StringBuffer与StringBuilder有什么区别

package String比较;
/*
 * StringBuffer与StringBuilder有什么区别
 * StringBuilder是JDK5增加的一个新类，功能几乎与StringBuffer相同。区别在于StringBuffer是线程安全的，
 * 多个线程同时操作StringBuffer时，能保证同步，而Stringbuilder是线程非安全的。如果只在单线程环境下使用，StringBuilder
 * 的效率会更高
 */

public class Test9 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        String strtemp = "watch_free";
        int time = 12000;
        @SuppressWarnings("unused")
        String str = "";
        long start1 = System.currentTimeMillis();
        for(int i= 0;i<time;i++){
            str+=strtemp;
        }
        long end1 = System.currentTimeMillis();
        System.out.println("String:"+(end1-start1)+"ms");//1218ms

        StringBuffer sbf = new StringBuffer();
        long start2 = System.currentTimeMillis();
        for(int i = 0;i<time;i++){
            sbf.append(strtemp);
        }
        long end2 = System.currentTimeMillis();
        System.out.println("StringBuffer:"+(end2-start2)+"ms");//2ms

         StringBuilder sbd = new StringBuilder();
         long start3 = System.currentTimeMillis();
         for(int i = 0;i<time;i++){
             sbd.append(strtemp);
         }
         long end3 = System.currentTimeMillis();
         System.out.println("StringBuilder:"+(end3-start3)+"ms");//1ms
    }

}