2D array in Java

Question

Which of the following is faster in Java? Is there any other way that is faster than any of these?

int[][] matrix = new int[50][50];

for (int k=0;k<10;k++){
// some calculations here before resetting the array to zero
      for (int i = 0; i <50; i++) {
            for (int j = 0; j <50; j++) {
                matrix[i][j]=0;
            }              
        }
}

Or this:

int[][] matrix = new int[50][50];

    for (int k=0;k<10;k++){
// some calculations here before resetting the array to zero 
          matrix = new int[50][50];
    }

What does the [profiler](http://stackoverflow.com/q/2064427/230513) say? — trashgod, Sep 24 '12 at 20:41
IIRC correctly, Java memory (unlike `C`) is zeroed when assigned, so the first code would be quicker with `int[][] matrix = new int[50][50];`. The second code repeats 10 times the same operation, creating 9 useless arrays by the way. — SJuan76, Sep 24 '12 at 20:42
The arrays are guaranteed to be initialized [with a default value](http://docs.oracle.com/javase/specs/jls/se5.0/html/arrays.html#10.3). If you're only zeroing out the array, then declaring/instantiating would be faster. — Makoto, Sep 24 '12 at 20:43
@SJuan76 They are not useless, I want the matrix to be reset to zero. There will be some operation inside for loop before declaring the matrix. — zista, Sep 24 '12 at 20:48
Obviously, zeroing out the array once rather than 10 times will be faster ... if you want a better answer, ask a better question. — meriton, Sep 24 '12 at 20:48
@meriton you clearly didn't understand the question. I am zeroing out the array 10 times in both scenarios. I guess someone voted this question down based on what you assumed. — zista, Sep 24 '12 at 20:51
@RohitJain because I will be doing some calculations and I want the matrix to reset to zero after the calculations in every loop (10 times) — zista, Sep 24 '12 at 20:53
Then you should perhaps say so in your question ... use the edit button ... — meriton, Sep 24 '12 at 20:55
@zesta.. I think second option would be better.. Because it is just creating new Array object every time, thus making the older arrays eligible for garbage collections.. Of course you don't want to iterate 2500 times to set each cells to Zero.. That is done automatically.. — Rohit Jain, Sep 24 '12 at 20:56
@meriton I thought it was obvious. Will be more descriptive next time. — zista, Sep 24 '12 at 21:06

allingeek · Accepted Answer · 2012-09-24T21:38:14.857

The fastest way to perform an action that leaves the variable, "matrix" in an equivalent state at the end of a run is int[][] matrix = new int[50][50];

However, none of these solutions are equivalent in terms of number of operations or memory thrash. The statement I've provided is what you are looking for.

Update: With your updated question where you are manipulating matrix and then resetting it.

Your second example will likely be faster on each iteration. The thought being that it is faster to allocate memory than iterate and set a variable 50^2 times. Though this is a question for a profiler. In general, zeroing out memory is something that is better optimized by the JVM than your application.

This being said, it is important to remember than memory allocation is not without caveats in extreme scenarios. If you allocate and trash memory too often, you may have a suboptimal GC experience.

2D array in Java

1 Answers1