截取中文字符串（面试题）

编写一个截取字符串的函数，输入为一个字符串和字节数，输出为按字节截取的字符串，但要保证汉字不被截取半个，如“我ABC”，4，应该截取“我AB”，输入“我ABC汉DEF”，6，应该输出“我ABC”，而不是“我ABC+汉的半个”。

首先要了解中文字符有多种编码及各种编码的特征。假设n为要截取的字节数。

public class DismantleORsplit
{
    public static void main(String[] args) throws Exception
    {
        String str = "我ABC汉def";;
        byte bytes[] = str.getBytes("GBK");
        byte bytes2[] = str.getBytes("UTF-8");
        for (int i=0; i<bytes.length; i++)
        {
            System.out.print(bytes[i] + " ");
        }
        int num = trimGBK(bytes, 3);
        int num2 = trimUTF(bytes2,3);
        System.out.println(num);
        
        System.out.println(str.substring(0, num));
        System.out.println(str.substring(0, num2));
    }

    private static int trimUTF(byte[] buf, int n)
    {
        int index = 0;
        int cPart = 0;
        for (int i=0; i<n; i++)
        {
            if (buf[i]<0)
            {    
                cPart++;
                if (cPart % 3 == 0)
                {
                    index++;
                }
            }
            else
            {
                index++;
            }
        }
        return index;
    }

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

}