Md5加密错误问题

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;

import cn.mpaike.interfaces.util.file.DigestUtil;

public class MD5 {

	private MD5() {}
	
	public final static String getMessageDigest(byte[] buffer) {
		char hexDigits[] = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "a", "b", "c", "d", "e", "f" };
		try {
			MessageDigest mdTemp = MessageDigest.getInstance("MD5");
			mdTemp.update(buffer);
			byte[] md = mdTemp.digest();
			int j = md.length;
			char str[] = new char[j * 2];
			int k = 0;
			for (int i = 0; i < j; i++) {
				byte byte0 = md[i];
				str[k++] = hexDigits[byte0 >>> 4 & 0xf];
				str[k++] = hexDigits[byte0 & 0xf];
			}
			return new String(str);
		} catch (Exception e) {
			return null;
		}
	}
	
	public static void main(String[] args) {
		String key = "8798798da测试中文加密";
		
		System.out.println("待加密字符串：" + key);
		String signB = MD5.getMessageDigest(key.getBytes()).toUpperCase();
		
		System.out.println("加密成果：" + sign);
	}
}

import java.io.UnsupportedEncodingException;
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