String join benchmark ( http://stackoverflow.com/q/33022822/4856258 )

JoinTest.java

import java.util.concurrent.TimeUnit;
import java.util.stream.*;
import java.util.*;

import org.openjdk.jmh.infra.Blackhole;
import org.openjdk.jmh.annotations.*;
 
@Warmup(iterations = 5, time = 500, timeUnit = TimeUnit.MILLISECONDS)
@Measurement(iterations = 10, time = 500, timeUnit = TimeUnit.MILLISECONDS)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.MICROSECONDS)
@Fork(3)
@State(Scope.Benchmark)
public class JoinTest {
  @Param({"1", "2", "10", "100", "1000"})
  private int n;

  List<String> input;

  @Setup
  public void setUp() {
    input = Stream.generate(() -> "foo").limit(n).collect(Collectors.toList());
  }

  @Benchmark
  public String stringJoin() {
    return String.join(",", input);
  }

  @Benchmark
  public String sbFirst() {
    StringBuilder sb = new StringBuilder();
    
    Iterator<String> iterator = input.iterator();
    
    // First time (no delimiter):
    if (iterator.hasNext()) {
        sb.append(iterator.next());
    }
    
    // Other times (with delimiter):
    while (iterator.hasNext()) {
        sb.append(",");
        sb.append(iterator.next());
    }
    return sb.toString();
  }

  @Benchmark
  public String sbFlag() {
    StringBuilder sb = new StringBuilder();
    
    boolean firstTime = true;
    
    for (String str : input) {
    
        if (firstTime) {
            firstTime = false;
        } else {
            sb.append(",");
        }
    
        sb.append(str);
    }  
    return sb.toString();
  }

  @Benchmark
  public String sbSetLength() {
    StringBuilder sb = new StringBuilder();
    
    for (String str : input) {
        sb.append(str).append(",");
    }  
    if(sb.length()>0) sb.setLength(sb.length()-1);
    return sb.toString();
  }
}

result.txt

# JMH 1.11 (released 33 days ago)
# VM version: JDK 1.8.0_60, VM 25.60-b23
# VM invoker: C:\Program Files\Java\jre1.8.0_60\bin\java.exe
...
Benchmark              (n)  Mode  Cnt   Score    Error  Units
JoinTest.sbFirst         1  avgt   30   0.029 ±  0.001  us/op
JoinTest.sbFirst         2  avgt   30   0.052 ±  0.001  us/op
JoinTest.sbFirst        10  avgt   30   0.262 ±  0.002  us/op
JoinTest.sbFirst       100  avgt   30   2.314 ±  0.011  us/op
JoinTest.sbFirst      1000  avgt   30  23.710 ±  0.516  us/op
JoinTest.sbFlag          1  avgt   30   0.033 ±  0.001  us/op
JoinTest.sbFlag          2  avgt   30   0.053 ±  0.001  us/op
JoinTest.sbFlag         10  avgt   30   0.249 ±  0.002  us/op
JoinTest.sbFlag        100  avgt   30   2.177 ±  0.015  us/op
JoinTest.sbFlag       1000  avgt   30  21.173 ±  0.109  us/op
JoinTest.sbSetLength     1  avgt   30   0.046 ±  0.002  us/op
JoinTest.sbSetLength     2  avgt   30   0.063 ±  0.001  us/op
JoinTest.sbSetLength    10  avgt   30   0.275 ±  0.001  us/op
JoinTest.sbSetLength   100  avgt   30   2.484 ±  0.016  us/op
JoinTest.sbSetLength  1000  avgt   30  23.260 ±  0.083  us/op
JoinTest.stringJoin      1  avgt   30   0.049 ±  0.002  us/op
JoinTest.stringJoin      2  avgt   30   0.077 ±  0.001  us/op
JoinTest.stringJoin     10  avgt   30   0.314 ±  0.002  us/op
JoinTest.stringJoin    100  avgt   30   2.720 ±  0.026  us/op
JoinTest.stringJoin   1000  avgt   30  25.546 ±  0.163  us/op