Non-constant 'String' can be replaced with 'StringBuilder'

Reports variables declared as java.lang.String that are repeatedly appended to. Such variables could be declared more efficiently as java.lang.StringBuffer or java.lang.StringBuilder.

Example:

  String s = "";
  for (int i = 0; i < names.length; i++) {
    String name = names[i] + (i == names.length - 1 ? "" : " ");
    s = s + name;
  }

Such a loop can be replaced with:

  StringBuilder s = new StringBuilder();
  for (int i = 0; i < names.length; i++) {
    String name = names[i] + (i == names.length - 1 ? "" : " ");
    s.append(name);
  }

Or even with:

String s = String.join(" ", names);

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.

NonConstantStringShouldBeStringBuffer

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

Use the option to make this inspection only report when the variable is appended to in a loop.

Inspection options

Here you can find the description of settings available for the Non-constant 'String' can be replaced with 'StringBuilder' inspection, and the reference of their default values.

Only warn when appending in a loop: Default value:
Selected

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 NonConstantStringShouldBeStringBuffer

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