Why is this an issue?

Strings are immutable objects, so concatenation doesn’t simply add the new String to the end of the existing string. Instead, in each loop iteration, the first String is converted to an intermediate object type, the second string is appended, and then the intermediate object is converted back to a String. Further, performance of these intermediate operations degrades as the String gets longer. Therefore, the use of StringBuilder is preferred.

Noncompliant code example

String str = "";
for (int i = 0; i < arrayOfStrings.length ; ++i) {
  str = str + arrayOfStrings[i];
}

Compliant solution

StringBuilder bld = new StringBuilder();
  for (int i = 0; i < arrayOfStrings.length; ++i) {
    bld.append(arrayOfStrings[i]);
  }
  String str = bld.toString();

Resources

Benchmarks

Method	size	Runtime	Average time	Error margin
plus	100	Temurin 21	4.19 µs/op	±0.34 µs/op
plus	1000	Temurin 21	377.08 µs/op	±17.36 µs/op
plus	10000	Temurin 21	40221.49 µs/op	±1342.76 µs/op
plus	100000	Temurin 21	5286840.53 µs/op	±185796.75 µs/op
stringBuilder	100	Temurin 21	0.97 µs/op	±0.03 µs/op
stringBuilder	1000	Temurin 21	10.25 µs/op	±1.64 µs/op
stringBuilder	10000	Temurin 21	93.27 µs/op	±16.05 µs/op
stringBuilder	100000	Temurin 21	1019.91 µs/op	±69.58 µs/op

Benchmarking code

The results were generated by running the following snippet with JMH.

@Param({"100", "1000", "10000", "100000"})
int size;
private String word = "append";

@Benchmark
public String plus() {
  String str = "";
  for (int i = 0; i < size; i++) {
    str = str + word;
  }
  return str;
}

@Benchmark
public String stringBuilder() {
  StringBuilder builder = new StringBuilder();
  for (int i = 0; i < size; i++) {
    builder.append(word);
  }
  return builder.toString();
}