Remove last character of a StringBuilder?

user_310991 0
Viewed 428319

When you have to loop through a collection and make a string of each data separated by a delimiter, you always end up with an extra delimiter at the end, e.g.

for (String serverId : serverIds) {
  sb.append(serverId);
   sb.append(",");
}

Gives something like : serverId_1, serverId_2, serverId_3,

I would like to delete the last character in the StringBuilder (without converting it because I still need it after this loop).

stringbuilder java
17 Answers

In this case,

sb.setLength(sb.length() - 1);

is preferable as it just assign the last value to '\0' whereas deleting last character does System.arraycopy

user_12497510

stringBuilder.Remove(stringBuilder.Length - 1, 1);

user_92514630

I found myself doing this quite a bit so I wrote a benchmark for the 3 main append delimiter techniques:

(benchmark with proper warmup and 100 rounds of 100,000 iterations)

"Append After"

static void appendAfter()
{
    sb.append('{');
    for (int i = 0; i < 10; i++)
    {
        sb.append('"');
        sb.append(i);
        sb.append('"');
        sb.append(':');
        sb.append(i);
        sb.append(',');
    }
    sb.setLength(sb.length() - 1);
    sb.append('}');
}

"Append Before"

static void appendBefore()
{
    sb.append('{');
    String delimiter = "";
    for (int i = 0; i < 10; i++)
    {
        sb.append(delimiter);
        sb.append('"');
        sb.append(i);
        sb.append('"');
        sb.append(':');
        sb.append(i);
        delimiter = ",";
    }
    sb.append('}');
}

"Append Maybe"

static void appendMaybe()
{
    sb.append('{');
    for (int i = 0; i < 10; i++)
    {
        sb.append('"');
        sb.append(i);
        sb.append('"');
        sb.append(':');
        sb.append(i);
        if (i < 9)
        {
            sb.append(',');
        }
    }
    sb.append('}');
}

I got the following results:

Platform Append After Append Before Append Maybe
Windows Server 2016, Java 11 - Hotspot 26ms 40ms 26ms
Windows Server 2016, Java 8 - Hotspot 27ms 36ms 21ms
Windows Server 2016, Java 11 - OpenJ9 63ms 81ms 59ms
Windows Server 2016, Java 8 - OpenJ9 66ms 64ms 55ms

Aside from being the fastest, I am of the opinion that the "Append Maybe" implementation shows the intent of the code the best. That is usually more important than the fraction of nanoseconds gained per iteration.

I left the benchmark code here in case anyone wanted to try it on their platform. Please contribute your results above if you do so!

user_10629920
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