Stream.concat() in Java

The concat() method of the Stream class in Java creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream. The syntax is as follows −

concat(Stream<? extends T> a, Stream<? extends T> b)

Here, a is the first stream, whereas b is the second stream. T is the type of stream elements.

Example

Following is an example to implement the concat() method of the Stream class −

import java.util.*;
import java.util.stream.IntStream;
import java.util.stream.Stream;
public class Demo {
   public static void main(String[] args) {
      Stream<String> streamOne = Stream.of("John");
      Stream<String> streamTwo = Stream.of("Tom");
      Stream.concat(streamOne, streamTwo).forEach(val -> System.out.println(val));
   }
}

Output

John
Tom

Example

Let us now see another example wherein we are working on multiple streams −

import java.util.stream.Collectors;
import java.util.stream.Stream;
import static java.util.stream.Stream.*;
public class Main {
   public static void main(String[] args) {
      Stream<Integer> streamOne = Stream.of(15, 40, 50);
      Stream<Integer> streamTwo = Stream.of(55, 70, 90);
      Stream<Integer> streamThree = Stream.of(110, 130, 150);
      Stream<Integer> streamFour = Stream.of(170, 200, 240, 300);
      Stream<Integer> res = Stream.concat(streamOne, concat(streamTwo, concat(streamThree, streamFour)));
      System.out.println( res.collect(Collectors.toList()) );
   }
}

Output

[15, 40, 50, 55, 70, 90, 110, 130, 150, 170, 200, 240, 300]