functional array update 'in place'; in 2 passes; order by 3,1,2; no other values

312.java

import static java.util.stream.Collectors.counting;
import static org.junit.Assert.assertArrayEquals;

import java.util.Arrays;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

import org.junit.Test;

/**
 * update array 'in place'; order by 3,1,2; no other values; in only 2 passes
 * 
 * 
 */


public class GH312SortTest {

    public static void sortIt(int[] arr) {

        //read array
        Map<Integer, Long> map =
                Arrays.stream(arr).boxed().collect(Collectors.toList()).stream().collect(
                        Collectors.groupingBy(
                                Function.identity(), counting()
                        )
                );

        int x1 = map.getOrDefault(1, 0L).intValue();
        int x3 = map.getOrDefault(3, 0L).intValue();

        //update array in place
        IntStream.range(0, x3).forEach(i -> arr[i] = 3);
        IntStream.range(x3, x3 + x1).forEach(i -> arr[i] = 1);
        IntStream.range(x3 + x1, arr.length).forEach(i -> arr[i] = 2);

    }

    @Test
    public void test() {
        int[] arr = new int[]{};
        GH312SortTest.sortIt(arr);
        assertArrayEquals(new int[]{}, arr);
    }

    @Test
    public void test1a() {
        int[] arr = new int[]{1};
        GH312SortTest.sortIt(arr);
        assertArrayEquals(new int[]{1}, arr);
    }

    @Test
    public void test1b() {
        int[] arr = new int[]{2};
        GH312SortTest.sortIt(arr);
        assertArrayEquals(new int[]{2}, arr);
    }

    @Test
    public void test2a() {
        int[] arr = new int[]{1,3};
        GH312SortTest.sortIt(arr);
        assertArrayEquals(new int[]{3,1}, arr);
    }

    @Test
    public void test3a() {
        int[] arr = new int[]{2, 2, 2, 3, 1, 1, 1, 1, 2, 3};
        GH312SortTest.sortIt(arr);
        assertArrayEquals(new int[]{3, 3, 1, 1, 1, 1, 2, 2, 2, 2}, arr);
    }

    @Test
    public void test3b() {
        int[] arr = new int[]{2, 2, 2, 2, 2, 2, 1, 1, 2, 2};
        GH312SortTest.sortIt(arr);
        assertArrayEquals(new int[]{1,1,2,2,2,2,2,2,2,2}, arr);
    }

    @Test
    public void test3c() {
        int[] arr = new int[]{2, 1, 3, 2, 1, 3, 2, 1, 3};
        GH312SortTest.sortIt(arr);
        assertArrayEquals(new int[]{3,3,3,1,1,1,2,2,2}, arr);
    }
}