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Java 8 Streams - Stream.flatMapToInt Examples

Java 8 Streams Java Java API 

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Interface:

java.util.stream.Stream

java.lang.AutoCloseableAutoCloseablejava.util.stream.BaseStreamBaseStreamjava.util.stream.StreamStreamLogicBig

Method:

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

This intermediate operation 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.


Examples


package com.logicbig.example.stream;

import java.util.Arrays;
import java.util.List;
import java.util.stream.IntStream;

public class FlatMapToIntExample {

    public static void main(String... args) {
        List<List<String>> listOfLists = Arrays.asList(
                Arrays.asList("1", "2"),
                Arrays.asList("5", "6"),
                Arrays.asList("3", "4")
        );

        IntStream intStream =
                listOfLists.stream()
                           .flatMapToInt(childList ->
                                   childList.stream()
                                            .mapToInt(Integer::new));
        //let's peek and find sum of the elements
        int sum = intStream.peek(System.out::println)
                           .sum();
        System.out.println("sum: " + sum);
    }
}

Output

1
2
5
6
3
4
sum: 21




Using Array of arrays. 

package com.logicbig.example.stream;

import java.util.Arrays;
import java.util.stream.IntStream;

public class FlatMapToIntExample2 {

    public static void main(String... args) {
        String[][] arrayOfArrays = {{"1", "2"}, {"5", "6"}, {"3", "4"}};


        IntStream intStream = Arrays.stream(arrayOfArrays)
                                    .flatMapToInt(childArray -> Arrays.stream(childArray)
                                                                      .mapToInt(Integer::new));
        int sum = intStream.peek(System.out::println)
                           .sum();
        System.out.println("sum: " + sum);
    }
}

Output

1
2
5
6
3
4
sum: 21




package com.logicbig.example.stream;

import java.util.Arrays;
import java.util.List;

public class FlatMapToIntExample3 {

    public static void main(String... args) {
        List<String[]> list = Arrays.asList(
                new String[]{"a", "b", "cc"},
                new String[]{"d", "ee", "fff"}
        );

        //converting to int array of each string's array length
        int[] ints = list.stream()
                         .flatMapToInt(stringArray -> Arrays.stream(stringArray)
                                                            .mapToInt(String::length))
                         .toArray();
        System.out.println(Arrays.toString(ints));

        //finding max int
        int i = list.stream()
                    .flatMapToInt(stringArray -> Arrays.stream(stringArray)
                                                       .mapToInt(String::length))
                    .max()
                    .orElse(-1);
        System.out.println(i);
    }
}

Output

[1, 1, 2, 1, 2, 3]
3

See Also