System out println

Question

I'm printing 2 lines to the console. They both print, but when the second one has printed, the first one changes to the second one, so the 2 lines are identical. I've never encountered this before. Why does the second print overwrite the first one, and how do I fix it?

public static void main(String args[]){
    new MergeSort(90000);

    System.out.println("Array to be mergesorted: " +Arrays.toString(array));

    long start = System.currentTimeMillis();

    mergeSort(array, 1, array.length);

    long end = System.currentTimeMillis();

    System.out.println("Result: " + Arrays.toString(array) );
}

The constructor:

public MergeSort(int n){
    Random rand = new Random();
    array = new int[n];
    for(int i = 0; i <array.length; i++){
        array[i] = rand.nextInt(101);
    }
}

Rest of code:

public static void merge(int[] A, int p, int q, int r){
    //
    //length of subarray 1
    int n1 = q-p+1;

    //length of subarray 2
    int n2 = r-q;

    int[] L = new int[n1+1];
    int[] R = new int[n2+1];

    for(int i = 0; i < n1; i++){
        L[i] = A[p+i-1];
    }

    for(int j=0; j< n2; j++){
        R[j] = A[q+j];
    }

    L[n1] = Integer.MAX_VALUE;
    R[n2] = Integer.MAX_VALUE;

    int i = 0;
    int j = 0;

    for(int k = p-1; k < r; k++){
        if(L[i] <= R[j]){
            A[k] = L[i];
            i++;
        }
        else{
                A[k] = R[j];
                j++;
            }
    }

}

public static void mergeSort(int[] A, int p, int r){
    if (p<r){
        int q = (int) Math.floor((r+p)/2);
        mergeSort(A, p, q);
        mergeSort(A, q+1, r);
        merge(A, p, q, r);
    }
}

Answer 1

This is due to the buffer limit of the console in your IDE. I cannot explain why exactly you are seeing duplicate output of certain strings other than to say it looks like theres a bug in how it clears out old characters in the buffer when it hits the limit.

I think Eclipse comes with a default of 80,000 character limit in its console output. Since you are printing 90,000 numbers between 1-100 two times that means your over shooting this buffer and then some.

To increase the buffer limit on the console:

• Right click on the output window in Eclipse and select Perferences
• Change "Console buffer size (characters)" to be your desired limit.

Ideally would change it to something higher than the maximum characters your printing out for this program. Maybe something like 800,000?

---

Heres a picture of the preferences window.

Edit: This question reminded me of another interesting question in which the answer to the question lied inside how the word wrapping was performed in the terminal output. Not really the same as this question but it is related and quite an interesting question/answer. Its worth a read and there's definitely a lesson to be learned in all of this.