Sunday, April 7, 2019

Predicates and BiPredicate Functional Interface - JAVA 8


Predicates and BiPredicate Functional Interface


In my previous blog, I’ve already talked about Lambda Expression and Functional Interface. In this blog, I’ll talk about predefined Functional Interface which has been introduced in JAVA 8 under Java.util.Function.

Predicate is a Functional Interface which accept single input and return response in either True or False. It is very similar to Predicates that we have learnt in School i.e. which takes a value and return only Boolean value. The functional method of Predicate is test(Object).

@FunctionalInterface

Public interface Predicate<T>

Here is a simple source code of java.util.function.Predicate
package java.util.function;
    
import java.util.Objects;
    
@FunctionalInterface
public interface Predicate<T> {
    boolean test(T t);
  }

Where Test (T t) is an abstract method where T is the type of the input to the predicate and will always return a Boolean Value.

Let’s understand this with examples

Example 1:





Example 2:





Example 3:

import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;

public class PredicateExmaple2 {

       public static void main(String[] args) {

             PredicateExmaple2 predicate = new PredicateExmaple2();

             Predicate<Student> p3 = student -> student.getAge() > 30;
             Predicate<Student> p4 = student -> student.getName().startsWith("A");

             List<Student> students = predicate.populateStudentList();
             for (Student student : students) {
                    if (p3.test(student) && p4.test(student)) {
                           System.out.println("Student Name: " + student.getName());
                           System.out.println("Student Age: " + student.getAge());
                    }
             }
       }

       class Student {
             private String name;
             private int age;

             public Student(String name, int age) {
                    super();
                    this.name = name;
                    this.age = age;
             }

             public String getName() {
                    return name;
             }

             public void setName(String name) {
                    this.name = name;
             }

             public int getAge() {
                    return age;
             }

             public void setAge(int age) {
                    this.age = age;
             }
       }

       public List<Student> populateStudentList() {
             List<Student> studentList = Arrays.asList(new Student("Abdul", 31), new Student("Waheed", 29),
                           new Student("DummyUser", 20));
             return studentList;

       }

}

Output:
Student Name: Abdul
Student Age: 31

Apart from test (T t) method , Predicate also has 3 default method and one static method.


Default Method Name
Explanation
and()
It does logical AND of the predicate on which it is called with another predicate. Example: predicate1.and(predicate2)
or()
It does logical OR of the predicate on which it is called with another predicate. Example: predicate1.or(predicate2)
negate()
It does boolean negation of the predicate on which it is invoked. Example: predicate1.negate()
isEqual()
Compare 2 instances of Predicate functional interface.















Lets understand it with few more example

package com.waheedtechblog.functionalinterface;

import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;

public class PredicateExmaple2 {

       public static void main(String[] args) {

             PredicateExmaple2 predicate = new PredicateExmaple2();

             Predicate<Student> p3 = student -> student.getAge() > 30;
             Predicate<Student> p4 = student -> student.getName().startsWith("A");
             Predicate<Student> p5 = Predicate.isEqual(new PredicateExmaple2().new Student("Abdul", 31));

             List<Student> students = predicate.populateStudentList();
             for (Student student : students) {
                    if (p3.test(student) && p4.test(student)) {
                           System.out.println("Student Name: " + student.getName());
                           System.out.println("Student Age: " + student.getAge());
                    }
             }
             System.out.println("\n");

             System.out.println("=============================");
             System.out.println("=====USING AND PREDICATE======");

             // Using AND PREDICATE
             for (Student student : students) {
                    if (p3.and(p4).test(student)) {
                           System.out.println("Student Name: " + student.getName());
                           System.out.println("Student Age: " + student.getAge());
                    }
             }
             System.out.println("\n");

             System.out.println("=============================");
             System.out.println("=====USING OR PREDICATE======");

             // Using OR PREDICATE
             for (Student student : students) {
                    if (p3.or(p4).test(student)) {
                           System.out.println("Student Name: " + student.getName());
                           System.out.println("Student Age: " + student.getAge());
                    }
             }
             System.out.println("\n");

