输入“我ABC汉DEF”和字节数6，应该输出“我ABC”，而不是“我ABC+汉的半个”。

public class CutOutHanzi {    public static void main(String[] args) throws UnsupportedEncodingException {        String str = "我ABC汉DEF";        cutOutGBK(str,6);    }    public static String cutOutGBK(String str,int n) throws UnsupportedEncodingException{
   //n为要截取的字节数        byte[] buf = str.getBytes("GBK");        int num = 0;        boolean isHanziFirstHalf = false;        for (int i = 0; i < n; i++) {            if(buf[i]<0 && !isHanziFirstHalf)                isHanziFirstHalf = true;            else{                num++;                isHanziFirstHalf = false;            }        }        return str.substring(0,num);    }}