Question

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Should I use Java's String.format() if performance is important?

I was wondering if is good to use String.format in Java apps instead of StringBuilder… so, I just write a simple test, like this:

public static void main(String[] args) {
        int i = 0;
        Long start = System.currentTimeMillis();
        while (i < 10000) {
            String s = String.format("test %d", i);
            i++;
        }
        System.out.println(System.currentTimeMillis() - start);
        i = 0;
        start = System.currentTimeMillis();
        while (i < 10000) {
            String s = new StringBuilder().append("test ").append(i).toString();
            i++;
        }
        System.out.println(System.currentTimeMillis() - start);
    }

And the results where:

238
15

So, if my test is valid, StringBuilder is faster than String.format. OK.
Now, I start thinking how String.format works. Is it a simple String concatenation, like "test " + i?

What the differences between StringBuilder concatenation and String.format? Is there a way simple as String.format and fast like StringBuilder?

Answer 1

I wrote a quick caliper benchmark to compare String.format() vs. StringBuilder, StringBuffer, normal String + operator, String.replace() and String.concat() methods:

public class StringFormatBenchmark extends SimpleBenchmark {

    public void timeStringFormat(int reps) {
        while (--reps >= 0) {
            String s = String.format("test %d", reps);
        }
    }

    public void timeStringBuilder(int reps) {
        while (--reps >= 0) {
            String s = new StringBuilder("test ").append(reps).toString();
        }
    }

    public void timeStringBuffer(int reps) {
        while (--reps >= 0) {
            String s = new StringBuffer("test ").append(reps).toString();
        }
    }

    public void timeStringPlusOperator(int reps) {
        while (--reps >= 0) {
            String s = "test " + reps;
        }
    }

    public void timeReplace(int reps) {
        while (--reps >= 0) {
            String s = "test {}".replace("{}", String.valueOf(reps));
        }
    }

    public void timeStringConcat(int reps) {
        while (--reps >= 0) {
            String s = "test ".concat(String.valueOf(reps));
        }
    }

    public static void main(String[] args) {
        new Runner().run(StringFormatBenchmark.class.getName());
    }

}

The results follow (Java 1.6.0_26-b03, Ubuntu, 32 bits):

Clearly String.format() is much slower (by an order of magnitude). Also StringBuffer is considerably slower than StringBuilder (as we were taught). Finally StringBuilder and String + operator are almost identical since they compile to very similar bytecode. String.concat() is a bit slower.

Also don’t use String.replace() if simple concatenation is sufficient.