Stream.concat() in Java

Stream.concat() method creates a concatenated stream in which the elements are all the elements of the first stream followed by all the elements of the second stream. The resulting stream is ordered if both of the input streams are ordered, and parallel if either of the input streams is parallel.

Syntax :

static <T> Stream<T> concat(Stream<? extends T> stream1, 
                            Stream<? extends T> stream2)

Where, T is the type of stream elements,
stream1 represents the first stream,
stream2 represents the second stream and
the function returns the concatenation of
the two input streams

The calls to Stream.concat(stream1, stream2) can be think of as forming a binary tree. The concatenation of all the input streams is at the root. The individual input streams are at the leaves. Below given are some examples of trees upto four input streams a, b, c and d.

For two streams a and b, the tree looks like :

For three streams a, b and c, the tree looks like :

For four streams a, b, c and d, the tree looks like :

Each additional input stream adds one layer of depth to the tree and one layer of indirection to reach all the other streams.

Note : The elements returned by Stream.concat() method is ordered. For example, the following two lines returns the same result:

Stream.concat(Stream.concat(stream1, stream2), stream3);
Stream.concat(stream1, Stream.concat(stream2, stream3));

But the result for the following two are different.

Stream.concat(Stream.concat(stream1, stream2), stream3); 
Stream.concat(Stream.concat(stream2, stream1), stream3);

Below are some examples to understand the implementation of the function in a better way.
Example 1 :

// Implementation of Stream.concat()
// method in Java 8 with 2 Streams
import java.util.*;
import java.util.stream.IntStream;
import java.util.stream.Stream;

class GFG {

    // Driver code
    public static void main(String[] args)
    {
        // Creating two Streams
        Stream<String> stream1 = Stream.of("Geeks", "for");
        Stream<String> stream2 = Stream.of("GeeksQuiz", "GeeksforGeeks");

        // concatenating both the Streams
        // with Stream.concat() function
        // and displaying the result
        Stream.concat(stream1, stream2)
            .forEach(element -> System.out.println(element));
    }
}

Output:

Geeks
for
GeeksQuiz
GeeksforGeeks

Example 2 :

// Implementation of Stream.concat()
// method in Java 8 with more than
// two Streams
import java.util.*;
import java.util.stream.IntStream;
import java.util.stream.Stream;

class GFG {

    // Driver code
    public static void main(String[] args)
    {

        // Creating more than two Streams
        Stream<String> stream1 = Stream.of("Geeks");
        Stream<String> stream2 = Stream.of("GeeksQuiz");
        Stream<String> stream3 = Stream.of("GeeksforGeeks");
        Stream<String> stream4 = Stream.of("GFG");

        // concatenating all the Streams
        // with Stream.concat() function
        // and displaying the result
        Stream.concat(Stream.concat(Stream.concat(stream1,
                             stream2), stream3), stream4)
            .forEach(element -> System.out.println(element));
    }
}

Output:

Geeks
GeeksQuiz
GeeksforGeeks
GFG

Example 3 :

// Implementation of Stream.concat()
// method in Java 8 with DoubleStream
import java.util.*;
import java.util.stream.Stream;
import java.util.stream.DoubleStream;

class GFG {

    // Driver code
    public static void main(String[] args)
    {

        // Creating two Streams
        DoubleStream Stream1 = DoubleStream.of(1520, 1620);
        DoubleStream Stream2 = DoubleStream.of(1720, 1820);

        // concatenating both the Streams and
        // displaying the result
        DoubleStream.concat(Stream1, Stream2)
            .forEach(element -> System.out.println(element));
    }
}

Output:

1520.0
1620.0
1720.0
1820.0

Example 4 :

// Implementation of Stream.concat()
// method in Java 8 and removing
// the duplicates
import java.util.*;
import java.util.stream.IntStream;
import java.util.stream.Stream;

class GFG {

    // Driver code
    public static void main(String[] args)
    {

        // Creating two Streams
        Stream<String> stream1 = Stream.of("Geeks", "for", "GeeksforGeeks");
        Stream<String> stream2 = Stream.of("GeeksQuiz", "GeeksforGeeks", "for");

        // concatenating both the Streams
        // with Stream.concat() function
        // and displaying the result after
        // removing the duplicates
        Stream.concat(stream1, stream2).distinct().forEach(element -> System.out.println(element));
    }
}

Output:

Geeks
for
GeeksforGeeks
GeeksQuiz

Example 5 :

// Implementation of Stream.concat()
// method in Java 8 with LongStream
import java.util.*;
import java.util.stream.Stream;
import java.util.stream.LongStream;

class GFG {

    // Driver code
    public static void main(String[] args)
    {

        // Creating two Streams
        LongStream Stream1 = LongStream.of(1520, 1620);
        LongStream Stream2 = LongStream.of(1720, 1820);

        // concatenating both the Streams and
        // displaying the result
        LongStream.concat(Stream1, Stream2)
            .forEach(element -> System.out.println(element));
    }
}

Output:

1520
1620
1720
1820