不截半个汉字

统计出现的字符串
 public static void main(String[] args) {
        String str ="我 a 爱中华 abc 我爱小苏 def";
//        String str ="我 ABC 汉";
        int num = 0;
        try {
            num = trimGBK(str.getBytes("GBK"),5);
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
        System.out.println(str.substring(0,num));


    }

    public static int trimGBK(byte[] buf,int n){
        int num = 0;
        boolean bChineseFirstHalf = false;
        for(int i=0;i<n;i++)
        {
            if(buf[i]<0&& !bChineseFirstHalf){
                bChineseFirstHalf= true;
            }else{
                num++;
                bChineseFirstHalf= false;
            }
        }
        return num;
    }