String VS StringBuffer

public class StringTest {

  static void bed(int t) {
    String str = "";
    for (int i = 0; i < t; i++) {
      str += "a";
    }
  }

  static void good(int t) {
    StringBuffer sb = new StringBuffer();
    for (int i = 0; i < t; i++) {
      sb.append("a");
    }
  }

  public static void main(String[] args) {
    int t = 50000;
    System.out.println("****** Bed Testting ******");
    long startTime = System.currentTimeMillis();
    long startMem = Runtime.getRuntime().freeMemory();

    bed(t);  // good , bed

    long endMem = Runtime.getRuntime().freeMemory();
    long endTime = System.currentTimeMillis();

    System.out.println("Use memory: " + (startMem - endMem));
    System.out.println("Use Time: " + (endTime - startTime));
  }

 * 2007-11-30 : OS Vista, JDK1.6, CPU intel T7300, RAM DDRII 2G
 * 
 * 10000　次
 * String : memory = 312800 bit, time = 192 ms
 * StringBuffer : memory = 69152 bit, time = 2 ms
 * 
 * 50000　次
 * String : memory = 3288344 bit, time = 6812 ms
 * StringBuffer : memory = 290352 bit, time = 6 ms 
 * *****************************************************
 * 
 * 2003-03-03 : OS winxp, JDK1.4, CPU intel P-M 1.5G, RAM DDR 1G
 * 
 * 10000　次
 * String : memory = 153456 bit, time = 811 ms
 * StringBuffer : memory = 74128 bit, time = 20 ms
 * 
 * 50000　次
 * String : memory = 1172744 bit, time = 49180 ms
 * StringBuffer : memory = 295328 bit, time = 50 ms