What is the concat() method of Stream Interface?

The concat() method is a static method of the Stream Interface that can be used to merge two streams into a single stream.

The merged stream contains all the elements of the first stream, followed by all the elements of the second stream.
If both the streams are ordered, then the merged stream will be ordered.
The merged stream is parallel if one of the input streams is parallel.
This method creates a lazily concatenated stream, which means the concat method is not evaluated unless the terminal operation is invoked.

Syntax

static <T> Stream<T> concat(Stream<? extends T> firstStream,
                            Stream<? extends T> secondStream)

Example

In the code above, we have:

Used the Stream.of() method to create two streams:
- stream1 with elements 1,2,3.
- stream2 with elements 4,5,6

Called the Stream.concat method with the created stream as an argument.
The concat method will return a stream that contains both the elements of stream1 and stream2.
Collected the stream as a List using the collect(Collectors.toList()) method. The argument Collectors.toList() tells the concat method to collect the stream as List.

Merge multiple streams

import java.util.stream.Stream;
import java.util.stream.Collectors;
class HelloWorld {
    public static void main(String[] args) {
        Stream<Integer> stream1 = Stream.of(1, 2, 3);
        Stream<Integer> stream2 = Stream.of(4, 5, 6);
        Stream<Integer> stream3 = Stream.of(7, 8, 9);
        
        Stream<Integer> mergedStream = Stream.concat(stream1, stream2); 
        mergedStream = Stream.concat(mergedStream, stream3);
        System.out.println( mergedStream.collect(Collectors.toList() ));
    }
}

In the code above, we have created three Stream objects. First, we merged stream1 and stream2 by calling:

Stream<Integer> mergedStream = Stream.concat(stream1, stream2);

Then, the mergedStream is merged with the stream3.

mergedStream = Stream.concat(mergedStream, stream3);

Now, the merged stream contains elements of all three streams.