Stream API filter() method in Java

Short Intro of Stream APIs in Java:

The Stream APIs were introduced in Java 8. It helps in writing code in functional manner, reduces boiler plate code and is lazy in nature. Stream APIs takes Collections as an input converts it into a sequence of data stream. Stream APIs have made parallel processing easier to use. It is a chain of operations that combine together to form a stream pipeline.

To understand the Stream API, Use Cases and Features visit here.

filter() method in Java Stream APIs:

As you know Stream operations are divided into intermediate and terminal operations the filter() method is an intermediate operation.

These type of intermediate operation returns a new stream.It is a lazy operation.

Lets see 1 simple example of filter() method, later on in this post we will understand more complex examples of filter method.

filter() method in Java Stream APIs example 1:

Task:

Given a list of integers filter out the integers in range of -1 to 10 .

Explanation:

Question demands to create a new list that has the elements from the input integer list, the constraint is that the element should be in the given range.

Solution:



import java.util.Arrays;
import java.util.List;

public class StreamAPIFilterMethod {

    public static void main(String args[]){

        List<Integer> result= new StreamAPIFilterMethod()
                .filterOutTheIntegers(
                        Arrays.asList(20,16,-1,0,76,9,10,32)
                );

        for (int i : result){
            System.out.print(i+" ");
        }

    }
    /*
    * filter() method example 1
    * Take an integer list and
    * filter out the integers
    * in range of -1 to 10
    * */

    public List<Integer> filterOutTheIntegers(
            List<Integer> integerList
    )
    {
       return integerList
               .stream()
               .filter(
                       value->value>=-1
                               &&
                               value<=10
               )
               .toList();
    }

}

Some more points for filter() method:

It is a Stateless operations it does not retain any state from previously seen element when processing a new element -- each element can be processed independently of operations on other elements.
It does not actually perform any filtering, but instead creates a new stream that, when traversed, contains the elements of the initial stream that match the given predicate.
Traversal of the pipeline source does not begin until the terminal operation of the pipeline is executed.

filter() method little more complex example:

Task:

You have given a list of users with there names and credti score. Print the names of the NTC users. Range for NTC score is -1 to 10.

Explanation:

Its similar to example 1 where we need to filter out some data, but here the input is not a simple int array but a list of object. And we just don't need to send the response but transform it .

So to do this we will use filter() method of Stream API in java along with map() method to transform our data.

Solution:



import java.util.Arrays;
import java.util.List;

public class StreamAPIFilterMethod {

    public static void main(String args[]){


        new StreamAPIFilterMethod()
                .getNamesOfNTCUserUsers(
                        Arrays.asList(
                                new User("user1",600),
                                new User("user2",7),
                                new User("user3",1),
                                new User("user4",876),
                                new User("user5",756)
                        )
                )
                .stream()
                .forEach(System.out::println);

    }


    public List<String> getNamesOfNTCUserUsers(
            List<User> users
    ){

        return users
                .stream()
                .filter(
                        user->
                                user.getCreditScore()>=-1
                                        && user.getCreditScore()<=10
                )
                .map(
                        user->user.getName()
                )
                .toList();

    }

}


class User{
    private String name;
    private int creditScore;

    public User(
            String name,
            int creditScore
    ) {
        this.name = name;
        this.creditScore = creditScore;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", creditScore=" + creditScore +
                '}';
    }

    public String getName() {
        return name;
    }

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

    public int getCreditScore()
    {
        return creditScore;
    }

    public void setCreditScore(
            int creditScore)
    {
        this.creditScore = creditScore;
    }
}

Output:

Stream API filter() method in Java | KodeSrc

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Stream API filter() method in Java | KodeSrc