Reusing StringBuilder or creating a new one? [closed]

Question

Given that it is better to reuse objects than create new ones when developing with Android, is it worth while deleting the contents of a StringBuilder and reusing it?

StringBuilder b = new StringBuilder();
//build up a string
b.delete(0, b.length());
//reuse it

Compared to:

StringBuilder b1 = new StringBuilder();
//build up a string
StringBuilder b2 = new StringBuilder();
//create a new one

Answer 1

Reusing doesn't need memory allocation, so it could be faster as @Mike Nakis mentioned. On the other hand it leads to ugly code: you use the same variable for two (or more) different purposes which could confuse readers, make maintenance harder. Unless you have a good reason to do that avoid it. I mean, you have a good reason if a profiling shows that creating new StringBuilder instances is a bottleneck in the application.

If you really have to reuse StringBuilders, try accessing them with different names with a helper method:

public static StringBuilder reuseForBetterPerformance(final StringBuilder sb) {
    sb.delete(0, sb.length());
    return sb;
}

Client code:

final StringBuilder b = new StringBuilder();
//build up a string
final StringBuilder c = reuseForBetterPerformance(b);
//reuse it
c.append(...)

It makes the code a little bit more readable, since the method name says why you're reusing the StringBuilder.

Answer 2

To add some hard numbers, I wrote a simple test application:

public static void main(String[] args) {
    int iterations = 1000000;
    int secondIterations = 25;

    long start = System.currentTimeMillis();
    for (int count = 0; count < iterations; count++) {
        StringBuilder builder = new StringBuilder();
        for (int secondCounter = 0; secondCounter < secondIterations; secondCounter++) {
            builder.append("aaassssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss");
        }
    }
    System.out.println("Recreation took: " + Long.toString(System.currentTimeMillis() - start) + "ms");

    start = System.currentTimeMillis();
    StringBuilder builder = new StringBuilder();
    for (int count = 0; count < iterations; count++) {
        builder.delete(0, builder.length());
        for (int secondCounter = 0; secondCounter < secondIterations; secondCounter++) {
            builder.append("aaassssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss");
        }
    }
    System.out.println("Reuse took: " + Long.toString(System.currentTimeMillis() - start) + "ms");
}

This gives the following output:

Recreation took: 10594ms
Reuse took: 1937ms

So, yes, as it seems reusing is indeed faster, at least if you test it in a tight loop with a small memory footprint. But please keep in mind that those numbers were generated by using 1 million iterations.

Keep two rules in mind:

• First go for readability and maintainability.
• Second...run a profiler.

This difference I see here is nothing I'd worry about until I really run into performance problems.

Answer 3

Yes, of course it is worthwhile, even in the great big world outside of Android.

According to the documentation about StringBuilder:

Every string builder has a capacity. As long as the length of the character sequence contained in the string builder does not exceed the capacity, it is not necessary to allocate a new internal buffer. If the internal buffer overflows, it is automatically made larger.

This means that deletion of the StringBuilder's contents will result in no memory allocation operation for its internal array of chars (it will just set its length to zero and return, maintaining its last capacity). Plus, you save the StringBuilder object itself.