TestSB.java

import java.util.stream.IntStream;

public class TestSB {

  private static String a = "";
  private static String b = "";

  private static int iterations = 100000;
  private static long start;

  public static void main(String[] args) {

    start = System.nanoTime();

    IntStream.range(0, iterations).forEach(i -> a += i);

    System.out.println("concat took: " + (System.nanoTime() - start) + " ms.");

    start = System.nanoTime();

    StringBuilder sb = new StringBuilder();

    IntStream.range(0, iterations).forEach(i -> sb.append(i));

    String s = sb.toString();

    System.out.println("sb took: " + (System.nanoTime() - start) + " ms.");
  }
}

// concat took: 8298702665 ms.
// sb took: 7196031 ms.