Why ‘+’ Can Beat StringBuilder in Java: Benchmarks and Best Practices

Normal Concatenation

Many developers believe that using

StringBuilder

is always faster than the

+

operator for concatenating strings. Since JDK 5, the Java compiler automatically rewrites

+

concatenations into

StringBuilder

calls, so the two approaches generate identical bytecode.

Benchmark: Simple Concatenation

A test class

StringTest

contains two methods—one using

+

and one using

StringBuilder

—each called 100,000 times in a JUnit test. The results are:

<code>testStringConcatenation01ByPlus，拼接字符串100000次，花费33秒
testStringConcatenation02ByStringBuilder，拼接字符串100000次，花费36秒
</code>

The difference is minimal; both methods take roughly the same time, confirming that for single concatenations the

+

operator is as efficient as

StringBuilder

.

Loop Concatenation

When concatenating inside a loop, using

+

repeatedly creates a new

StringBuilder

instance each iteration, leading to much higher overhead. A benchmark comparing looped

+

versus a single

StringBuilder

instance shows a dramatic performance gap.

<code>testLoopStringConcatenation03ByPlus，拼接字符串10000次，花费463秒
testLoopStringConcatenation04ByStringBuilder，拼接字符串10000次，花费13秒
</code>

The

StringBuilder

approach is orders of magnitude faster for looped concatenation.

Conclusion

For simple, non‑looped string concatenation, using the

+

operator is concise and performs as well as

StringBuilder

.

For concatenation inside loops, prefer

StringBuilder

to avoid excessive object creation and achieve better performance.