Benchmark string inserting using concatenation vs StringBuilder

BenchmarkInsertUsingConcatVsStringBuilder.java

package benchmark;

import java.util.Random;
import java.util.UUID;
import java.util.concurrent.ThreadLocalRandom;
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.Scope;
import org.openjdk.jmh.annotations.State;
import org.openjdk.jmh.annotations.Threads;
import org.openjdk.jmh.annotations.Warmup;

@Fork(1)
@Threads(1)
@State(Scope.Benchmark)
@BenchmarkMode(Mode.Throughput)
@Warmup(iterations = 5, time = 1)
@Measurement(iterations = 5, time = 3)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
public class BenchmarkInsertUsingConcatVsStringBuilder {
	
	public static String insertUsingConcatenation(String str, String insert) {
		final int position = 10;
	    return str.substring(0, position) + insert + str.substring(position);
	}
	
	public static String insertUsingStringBuilder(String str, String insert) {
		final int position = 10;
	    StringBuilder builder = new StringBuilder(str);
	    builder.insert(position, insert);
	    return builder.toString();
	}

	private static final String string = "00000000000000000000000000"; // ZERO ULID
	
	@Benchmark
	public String insert_using_concatenation() {
		return insertUsingConcatenation(string, "-AAAAAAA-");
	}

	@Benchmark
	public String insert_using_string_builder() {
		return insertUsingStringBuilder(string, "-AAAAAAA-");
	}
}

BENCHMARK RESULT:

    Benchmark                                Mode  Cnt      Score      Error   Units
    Throughput.insert_using_concatenation   thrpt    5  13206,744 ± 1158,203  ops/ms
    Throughput.insert_using_string_builder  thrpt    5  38780,207 ± 2134,693  ops/ms (2.9x)

Conclusion: the method that uses string builder is almost 3x faster than the method using concatenation.