Question

I have a mergesort function I am testing using JUnit. Below is one of my test cases:

@Test
    //test using randomly generated numbers
    public void MergeSortTest002()
    {
        long seed = System.currentTimeMillis();
        Random rng = new Random(seed);
        Integer[] TestArray = new Integer[1000];
        int MAX_VALUE = Integer.MAX_VALUE;

        for(int i=0; i<1000; i++) {
            //this will generate positive and negative numbers from
            // -MAX_VALUE/2 to +MAX_VALUE/2
            Integer newNum = rng.nextInt(MAX_VALUE/2) - MAX_VALUE;
            TestArray[i] = newNum;
        }

        Integer[] correctArray = TestArray;
        Arrays.sort(correctArray);
        MergeSort.mergeSort(TestArray);

        Assert.assertArrayEquals(correctArray,TestArray);
    }

The strange thing is, even if I comment out the line where I call my mergeSort function, the test still passes.

I figured one of two things is happening: Either assertArrayEquals does not care about the order of elements (unlikely), of when I’m copying TestArray to correctArray, it’s copying by reference, and thus calling Arrays.sort on correctArray is sorting TestArray as well.

Could anyone confirm which of the two is happening, and what should be the solution? Is there a Assert that keeps order in mind, or is there a way to copy arrays by values rather than reference without writing an explicit for loop?

Answer 1

The problem is right here:

Integer[] correctArray = TestArray;

You’re copying the reference to your array, not the contents of the array.

To fix, change that line to:

Integer[] correctArray = TestArray.clone();