Java 8 Predicate with Examples

A Functional Interface is an Interface which allows only one Abstract method within the Interface scope. There are some predefined functional interface in Java like Predicate, consumer, supplier etc. The return type of a Lambda function (introduced in JDK 1.8) is a also functional interface.

The Functional Interface PREDICATE is defined in the java.util.function package. It improves manageability of code, helps in unit-testing them separately, and contain some methods like:

1. isEqual(Object targetRef) : Returns a predicate that tests if two arguments are equal according to Objects.equals(Object, Object).

   static  Predicate isEqual(Object targetRef)
   Returns a predicate that tests if two arguments are 
   equal according to Objects.equals(Object, Object).
   T : the type of arguments to the predicate
   Parameters:
   targetRef : the object reference with which to 
   compare for equality, which may be null
   Returns: a predicate that tests if two arguments 
   are equal according to Objects.equals(Object, Object)
   

2. and(Predicate other) : Returns a composed predicate that represents a short-circuiting logical AND of this predicate and another.

   default Predicate and(Predicate other)
   Returns a composed predicate that represents a 
   short-circuiting logical AND of this predicate and another.
   Parameters:
   other: a predicate that will be logically-ANDed with this predicate
   Returns : a composed predicate that represents the short-circuiting 
   logical AND of this predicate and the other predicate
   Throws: NullPointerException - if other is null

3. negate() : Returns a predicate that represents the logical negation of this predicate.

   default Predicate negate()
   Returns:a predicate that represents the logical 
   negation of this predicate

4. or(Predicate other) : Returns a composed predicate that represents a short-circuiting logical OR of this predicate and another.

   default Predicate or(Predicate other)
   Parameters:
   other : a predicate that will be logically-ORed with this predicate
   Returns:
   a composed predicate that represents the short-circuiting 
   logical OR of this predicate and the other predicate
   Throws : NullPointerException - if other is null

5. test(T t) : Evaluates this predicate on the given argument.boolean test(T t)

   test(T t) 
   Parameters:
   t - the input argument
   Returns:
   true if the input argument matches the predicate, otherwise false

Examples

Example 1: Simple Predicate

Java

// Java program to illustrate Simple Predicate

import java.util.function.Predicate;
public class PredicateInterfaceExample1 {
    public static void main(String[] args)
    {
        // Creating predicate
        Predicate<Integer> lesserthan = i -> (i < 18);

        // Calling Predicate method
        System.out.println(lesserthan.test(10));
    }
}

Output:

True

Example 2: Predicate Chaining

Java

// Java program to illustrate Predicate Chaining

import java.util.function.Predicate;
public class PredicateInterfaceExample2 {
    public static void main(String[] args)
    {
        Predicate<Integer> greaterThanTen = (i) -> i > 10;

        // Creating predicate
        Predicate<Integer> lowerThanTwenty = (i) -> i < 20;
        boolean result = greaterThanTen.and(lowerThanTwenty).test(15);
        System.out.println(result);

        // Calling Predicate method
        boolean result2 = greaterThanTen.and(lowerThanTwenty).negate().test(15);
        System.out.println(result2);
    }
}

Output:

True
False

Example 3: Predicate in to Function

Java

// Java program to illustrate
// passing Predicate into function

import java.util.function.Predicate;
class PredicateInterfaceExample3 {
    static void pred(int number, Predicate<Integer> predicate)
    {
        if (predicate.test(number)) {
            System.out.println("Number " + number);
        }
    }
    public static void main(String[] args)
    {
        pred(10, (i) -> i > 7);
    }
}

Output:

Number 10

Example 4: Predicate OR

Java

// Java program to illustrate OR Predicate

import java.util.function.Predicate;
class PredicateInterfaceExample4 {
    public static Predicate<String> hasLengthOf10 = new Predicate<String>() {
        @Override
        public boolean test(String t)
        {
            return t.length() > 10;
        }
    };

    public static void predicate_or()
    {

        Predicate<String> containsLetterA = p -> p.contains("A");
        String containsA = "And";
        boolean outcome = hasLengthOf10.or(containsLetterA).test(containsA);
        System.out.println(outcome);
    }
    public static void main(String[] args)
    {
        predicate_or();
    }
}

Output:

