Reverse loop versus Forward loop in Performance – Java

Forward Looping

Forward looping is loop from starting element and continues to the end of the elements. 0..1..2..3..End. Most of the time, may be i can said all the time we are using this type of looping method to loop through an array, List or collection.

for (int i=0; i< lList.size(); i++)

Reverse Looping

Reverse looping is loop from end of the element and continues to the starting elements. End..9..8..0. Most of the time, may be i can said all the time we are not using this type of looping method.

for (int i=lList.size()-1; i > 0; i--)

Performance Test

Here i create two tiny programs to use forward loop and reverse loop to traverse 15 millions of data, and display the elapse time in output.

1. Forward Looping

import java.util.Arrays;

import java.util.Date;

import java.util.List;

public class LoopTestForward {

  public static void main(String[] argv) {

          String sArray[] = createArray();

          //convert array to list

          List lList = Arrays.asList(sArray);

          int iListSize = lList.size();

          //Forward Loop Testing

          System.out.println("\n--------- Forward Loop --------\n");

          long lForwardStartTime = new Date().getTime();

          System.out.println("Start: " + lForwardStartTime);

          //for loop

          for (int i=0; i< iListSize; i++){

                 String stemp = (String)lList.get(i);

          }

          long lForwardEndTime = new Date().getTime();

          System.out.println("End: " + lForwardEndTime);

          long lForwardDifference = lForwardEndTime - lForwardStartTime;

          System.out.println("Forward Looping - Elapsed time in milliseconds: " + lForwardDifference);

          System.out.println("\n-------END-------");

  }

  static String [] createArray(){

          String sArray[] = new String [15000000];

          for(int i=0; i<15000000; i++)

                 sArray[i] = "Array " + i;

          return sArray;

  }

}

2. Reverse Looping

import java.util.Arrays;

import java.util.Date;

import java.util.List;

public class LoopTestReverse {

  public static void main(String[] argv) {

          String sArray[] = createArray();

          //convert array to list

          List lList = Arrays.asList(sArray);

          int iListSize = lList.size();

          //Reverse Loop Testing

          System.out.println("\n--------- Reverse Loop --------\n");

          long lReverseStartTime = new Date().getTime();

          System.out.println("Start: " + lReverseStartTime);

          //for loop

          for (int i=iListSize-1; i > 0; i--){

                 String stemp = (String)lList.get(i);

          }

          long lReverseEndTime = new Date().getTime();

          System.out.println("End: " + lReverseEndTime);

          long lReverseDifference = lReverseEndTime - lReverseStartTime;

          System.out.println("For - Elapsed time in milliseconds: " + lReverseDifference);

          System.out.println("\n-------END-------");

  }

  static String [] createArray(){

          String sArray[] = new String [15000000];

          for(int i=0; i<15000000; i++)

                 sArray[i] = "Array " + i;

          return sArray;

  }

}

D:\test>java -Xms1024m -Xmx1024m LoopTestFoward

D:\test>java -Xms1024m -Xmx1024m LoopTestReverse

Performance Test Result (in milliseconds)

Conclusion

The result show there’s not much different between forward and reverse looping in 1 million of data. However when data grow huge, the performance of reverse looping is slightly faster than forward looping around 15%.

May be you will question about who so stupid to load 1 million of data into a single List or Collection. Please imagine a multi-thread system environment, where 100k people concurrent access your “forward loop x 10″, it already over 1 million. Ya i know sometime the different is negligible, but i do believe believe “Reverse loop” is a good habit in programming , it can increase the performance. It’s sound weird …but performance show.