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:

testStringConcatenation01ByPlus，拼接字符串100000次，花费33秒
testStringConcatenation02ByStringBuilder，拼接字符串100000次，花费36秒

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.

testLoopStringConcatenation03ByPlus，拼接字符串10000次，花费463秒
testLoopStringConcatenation04ByStringBuilder，拼接字符串10000次，花费13秒

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.