Why is this an issue?

It is inefficient to build a String from a StringBuilder or StringBuffer just to check if it’s empty. Instead, directly use the .isEmpty method.

Noncompliant code example

StringBuilder sb = new StringBuilder();
// ...
if ("".equals(sb.toString()) { // Noncompliant
  // ...
}
if (sb.toString().isEmpty()) { // Noncompliant
  // ...
}

Compliant solution

StringBuilder sb = new StringBuilder();
// ...
if (sb.isEmpty()) {
  // ...
}
if (sb.isEmpty()) {
  // ...
}

Benchmarks

Method	stringBuilderSize	Runtime	Average time	Error margin
isEmpty	10	Temurin 21	6.57 ns/op	±0.38 ns/op
isEmpty	100	Temurin 21	6.68 ns/op	±0.10 ns/op
isEmpty	1000	Temurin 21	6.80 ns/op	±0.12 ns/op
length	10	Temurin 21	6.83 ns/op	±0.12 ns/op
length	100	Temurin 21	6.66 ns/op	±0.11 ns/op
length	1000	Temurin 21	6.67 ns/op	±0.07 ns/op
toStringEquals	10	Temurin 21	13.92 ns/op	±0.18 ns/op
toStringEquals	100	Temurin 21	59.09 ns/op	±0.53 ns/op
toStringEquals	1000	Temurin 21	465.79 ns/op	±5.86 ns/op
toStringIsEmpty	10	Temurin 21	13.83 ns/op	±0.23 ns/op
toStringIsEmpty	100	Temurin 21	60.06 ns/op	±3.42 ns/op
toStringIsEmpty	1000	Temurin 21	484.58 ns/op	±4.24 ns/op

Benchmarking code

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

@Param({"10", "100", "1000"})
int stringBuilderSize;


private StringBuilder sb;

@Setup(Level.Iteration)
public void setup() {
  sb = new StringBuilder();
  IntStream.range(0, stringBuilderSize).forEach(i -> sb.append("word"));
}

@Benchmark
public StringBuilder toStringEquals() {
  if ("".equals((sb.toString()))) {
    return sb;
  }
  return new StringBuilder();
}

@Benchmark
public StringBuilder toStringIsEmpty() {
  if (sb.toString().isEmpty()) {
    return sb;
  }
  return new StringBuilder();
}

@Benchmark
public StringBuilder length() {
  if (sb.length() == 0) {
    return sb;
  }
  return new StringBuilder();
}

@Benchmark
public StringBuilder isEmpty() {
  if (sb.isEmpty()) {
    return sb;
  }
  return new StringBuilder();
}