String concatenation in loop

Reports String concatenation in loops.

As every String concatenation copies the whole string, usually it is preferable to replace it with explicit calls to StringBuilder.append() or StringBuffer.append().

Example:

  String str = "";
  for(int i=0; i<10; i++) {
    str += i;
  }

After the quick-fix is applied:

  String str = "";
  StringBuilder strBuilder = new StringBuilder(str);
  for(int i = 0; i<10; i++) {
    strBuilder.append(i);
  }
  str = strBuilder.toString();

Locating this inspection

By ID

Can be used to locate inspection in e.g. Qodana configuration files, where you can quickly enable or disable it, or adjust its settings.

StringContatenationInLoop

Via Settings dialog

Path to the inspection settings via IntelliJ Platform IDE Settings dialog, when you need to adjust inspection settings directly from your IDE.

Settings or Preferences | Editor | Inspections | Java | Performance

Sometimes, the quick-fixes allow you to convert a String variable to a StringBuilder or introduce a new StringBuilder. Be careful if the original code specially handles the null value, as the replacement may change semantics. If null is possible, null-safe fixes that generate necessary null-checks are suggested. Also, it's not guaranteed that the automatic replacement will always be more performant.

Suppressing Inspection

You can suppress this inspection by placing the following comment marker before the code fragment where you no longer want messages from this inspection to appear:

//noinspection StringContatenationInLoop

More detailed instructions as well as other ways and options that you have can be found in the product documentation:

IntelliJ IDEA 2025.2

Inspection Details
By default bundled with:	IntelliJ IDEA 2025.2, Qodana for JVM 2025.2,

Last modified: 18 September 2025