Checking performance of a code in Java

Below Example compares the speed of String and StringBuilder Concat Operation

public class PerformanceDemo {
	public static void main(String[] args) {
		long now = System.currentTimeMillis();
		slow();
		System.out.println("slow elapsed " + (System.currentTimeMillis() - now) + " ms");

		now = System.currentTimeMillis();
		fast();
		System.out.println("fast elapsed " + (System.currentTimeMillis() - now) + " ms");
	}

	private static void fast() {
		StringBuilder s = new StringBuilder();
		for (int i = 0; i < 100000; i++)
			s.append("*");
	}

	private static void slow() {
		String s = "";
		for (int i = 0; i < 100000; i++)
			s += "*";
	}
}