java stringBuilder vs stringConcat with JMH

java-string-benchmark.txt

# JMH version: 1.28
# VM version: JDK 11.0.10, OpenJDK 64-Bit Server VM, 11.0.10+9-Ubuntu-0ubuntu1.20.04
# VM invoker: /usr/lib/jvm/java-11-openjdk-amd64/bin/java
# VM options: <none>
# Blackhole mode: full + dont-inline hint
# Warmup: 3 iterations, 10 s each
# Measurement: 5 iterations, 10 s each
# Timeout: 10 min per iteration
# Threads: 1 thread, will synchronize iterations
# Benchmark mode: Average time, time/op
# Benchmark: org.sample.MyBenchmark.stringBuilder
# Parameters: (count = 100)

# Run progress: 0.00% complete, ETA 00:05:20
# Fork: 1 of 1
# Warmup Iteration   1: 0.403 ms/op
# Warmup Iteration   2: 0.987 ms/op
# Warmup Iteration   3: 1.331 ms/op
Iteration   1: 1.639 ms/op
Iteration   2: 1.882 ms/op
Iteration   3: 2.250 ms/op
Iteration   4: 2.111 ms/op
Iteration   5: 2.323 ms/op


Result "org.sample.MyBenchmark.stringBuilder":
  2.041 ±(99.9%) 1.079 ms/op [Average]
  (min, avg, max) = (1.639, 2.041, 2.323), stdev = 0.280
  CI (99.9%): [0.962, 3.120] (assumes normal distribution)


# JMH version: 1.28
# VM version: JDK 11.0.10, OpenJDK 64-Bit Server VM, 11.0.10+9-Ubuntu-0ubuntu1.20.04
# VM invoker: /usr/lib/jvm/java-11-openjdk-amd64/bin/java
# VM options: <none>
# Blackhole mode: full + dont-inline hint
# Warmup: 3 iterations, 10 s each
# Measurement: 5 iterations, 10 s each
# Timeout: 10 min per iteration
# Threads: 1 thread, will synchronize iterations
# Benchmark mode: Average time, time/op
# Benchmark: org.sample.MyBenchmark.stringBuilder
# Parameters: (count = 10000)

# Run progress: 25.00% complete, ETA 00:04:00
# Fork: 1 of 1
# Warmup Iteration   1: 6.367 ms/op
# Warmup Iteration   2: 17.183 ms/op
# Warmup Iteration   3: 22.627 ms/op
Iteration   1: 27.120 ms/op
Iteration   2: 29.092 ms/op
Iteration   3: 31.841 ms/op
Iteration   4: 34.816 ms/op
Iteration   5: 36.769 ms/op


Result "org.sample.MyBenchmark.stringBuilder":
  31.928 ±(99.9%) 15.274 ms/op [Average]
  (min, avg, max) = (27.120, 31.928, 36.769), stdev = 3.967
  CI (99.9%): [16.653, 47.202] (assumes normal distribution)


# JMH version: 1.28
# VM version: JDK 11.0.10, OpenJDK 64-Bit Server VM, 11.0.10+9-Ubuntu-0ubuntu1.20.04
# VM invoker: /usr/lib/jvm/java-11-openjdk-amd64/bin/java
# VM options: <none>
# Blackhole mode: full + dont-inline hint
# Warmup: 3 iterations, 10 s each
# Measurement: 5 iterations, 10 s each
# Timeout: 10 min per iteration
# Threads: 1 thread, will synchronize iterations
# Benchmark mode: Average time, time/op
# Benchmark: org.sample.MyBenchmark.stringConcat
# Parameters: (count = 100)

# Run progress: 50.00% complete, ETA 00:02:40
# Fork: 1 of 1
# Warmup Iteration   1: 4.259 ms/op
# Warmup Iteration   2: 8.805 ms/op
# Warmup Iteration   3: 11.861 ms/op
Iteration   1: 13.923 ms/op
Iteration   2: 16.093 ms/op
Iteration   3: 18.706 ms/op
Iteration   4: 21.460 ms/op
Iteration   5: 21.314 ms/op


