java/Android获取单个文件的MD5值，解决首位0被省略问题，解决超大文件问题，cmd命令行查看文件md5码

code来源：https://betheme.net/news/txtlist_i67135v.html?action=onClick
cmd命令行查看文件md5码：certutil -hashfile a.txt md5，不加后面的md5，查看的默认是sha1码。

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package com.tools.util;   import java.io.File; import java.io.FileInputStream; import java.nio.MappedByteBuffer; import java.nio.channels.FileChannel; import java.security.MessageDigest;   public class MD5Util {     private static MappedByteBuffer[] mappedByteBuffers;     private static int bufferCount;       /**      * 获取单个文件的MD5值！      *      * @param file      * @return 解决首位0被省略问题      * 解决超大文件问题      */     public static String getFileMD5(File file) {           StringBuffer stringbuffer = null;         try {             char[] hexDigits = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};             FileInputStream in = new FileInputStream(file);             FileChannel ch = in.getChannel();               long fileSize = ch.size();             bufferCount = (int) Math.ceil((double) fileSize / (double) Integer.MAX_VALUE);             mappedByteBuffers = new MappedByteBuffer[bufferCount];               long preLength = 0;             long regionSize = Integer.MAX_VALUE;             for (int i = 0; i < bufferCount; i++) {                 if (fileSize - preLength < Integer.MAX_VALUE) {                     regionSize = fileSize - preLength;                 }                 mappedByteBuffers[i] = ch.map(FileChannel.MapMode.READ_ONLY, preLength, regionSize);                 preLength += regionSize;             }               MessageDigest messagedigest = MessageDigest.getInstance("MD5");               for (int i = 0; i < bufferCount; i++) {                 messagedigest.update(mappedByteBuffers[i]);             }             byte[] bytes = messagedigest.digest();             int n = bytes.length;             stringbuffer = new StringBuffer(2 * n);             for (int l = 0; l < n; l++) {                 byte bt = bytes[l];                 char c0 = hexDigits[(bt & 0xf0) >> 4];                 char c1 = hexDigits[bt & 0xf];                 stringbuffer.append(c0);                 stringbuffer.append(c1);             }         } catch (Exception e) {             e.printStackTrace();         }         return stringbuffer.toString();       } }