Java StringBuilder Argument Performance

Your first example:

String line = "foo" + Integer.toString(i) + "bar\n";
sb.append(line);

is equivalent to this:

String line = new StringBuilder("foo").append(Integer.toString(i)).append("bar\n").toString();
sb.append(line);

Your third example:

sb.append(String.format("foo %i bar", i\n));

is the most wasteful, because it’s equivalent to this:

StringBuilder builder = new StringBuilder();
Formatter formatter = new Formatter(builder);
formatter.format("foo %i bar", i);
String value = formatter.out().toString();
sb.append(value);

Your second example has the best performance, but the use of Integer.toString is redundant and creates an unnecessary String:

sb.append("foo").append(i).append("bar\n");

In this form, no new objects are created (except whenever StringBuilder has to expand its internal buffer).

The difference will be quite small, often too small to notice. A modern system that can run Microsoft Office or Doom Eternal probably isn’t going to slow down noticeably from creating strings. And it’s entirely possible that the just-in-time compiler will turn append(Integer.valueOf(i)) into append(i) at runtime.

If you want guidance on the best practice, the answer is: don’t use + or String.format at all in a loop.

1. As Basil Bourque points out in a comment, your question smells of premature optimization. The "which is most efficient" question actually has a second meaning that you didn't consider: which is the most efficient use of your time.

2. The answer to the "which is more efficient" question that you actually asked is going to depend on the JVM version that you use. In some cases there will be differences. In others, the JIT compiler (mostly) is likely to optimize away the differences.

3. The way to find out which is actually faster, and what the actual differences are is to write a proper micro-benchmark ... and run it on various hardware and various Java versions.

   Important! Read this: How do I write a correct micro-benchmark in Java?

4. I predict that the differences between the first two will be small to non-existent, and that the format approach will be slower.

5. The big picture is that the differences between the 3 versions are probably too small to matter in most real world applications. You are probably wasting your time considering which of these versions is fastest. There are probably better things to spend your time on.

Point 2) means that we can't >tell< you which version is more efficient. Point 3) means it will cost you effort to get a definitive answer. Points 4) and 5) mean that it is probably not worth the effort getting a definitive answer. (Not least because the answer will only be definitive for the precise example that you test, on the specific platforms that you test on.)

The only other things to point out are that using StringBuilder is correct if it is essential to assemble the entire string in memory. But if not:

• It may be more efficient to write directly to a BufferedWriter or similar. There are second-order effects to assembling a large character string in memory in terms of allocating large objects and its impact on GC behavior and memory caches and the like.

• Any solution that entails assembling something large in the heap does not scale. If the thing you are assembling gets too large, you run out of heap space and ultimately physical RAM space.

I think it's a bit faster to write a simple test that shows the performance than to actually write this question :)

@Test
    void testBuildingString() {
        var start1 = Instant.now();

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 100000; i++) {
            String line = "foo" + Integer.toString(i) + "bar\n";
            sb.append(line);
        }

        var end1 = Instant.now();
        var start2 = Instant.now();

        StringBuilder sb2 = new StringBuilder();
        for (int i = 0; i < 100000; i++) {
            sb2.append("foo").append(Integer.toString(i)).append("bar\n");
        }

        var end2 = Instant.now();
        var start3 = Instant.now();
        StringBuilder sb3 = new StringBuilder();
        for (int i = 0; i < 100000; i++) {
            sb3.append(String.format("foo %d bar", i));
        }
        var end3 = Instant.now();

        System.out.println("Time 1 = " + Duration.between(start1, end1));
        System.out.println("Time 2 = " + Duration.between(start2, end2));
        System.out.println("Time 3 = " + Duration.between(start3, end3));
    }

And the results are as follows, and as expected:

Time 1 = PT0.0299933S
Time 2 = PT0.0059999S
Time 3 = PT0.2450021S

Your 2nd option is the fastest, around 5 times faster than the first. String.format is the slowest of the bunch. Note that this is 100,000 iterations. So on 1 iteration this will be lestt that 0.0000001s difference