算法之截取带汉字的字符串

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);
    }
}