Test cases for Java does not detect slow solutions. :/

Furthermore - there are accepted java solutions which produces wrong answers for some cases.

Please add following test case:

import org.junit.Assert;
import org.junit.Test;

import java.util.ArrayList;
import java.util.List;

public class RectanglesUnionTest {
    @Test
    public void testHugeOverlap() {
        List<int[]> rectangles = new ArrayList<>();

        for (int i = 0; i < 10000; i++) {
            rectangles.add(new int[]{i, i, 1000 + i, 1000 + i});
        }

        Assert.assertEquals(20988001, RectanglesUnion.calculateSpace(rectangles.toArray(new int[][]{})));
    }

}

cheater! ;)

^^

Description of Kata should be somehow polished. ;)

It could be passed without using splice.

Negative test case isn't present. Code, which uses filter method passes.