Improving for-each performance

Question

I have the following CountDownTimer running constantly at my Android Application.

CountDownTimer timer_status;
timer_status = new CountDownTimer(1000,1000) {

    List<Integer> lst = new ArrayList<Integer>();
    int k=0;
    @Override
    public void onTick(long millisUntilFinished) {


        String S = "";
        k=0;
        byte[] writeBuffer = new byte[8];

        for (int value : lst) {

            if(k==8) {
                k=0;
                S="";
             }

            S = S + String.format("%02x ", value).toUpperCase();
            writeBuffer[k] = (byte) value;
            k++;

        }

        editSent.setText(S);                            
}

Every loop, the application creates a new byte[] buffer from a List of Integers.

It work's fine, but is generating GC_FOR_ALLOC at LogCat.

03-05 11:12:41.330: D/dalvikvm(21178): GC_FOR_ALLOC freed 511K, 19% free 4662K/5720K, paused 14ms, total 14ms
03-05 11:12:42.250: D/dalvikvm(21178): GC_FOR_ALLOC freed 511K, 19% free 4662K/5720K, paused 15ms, total 15ms

GC_FOR_ALLOC is triggered because there wasn't enough memory left on the heap to perform an allocation. Might be triggered when new objects are being created.

How can I improve this foreach loop leaving enough memory on the heap?

Answer 1

 StringBuilder sb = new StringBuilder(); 
 k=0; 
 byte[] writeBuffer = new byte[8];

 for (int value : lst) { 
 if(k==8) { k=0; sb.setLength(0); 
 } 
 sb.append(String.format("%02x ", value)); 

 writeBuffer[k] = (byte) value; k++; 
 }

 editSent.setText(sb.toString().toUpperCase());

Apart from StringBuilder , I would like to point out that normal for has better performances than foreach in Android because the foreach loop creates a hidden Iterator object. So avoid using for-each in performance critical code in Android.

Also, if possible use normal Array for primitive types rather than ArrayList as autoboxing/unboxing would take place in each loop which would affect your performance.

Answer 2

Using a StringBuilder is the one thing (see comment).

You can also avoid the local variables S and writeBuffer and use class variables instead, which effectively reuses both with each call instead of passing them to the garbage collection.