LongStream.Builder build() in Java

LongStream.Builder build() builds the stream, transitioning this builder to the built state.

Syntax :

LongStream build()

Return Value: This method returns the built stream.

Note: A stream builder has a lifecycle, which starts in a building phase, during which elements can be added, and then transitions to a built phase, after which elements may not be added. The built phase begins when the build() method is called, which creates an ordered stream whose elements are the elements that were added to the stream builder, in the order they were added.

Below are the examples to illustrate build() method:

Example 1:

// Java code to show the implementation
// of LongStream.Builder build()

import java.util.stream.LongStream;

class GFG {

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

        // Creating a Stream in building phase
        LongStream.Builder b = LongStream.builder();

        // Adding elements into the stream
        b.add(1L);
        b.add(2L);
        b.add(3L);
        b.add(4L);

        // Constructing the built stream using build()
        // This will enter the stream in built phase
        b.build().forEach(System.out::println);
    }
}

Output:

1
2
3
4

Example 2: Trying to add element in after calling build() method (when stream is in built phase).

// Java code to show the implementation
// of LongStream.Builder build()

import java.util.stream.LongStream;

class GFG {

    // Driver code
    public static void main(String[] args)
    {
        // Creating a Stream in building phase
        LongStream.Builder b = LongStream.builder();

        // Adding elements into the stream
        b.add(1L);
        b.add(2L);
        b.add(3L);
        b.add(4L);

        // Constructing the built stream using build()
        // This will enter the stream in built phase
        // Now no more elements can be added to this stream
        b.build().forEach(System.out::println);

        // Trying to add more elements in built phase
        // This will cause exception
        try {
            b.add(50L);
        }
        catch (Exception e) {
            System.out.println("\nException: " + e);
        }
    }
}

Output:

1
2
3
4

Output:

1
2
3
4

Exception: java.lang.IllegalStateException