StringBuffer问题

Java code

public static void main(String[] args)
    {
        String a = "{\"deviceList\":[{\"deviceAliasName\":\"QQ\",\"deviceID\":\"lifanghua11\",\"deviceType\":\"1\",\"terminalType\":\"terminaltype\"}],\"returnCode\":\"0\",\"returnDesc\":\"Success\"}";
        StringBuffer buffer = new StringBuffer(a);

        buffer.insert(a.indexOf("["), "@");
        buffer.insert(a.lastIndexOf("]")
            + NumContants.TWO.getValue(), "@");
        System.out.println(a.toString());
    }

为什么输出地结果是:{"deviceList":[{"deviceAliasName":"QQ","deviceID":"lifanghua11","deviceType":"1","terminalType":"terminaltype"}],"returnCode":"0","returnDesc":"Success"}
@没有加上去.新人,求指点

------解决方案--------------------
System.out.println(buffer);
------解决方案--------------------
StringBuffer对象已经改变了，但是String a是没有改变的，呵呵。
楼主小心了。
------解决方案--------------------
楼主粗心了，你最后输出的是a啊，不是buffer，要检查检查……
------解决方案--------------------
System.out.println(buffer.toString());
------解决方案--------------------
String a = "{\"deviceList\":[{\"deviceAliasName\":\"QQ\",\"deviceID\":\"lifanghua11\",\"deviceType\":\"1\",\"terminalType\":\"terminaltype\"}],\"returnCode\":\"0\",\"returnDesc\":\"Success\"}";
StringBuffer buffer = new StringBuffer(a);

buffer.insert(a.indexOf("["), "@");
buffer.insert(a.lastIndexOf("]")
+ NumContants.TWO.getValue(), "@");
System.out.println(a.toString());
代码没有问题，输出的不是自己的结果。。。