String concat in loop — bytecode and performance fix?

Concatenation in loops

Using + on String in a loop creates a new String each iteration—quadratic copying. The compiler may optimize single-expression concat with invokedynamic string concat (JDK 9+), but loops still need StringBuilder or joining collectors.

• Bytecode: repeated concat → StringBuilder in loop at compile time for simple cases, not always for complex loops.
• Prefer StringBuilder or String.join / Collectors.joining.
• Logging: pass parameters to SLF4J {} placeholders—no manual concat.
• Microbenchmark with JMH before "optimizing" small loops.

StringBuilder sb = new StringBuilder();
for (String part : parts) sb.append(part).append(',');
String csv = sb.substring(0, sb.length() - 1);

// Or: String joined = String.join(",", parts);