问题
I hear that the System.out.println(); Java statement is costly (it consumes a lot of time)
So I try to evaluate its cost:
When I evaluate 5 statements... The cost = 1.0
So I expect the cost of 1 statement = 0.2
But actually I found The cost = 0.0 !!
double t1 = 0;
double t2 = 0;
t1 = System.currentTimeMillis();
System.out.println("aa");
System.out.println("aa");
System.out.println("aa");
System.out.println("aa");
System.out.println("aa");
t2 = System.currentTimeMillis();
System.out.println("The cost = " + (t2-t1) );
// The cost = 1.0
t1 = System.currentTimeMillis();
System.out.println("aa");
t2 = System.currentTimeMillis();
System.out.println("The cost = " + (t2-t1) );
// The cost = 0.0
// Expected : 1.0/5 = 0.2 -- But Actual : 0.0
Why that?
回答1:
System#currentTimeMillis returns long and not double. Thus you're loosing .2.
Testing 5 statements is not a good idea, specially when you almost don't feel the time it takes to perform it. I advise you to have more than 5 statements to test and then reduce the amount to something more than 1 as well.
You want to do a precise measurements time, it's better to use System#nanoTime, since it gives time in nano seconds:
long startTime = System.nanoTime();
// ... the code being measured ...
long estimatedTime = System.nanoTime() - startTime;
Look for "nanoTime vs currentTimeMillis" and you'll get hundreds of articles.
回答2:
If the execution time is less than one millisecond, you will see nothing, because the clock hasn't 'ticked' yet. For this type of micro benchmarking, you should use something like:
long t1 = System.nanoTime()
for your time measurements. (this measures time in nanosecond units, although not always with that much granularity)
回答3:
When i tested your code, the output is always 0.0. Pls try your benchmark with System.nanoTime();
Or do it with more System.out.println()
double t1 = 0;
double t2 = 0;
t1 = System.currentTimeMillis();
for(int i = 0; i<1000; i++)
System.out.println("aa");
t2 = System.currentTimeMillis();
System.out.println("The cost = " + (t2-t1) );
t1 = System.currentTimeMillis();
for(int i = 0; i<100; i++)
System.out.println("aa");
t2 = System.currentTimeMillis();
System.out.println("The cost = " + (t2-t1) );
Output: 62.0 and 5.0
And this difference of 1.2 ms is because the loop gets faster while runtime.
回答4:
Also, the time to complete System.out.println will be different on a more or less powerful computer, and how busy that computer is performing other tasks at that particular moment.
来源：https://stackoverflow.com/questions/20683361/consumed-time-of-system-out-println-java-statement