberForEvenOdd + " is Even: " + isEven);

    }

Output:
5 is Even: false

See more examples

Function in Java 8:

A Function is a functional interface in Java used for representing a function that takes one argument and produces a result. It is part of the java.util.function package and is denoted by the apply() method, which transforms the input argument into an output of any data type.

@FunctionalInterface
public interface Function<T, R> {
    R apply(T t);
}


Example: Converting an Integer to a String

package org.code2care.examples;

import java.util.function.Function;

public class FunctionExample {

    public static void main(String[] args) {

        Function<Integer, String> intToString = num -> "The number is: " + num;

        int testNumber = 5;
        String result = intToString.apply(testNumber);

        System.out.println(result);
    }
}

Output:
The number is: 5

✌ tl;dr summary table

	Predicate	Function
Purpose:	Determines a condition as true/false	Transforms data
Functional Interface:	Predicate<T>	Function<T, R>
Method Name:	test(T t)	apply(T t)
Returns:	boolean	Result of any data type
Example Usage:	Checking if a number is even or odd	Converting an integer to a string