What is the Stream filter() method in Java?

Overview

The Stream filter() method is used to select elements as per the Predicate passed as argument. The Predicate is a functional interface and can be used as an assignment target for a lambda expression or method reference.

This answer explains how the Stream filter() method works with an example.

Syntax

Main.java

Person.java

import java.util.*; 
import java.util.function.*;
public class Main { 
  public static void main(String[] args) { 
    List<Person> list = new ArrayList<>(); 
    
    // Adding elements in the list 
    list.add(new Person("John", 32, 'M')); 
    list.add(new Person("Jennie", 30, 'F')); 
    list.add(new Person("Mike", 35, 'M')); 
    list.add(new Person("Teddy", 38, 'M')); 
    list.add(new Person("Alice", 40, 'F')); 
        
    // Instantiating the Predicate interface 
    // referring isMale() method of Person class 
    // as its functional interface method 
    Predicate<Person> male = Person::isMale; 
   
    // Using Stream filter() to find male persons 
    list.stream() 
        .filter(male) 
        .forEach(System.out::println);  
  }
}

Explanation

Person.java

Line 1–4: We create a class named Person, which has three fields—name, age, and gender.
Line 4–6: We define a constructor to instantiate Person object.
Line 12–25: We define the getter methods of each of the fields.
Line 27–34: We override the toString() method to print the Person object in a more user friendly manner.
Line 37–42: We define the isMale method of the Person class. Note that isMale() method is a static method.

Main.java

Line 3: We create a class named Main.
Line 4: We declare the main() method.
Line 5: We create an ArrayList of Person objects.
Line 8–12: We add five Person objects to the ArrayList created in Line 5.
Line 18: We declare a reference variable male of type Predicate interface, which refers to the isMale() method of the Person class.
Line 21–22: First, we obtain a stream from the list using the stream() method. Then we use the filter() method with the Predicate object to filter all Person objects in the list whose gender is male.
Line 23: We print all the elements in the filtered stream using the forEach() method.

What is the Stream filter() method in Java?

Overview

Syntax

Parameters

Return value

Example

Explanation