1.三者在执行速度方面的比较：StringBuilder > StringBuffer > String

2.String <（StringBuffer，StringBuilder）的原因

String：字符串常量

StringBuffer：字符串变量

StringBuilder：字符串变量
3.StringBuilder与 StringBuffer

StringBuilder：线程非安全的

StringBuffer：线程安全的

当我们在字符串缓冲去被多个线程使用是，JVM不能保证StringBuilder的操作是安全的，虽然他的速度最快，但是可以保证StringBuffer是可以正确操作的。当然大多数情况下就是我们是在单线程下进行的操作，所以大多数情况下是建议用StringBuilder而不用StringBuffer的，就是速度的原因。

对于三者使用的总结： 1.如果要操作少量的数据用 = String

2.单线程操作字符串缓冲区 下操作大量数据 = StringBuilder

3.多线程操作字符串缓冲区 下操作大量数据 = StringBuffer

参考https://www.cnblogs.com/A_ming/archive/2010/04/13/1711395.html

关于速度
http://blog.csdn.net/mad1989/article/details/26389541

public class StringTest {

    public static String BASEINFO = "Mr.Y";
    public static final int COUNT = 2000000;

    /**
     * 执行一项String赋值测试
     */
    public static void doStringTest() {

        String str = new String(BASEINFO);
        long starttime = System.currentTimeMillis();
        for (int i = 0; i < COUNT / 100; i++) {
            str = str + "miss";
        }
        long endtime = System.currentTimeMillis();
        System.out.println((endtime - starttime)
                + " millis has costed when used String.");
    }

    /**
     * 执行一项StringBuffer赋值测试
     */
    public static void doStringBufferTest() {

        StringBuffer sb = new StringBuffer(BASEINFO);
        long starttime = System.currentTimeMillis();
        for (int i = 0; i < COUNT; i++) {
            sb = sb.append("miss");
        }
        long endtime = System.currentTimeMillis();
        System.out.println((endtime - starttime)
                + " millis has costed when used StringBuffer.");
    }

    /**
     * 执行一项StringBuilder赋值测试
     */
    public static void doStringBuilderTest() {

        StringBuilder sb = new StringBuilder(BASEINFO);
        long starttime = System.currentTimeMillis();
        for (int i = 0; i < COUNT; i++) {
            sb = sb.append("miss");
        }
        long endtime = System.currentTimeMillis();
        System.out.println((endtime - starttime)
                + " millis has costed when used StringBuilder.");
    }

    /**
     * 测试StringBuffer遍历赋值结果
     * 
     * @param mlist
     */
    public static void doStringBufferListTest(List<String> mlist) {
        StringBuffer sb = new StringBuffer();
        long starttime = System.currentTimeMillis();
        for (String string : mlist) {
            sb.append(string);
        }
        long endtime = System.currentTimeMillis();
        System.out.println(sb.toString() + "buffer cost:"
                + (endtime - starttime) + " millis");
    }

    /**
     * 测试StringBuilder迭代赋值结果
     * 
     * @param mlist
     */
    public static void doStringBuilderListTest(List<String> mlist) {
        StringBuilder sb = new StringBuilder();
        long starttime = System.currentTimeMillis();
        for (Iterator<String> iterator = mlist.iterator(); iterator.hasNext();) {
            sb.append(iterator.next());
        }

        long endtime = System.currentTimeMillis();
        System.out.println(sb.toString() + "builder cost:"
                + (endtime - starttime) + " millis");
    }

    public static void main(String[] args) {
        doStringTest();
        doStringBufferTest();
        doStringBuilderTest();

        List<String> list = new ArrayList<String>();
        list.add(" I ");
        list.add(" like ");
        list.add(" BeiJing ");
        list.add(" tian ");
        list.add(" an ");
        list.add(" men ");
        list.add(" . ");

        doStringBufferListTest(list);
        doStringBuilderListTest(list);
    }

}