Question

How to flatten a 2D array into a sorted one-dimensional array with unique values in Java

Answer 1

import java.util.Set;
import java.util.Arrays;
import java.util.Comparator;
import java.util.stream.Collectors;
 
public class Flatten2DArrayIntoASorted1DArrayWithUniqueValues_Java {
    public static void main(String[] args) {
        int[][] array2d = {
            {4, 3, 3, 2, 4},
            {30, 10, 10},
            {10, 30},
            {1, 1, 6, 7, 7, 7, 8},
        };
 
        Set<Integer> set = Arrays.stream(array2d)
                                  .flatMapToInt(Arrays::stream)
                                  .distinct()
                                  .boxed()
                                  .sorted(Comparator.naturalOrder())
                                  .collect(Collectors.toSet());
 
        System.out.println(set.stream().map(String::valueOf).collect(Collectors.joining(", ")));
         
        for (Integer n : set) {
            System.out.print(n + " ");
        }
    }
}
 
 
 
/*
run:
 
1, 2, 3, 4, 6, 7, 8, 10, 30
1 2 3 4 6 7 8 10 30 
 
*/

 

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Answer 2

import java.util.Set;
import java.util.LinkedHashSet;
import java.util.Arrays;
import java.util.Comparator;
import java.util.stream.Collectors;

public class Flatten2DArrayIntoASorted1DArrayWithUniqueValues {

    // Function that flattens a 2D int array, removes duplicates, and returns a sorted Set
    public static Set<Integer> flattenSortUnique(int[][] array2d) {

        // Stream each row, flatten into one IntStream, remove duplicates,
        // convert to boxed Integer stream, sort, and collect into a LinkedHashSet
        // (LinkedHashSet preserves the sorted order)
        return Arrays.stream(array2d)
                     .flatMapToInt(Arrays::stream)
                     .distinct()
                     .boxed()
                     .sorted(Comparator.naturalOrder())
                     .collect(Collectors.toCollection(LinkedHashSet::new));
    }

    public static void main(String[] args) {
        int[][] array2d = {
            {4, 3, 3, 2, 4},
            {30, 10, 10},
            {10, 30},
            {1, 1, 6, 7, 7, 7, 8},
        };

        Set<Integer> set = flattenSortUnique(array2d);

        // Print values as comma-separated list
        System.out.println(
            set.stream()
               .map(String::valueOf)
               .collect(Collectors.joining(", "))
        );

        // Print values with spaces
        for (Integer n : set) {
            System.out.print(n + " ");
        }
    }
}


/*
run:

1, 2, 3, 4, 6, 7, 8, 10, 30
1 2 3 4 6 7 8 10 30

*/

 

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