Java Face test---intercept strings by byte __java

Write a function that intercepts the string, enter it as a string and a number of bytes, the output is a byte-truncated string, but to ensure that the Chinese character is not intercepted in half, such as "I ABC", 4, should intercept "I ab", enter "my ABC-def", 6, should output "my ABC" instead of "I abc+ Half of the Han. "

public static void Main (string[] args) throws exception{
		//string str = "Oh ah ah ah ah ah afasfd";
		String str = "I abc Han";
		int num = TRIMGBK (str.getbytes ("GBK"), 5);
		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;
	}