How to Measure Time Elapsed in Java

How do I measure time elapsed in Java?

Unfortunately, none of the ten answers posted so far are quite right.

If you are measuring elapsed time, and you want it to be correct, you must use System.nanoTime(). You cannot use System.currentTimeMillis(), unless you don't mind your result being wrong.

The purpose of nanoTime is to measure elapsed time, and the purpose of currentTimeMillis is to measure wall-clock time. You can't use the one for the other purpose. The reason is that no computer's clock is perfect; it always drifts and occasionally needs to be corrected. This correction might either happen manually, or in the case of most machines, there's a process that runs and continually issues small corrections to the system clock ("wall clock"). These tend to happen often. Another such correction happens whenever there is a leap second.

Since nanoTime's purpose is to measure elapsed time, it is unaffected by any of these small corrections. It is what you want to use. Any timings currently underway with currentTimeMillis will be off -- possibly even negative.

You may say, "this doesn't sound like it would ever really matter that much," to which I say, maybe not, but overall, isn't correct code just better than incorrect code? Besides, nanoTime is shorter to type anyway.

Previously posted disclaimers about nanoTime usually having only microsecond precision are valid. Also it can take more than a whole microsecond to invoke, depending on circumstances (as can the other one), so don't expect to time very very small intervals correctly.

How to measure elapsed time

When the game starts:

long tStart = System.currentTimeMillis();

When the game ends:

long tEnd = System.currentTimeMillis();
long tDelta = tEnd - tStart;
double elapsedSeconds = tDelta / 1000.0;

How do I time a method's execution in Java?

There is always the old-fashioned way:

long startTime = System.nanoTime();
methodToTime();
long endTime = System.nanoTime();

long duration = (endTime - startTime);  //divide by 1000000 to get milliseconds.

How do I calculate the elapsed time of an event in Java?

I would avoid using System.currentTimeMillis() for measuring elapsed time. currentTimeMillis() returns the 'wall-clock' time, which may change (eg: daylight savings, admin user changing the clock) and skew your interval measurements.

System.nanoTime(), on the other hand, returns the number of nanoseconds since 'some reference point' (eg, JVM start up), and would therefore not be susceptible to system clock changes.

How can I calculate a time difference in Java?

String time1 = "16:00:00";
String time2 = "19:00:00";

SimpleDateFormat format = new SimpleDateFormat("HH:mm:ss");
Date date1 = format.parse(time1);
Date date2 = format.parse(time2);
long difference = date2.getTime() - date1.getTime(); 

Difference is in milliseconds.

I modified sfaizs post.

How to measure the elapsed time of ASYNC sent HTTPRequest with Java 11 standard HTTPClient?

Something like the following should do the trick:

long start = System.nanoTime();
CompletableFuture<HttpResponse<String>> httpResponseCF = 
    this.httpClient.sendAsync(httpRequest, BodyHandlers.ofString()).
    whenComplete((r,t) -> System.out.println("elapsed ns: "
                           + (System.nanoTime() - start)));

Disclaimer: I haven't actually compiled the above...
It's also not the exact time but the best approximation you can get. As with everything, the exact time is impossible to measure.

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