True

Example 5: Predicate AND

Java

// Java program to illustrate AND Predicate

import java.util.function.Predicate;
import java.util.Objects;

class PredicateInterfaceExample5 {
    public static Predicate<String> hasLengthOf10 = new Predicate<String>() {
        @Override
        public boolean test(String t)
        {
            return t.length() > 10;
        }
    };

    public static void predicate_and()
    {
        Predicate<String> nonNullPredicate = Objects::nonNull;

        String nullString = null;

        boolean outcome = nonNullPredicate.and(hasLengthOf10).test(nullString);
        System.out.println(outcome);

        String lengthGTThan10 = "Welcome to the machine";
        boolean outcome2 = nonNullPredicate.and(hasLengthOf10).
        test(lengthGTThan10);
        System.out.println(outcome2);
    }
    public static void main(String[] args)
    {
        predicate_and();
    }
}

Output:

False
True

Example 6: Predicate negate()

Java

// Java program to illustrate
// negate Predicate

import java.util.function.Predicate;
class PredicateInterfaceExample6 {
    public static Predicate<String> hasLengthOf10 = new Predicate<String>() {
        @Override
        public boolean test(String t)
        {
            return t.length() > 10;
        }
    };

    public static void predicate_negate()
    {

        String lengthGTThan10 = "Thunderstruck is a 2012 children's "
                                + "film starring Kevin Durant";

        boolean outcome = hasLengthOf10.negate().test(lengthGTThan10);
        System.out.println(outcome);
    }
    public static void main(String[] args)
    {
        predicate_negate();
    }
}

Output:

False

Example 7: Predicate in Collection

Java

// Java program to demonstrate working of predicates
// on collection. The program finds all admins in an
// arrayList of users.
import java.util.function.Predicate;
import java.util.*;
class User
{
    String name, role;
    User(String a, String b) {
        name = a;
        role = b;
    }
    String getRole() { return role; }
    String getName() { return name; }
    public String toString() {
       return "User Name : " + name + ", Role :" + role;
    }

    public static void main(String args[])
    {
        List<User> users = new ArrayList<User>();
        users.add(new User("John", "admin"));
        users.add(new User("Peter", "member"));
        List admins = process(users, (User u) -> u.getRole().equals("admin"));
        System.out.println(admins);
    }

    public static List<User> process(List<User> users,
                            Predicate<User> predicate)
    {
        List<User> result = new ArrayList<User>();
        for (User user: users)
            if (predicate.test(user))
                result.add(user);
        return result;
    }
}

Output:

[User Name : John, Role :admin]

The same functionality can also be achieved by using Stream API
and lambda functions offered since JDK 1.8 on top of the Collections API.

The Stream API allows "streaming" of collections for dynamic processing. Streams allow concurrent and parallel computation on data (using internal iterations), to support database-like operations such as grouping and filtering the data (similar to GROUP BY and WHERE clause in SQL). This allows the developers to focus on "what data is needed" instead of "how data is needed" since streaming hides the details of the implementation and provides the result. This is done by providing predicates as inputs to functions operating at runtime upon the streams of collections. In the following example, we illustrate how Stream API can be used along with predicates to filter the collections of data as achieved in Example 7.

Java

// Java program to demonstrate working of predicates
// on collection. The program finds all admins in an
// arrayList of users.
import java.util.function.Predicate;
import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;

class User
{
    String name, role;
    User(String a, String b) {
        name = a;
        role = b;
    }
    String getRole() { return role; }
    String getName() { return name; }
    public String toString() {
    return "User Name : " + name + ",
    Role :" + role;
    }

    public static void main(String args[])
    {
        List<User> users =
            new ArrayList<User>();
        users.add(new User("John", "admin"));
        users.add(new User("Peter", "member"));

    // This line uses Predicates to filter
    // out the list of users with the role "admin".
    // List admins = process(users, (User u) ->
    // u.getRole().equals("admin"));

    // Replacing it with the following line
    // using Stream API and lambda functions
    // produces the same output

    // the input to the filter() is a lambda
    // expression that returns a predicate: a
    // boolean value for each user encountered
    // (true if admin, false otherwise)
    List admins = users.stream()
    .filter((user) -> user.getRole().equals("admin"))
    .collect(Collectors.toList());

    System.out.println(admins);
    }
}

Output:

[User Name : John, Role :admin]