             System.out.println("=============================");
             System.out.println("=====USING NEGATE PREDICATE======");
             for (Student student : students) {
                    if (p3.negate().test(student)) {
                           System.out.println("Student Name: " + student.getName());
                           System.out.println("Student Age: " + student.getAge());
                    }
             }
             System.out.println("\n");

             System.out.println("=============================");
             System.out.println("=====USING ISEQUAL PREDICATE======");
//Override equals method in Student class
             for (Student student : students) {
                    if (p5.test(student)) {
                           System.out.println("Student Name: " + student.getName());
                           System.out.println("Student Age: " + student.getAge());
                    }
             }

       }

       class Student {
             private String name;
             private int age;

             public Student(String name, int age) {
                    super();
                    this.name = name;
                    this.age = age;
             }

             public String getName() {
                    return name;
             }

             public void setName(String name) {
                    this.name = name;
             }

             public int getAge() {
                    return age;
             }

             public void setAge(int age) {
                    this.age = age;
             }

             public boolean equals(Object obj) {
                    Student student = (Student) obj;
                    if ((this.getName().equals(student.getName()) && this.getAge() == student.getAge())) {
                           return true;
                    } else {
                           return false;
                    }
             }
       }

       public List<Student> populateStudentList() {
             List<Student> studentList = Arrays.asList(new Student("Abdul", 31), new Student("Waheed", 29),
                           new Student("DummyUser", 20), new Student("Adam", 25));
             return studentList;

       }

}

Output:

Student Name: Abdul
Student Age: 31


=============================
=====USING AND PREDICATE======
Student Name: Abdul
Student Age: 31


=============================
=====USING OR PREDICATE======
Student Name: Abdul
Student Age: 31
Student Name: Adam
Student Age: 25


=============================
=====USING NEGATE PREDICATE======
Student Name: Waheed
Student Age: 29
Student Name: DummyUser
Student Age: 20
Student Name: Adam
Student Age: 25


=============================
=====USING ISEQUAL PREDICATE======
Student Name: Abdul
Student Age: 31


BiPredicate Functional Interface


It is very similar to Predicate Functional Interface. The only difference is just that it will accept two argument instead of one and as usual will return the Boolean Value.

@FunctionalInterface
public interface BiPredicate<T,U>


Let’s see how can we achieve Example 4 using BiPredicate Interface

import java.util.Arrays;
import java.util.List;
import java.util.function.BiPredicate;

public class BiPredicateExample {

       public static void main(String[] args) {

             BiPredicateExample predicate = new BiPredicateExample();

             BiPredicate<Integer, String> b1 = (age, name) -> age > 30 && name.startsWith("A");

             List<Student> students = predicate.populateStudentList();
             for (Student student : students) {
                    if (b1.test(student.getAge(), student.getName())) {
                           System.out.println("Student Name: " + student.getName());
                           System.out.println("Student Age: " + student.getAge());
                    }
             }
       }

       class Student {
             private String name;
             private int age;

             public Student(String name, int age) {
                    super();
                    this.name = name;
                    this.age = age;
             }

             public String getName() {
                    return name;
             }

             public void setName(String name) {
                    this.name = name;
             }

             public int getAge() {
                    return age;
             }

             public void setAge(int age) {
                    this.age = age;
             }

             public boolean equals(Object obj) {
                    Student student = (Student) obj;
                    if ((this.getName().equals(student.getName()) && this.getAge() == student.getAge())) {
                           return true;
                    } else {
                           return false;
                    }
             }
       }

       public List<Student> populateStudentList() {
             List<Student> studentList = Arrays.asList(new Student("Abdul", 31), new Student("Waheed", 29),
                           new Student("DummyUser", 20), new Student("Adam", 25));
             return studentList;

       }

}

Output:

Student Name: Abdul
Student Age: 31
  

That’s it for Predicate and BiPredicate Functional Interface, Will talk about other Predefined Functional Interface in my next blog.


Happy Coding…!!!





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