Stream flatMapToInt() in Java with examples

Stream flatMapToInt(Function mapper) returns an IntStream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element. Stream flatMapToInt(Function mapper) is an intermediate operation. These operations are always lazy. Intermediate operations are invoked on a Stream instance and after they finish their processing, they give a Stream instance as output.

Note : Each mapped stream is closed after its contents have been placed into this stream. If a mapped stream is null, an empty stream is used, instead.
Syntax :

IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper)

Where, IntStream is a sequence of primitive 
int-valued elements and T is the type 
of stream elements. mapper is a stateless function 
which is applied to each element and the function
returns the new stream.

Example 1 : flatMapToInt() function with operation of parsing string to Integer.

// Java code for Stream flatMapToInt
// (Function mapper) to get an IntStream
// consisting of the results of replacing
// each element of this stream with the
// contents of a mapped stream.
import java.util.*;
import java.util.stream.IntStream;

class GFG {

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

        // Creating a list of Strings
        List<String> list = Arrays.asList("1", "2", "3",
                                          "4", "5");

        // Using Stream flatMapToInt(Function mapper)
        list.stream().flatMapToInt(num -> IntStream.of(Integer.parseInt(num))).
        forEach(System.out::println);
    }
}

Output :

1
2
3
4
5

Example 2 : flatMapToInt() function with operation of mapping string with their length.

// Java code for Stream flatMapToInt
// (Function mapper) to get an IntStream
// consisting of the results of replacing
// each element of this stream with the
// contents of a mapped stream.
import java.util.*;
import java.util.stream.IntStream;

class GFG {

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

        // Creating a List of Strings
        List<String> list = Arrays.asList("Geeks", "GFG",
                                          "GeeksforGeeks", "gfg");

        // Using Stream flatMapToInt(Function mapper)
        // to get length of all strings present in list
        list.stream().flatMapToInt(str -> IntStream.of(str.length())).
        forEach(System.out::println);
    }
}

Output :

5
3
13
3