Result "org.sample.MyBenchmark.stringConcat":
  18.299 ±(99.9%) 12.664 ms/op [Average]
  (min, avg, max) = (13.923, 18.299, 21.460), stdev = 3.289
  CI (99.9%): [5.636, 30.963] (assumes normal distribution)


# JMH version: 1.28
# VM version: JDK 11.0.10, OpenJDK 64-Bit Server VM, 11.0.10+9-Ubuntu-0ubuntu1.20.04
# VM invoker: /usr/lib/jvm/java-11-openjdk-amd64/bin/java
# VM options: <none>
# Blackhole mode: full + dont-inline hint
# Warmup: 3 iterations, 10 s each
# Measurement: 5 iterations, 10 s each
# Timeout: 10 min per iteration
# Threads: 1 thread, will synchronize iterations
# Benchmark mode: Average time, time/op
# Benchmark: org.sample.MyBenchmark.stringConcat
# Parameters: (count = 10000)

# Run progress: 75.00% complete, ETA 00:01:20
# Fork: 1 of 1
# Warmup Iteration   1: 1128.453 ms/op
# Warmup Iteration   2: 1100.954 ms/op
# Warmup Iteration   3: 1462.695 ms/op
Iteration   1: 1983.978 ms/op
Iteration   2: 2379.345 ms/op
Iteration   3: 2521.124 ms/op
Iteration   4: 2744.632 ms/op
Iteration   5: 2943.203 ms/op


Result "org.sample.MyBenchmark.stringConcat":
  2514.456 ±(99.9%) 1410.239 ms/op [Average]
  (min, avg, max) = (1983.978, 2514.456, 2943.203), stdev = 366.234
  CI (99.9%): [1104.218, 3924.695] (assumes normal distribution)


# Run complete. Total time: 00:05:32

REMEMBER: The numbers below are just data. To gain reusable insights, you need to follow up on
why the numbers are the way they are. Use profilers (see -prof, -lprof), design factorial
experiments, perform baseline and negative tests that provide experimental control, make sure
the benchmarking environment is safe on JVM/OS/HW level, ask for reviews from the domain experts.
Do not assume the numbers tell you what you want them to tell.

Benchmark                  (count)  Mode  Cnt     Score      Error  Units
MyBenchmark.stringBuilder      100  avgt    5     2.041 ±    1.079  ms/op
MyBenchmark.stringBuilder    10000  avgt    5    31.928 ±   15.274  ms/op
MyBenchmark.stringConcat       100  avgt    5    18.299 ±   12.664  ms/op
MyBenchmark.stringConcat     10000  avgt    5  2514.456 ± 1410.239  ms/op

MyBenchmark.java

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package org.sample;

import java.util.concurrent.TimeUnit;

import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.Fork;
import org.openjdk.jmh.annotations.Measurement;
import org.openjdk.jmh.annotations.Mode;
import org.openjdk.jmh.annotations.OutputTimeUnit;
import org.openjdk.jmh.annotations.Param;
import org.openjdk.jmh.annotations.Scope;
import org.openjdk.jmh.annotations.Setup;
import org.openjdk.jmh.annotations.State;
import org.openjdk.jmh.annotations.Warmup;
import org.openjdk.jmh.runner.Runner;
import org.openjdk.jmh.runner.RunnerException;
import org.openjdk.jmh.runner.options.Options;
import org.openjdk.jmh.runner.options.OptionsBuilder;

@BenchmarkMode({ Mode.AverageTime })
@State(Scope.Thread)
@Fork(1)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@Warmup(iterations = 3)
@Measurement(iterations = 5)
public class MyBenchmark {

    @Param({ "100", "10000" })
    private int count;
    private String s;
    private StringBuilder sb;

    @Setup
    public void setUp() {
        s = "";
        sb = new StringBuilder();
    }

    @Benchmark
    public String stringConcat() {
        for (int i = 0; i < count; i++) {
            s += i;
        }
        return s;
    }

    @Benchmark
    public String stringBuilder() {
        for (int i = 0; i < count; i++) {
            sb.append(i);

        }
        return sb.toString();
    }

    public static void main(String[] args) throws RunnerException {
        Options options = new OptionsBuilder().include(MyBenchmark.class.getSimpleName()).build();
        new Runner(options).run();
    }
}