Java string concatenation vs string builder

gistfile1.java

package tostring;

public class Main {

	public static void main(String[] args) {
		int n = 1000, iterations = 10000;
		long len, t0, t1;
		
		// string builder: < 1 second
		len = 0;
		t0 = System.currentTimeMillis();
		for (int j = 0; j < iterations; j++) {
			StringBuilder builder = new StringBuilder();
			for (int i = 0; i < n; i++) {
				builder.append(i);
			}
			len += builder.toString().length();
		}
		t1 = System.currentTimeMillis();
		System.out.println(len + " " + (t1 - t0));

		// string concatenation: 10 seconds
		len = 0;
		t0 = System.currentTimeMillis();
		for (int j = 0; j < iterations; j++) {
			String res = "";
			for (int i = 0; i < n; i++) {
				res += i;
			}
			len += res.length();
		}
		t1 = System.currentTimeMillis();
		System.out.println(len + " " + (t1 - t0));
	}

}