Calculate elapsed time with System.currentTimeMillis() in java

Question

For some reason I can get the start time and the stop time in my program (which is normally a difference of 3 milliseconds) but when I print the result of the getElapsedTime() method it always comes back with 0. Above the listed code is the creation of a StopWatch object named sortTime.

Perhaps there is a problem with rounding? Perhaps there is a problem with the getElapsedTime() method itself?

    System.out.println(sortTime.start()); // Start Timer

    java.util.Arrays.sort(numbers); // Sort the array

    System.out.println(sortTime.stop());// stop timer

    System.out.println("the elapsed time is: " + sortTime.getElapsedTime());
    }
} //end of main class (used for testing)

//Define Class
 class StopWatch {
 long startTime;
 long endTime;

 //Constructors
 public StopWatch () { 
 }

 //Methods
long start() {
    long startTime = System.currentTimeMillis();
    return startTime; 
}
long stop() {
    long endTime = System.currentTimeMillis();
    return endTime; 
}
long getElapsedTime() {
        return (endTime - startTime);
    }
     }

For example, when I run the program the result is:

1442877972466
1442877972469
the elapsed time is: 0

http://stackoverflow.com/questions/4623334/question-about-variable-scope-and-shadowing-in-java?rq=1 — Sotirios Delimanolis, Sep 21 '15 at 23:33
related: to measure elapsed time don't use `System.currentTimeMillis()` because it can be wrong. http://stackoverflow.com/a/1776053 — zapl, Sep 22 '15 at 00:12

score 4 · Accepted Answer · answered Sep 21 '15 at 23:26

You are creating local variables with the same name as class variables:

long start() {
    long startTime = System.currentTimeMillis();
    return startTime; 
}

The use of long startTime in this function makes a local variable that is different from the class member named startTime. Change this to:

long start() {
    startTime = System.currentTimeMillis();
    return startTime; 
}

score 3 · Answer 2 · answered Sep 21 '15 at 23:26

startTime and endTime have been redeclared/shadowed as local variables, leaving the instance fields with their default values (of 0)

long start() {
    long startTime = System.currentTimeMillis();
    return startTime; 
}
long stop() {
    long endTime = System.currentTimeMillis();
    return endTime; 
}

Instead, make use of the instance fields...

long start() {
    startTime = System.currentTimeMillis();
    return startTime; 
}
long stop() {
    endTime = System.currentTimeMillis();
    return endTime; 
}

Calculate elapsed time with System.currentTimeMillis() in java

2 Answers2