请教一段代码的解释：java 算文件的MD5值是怎么用十六进制表示的？

Question

[code="java"]
import java.io.*;
import java.security.*;

public class HashFile {

/**
 * @param args
 */
public static char[] hexChar = { '0', '1', '2', '3', '4', '5', '6', '7',
        '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };

public static void main(String[] args) {
    String fileName = "F:\\soft\\Firefox.exe";
    String hashType = "MD5";
    try {
        System.out.println(hashType + " == " + getHash(fileName, hashType));

    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

public static String getHash(String fileName, String hashType)
        throws Exception {
    InputStream fis;
    fis = new FileInputStream(fileName);
    byte[] buffer = new byte[1024];
    MessageDigest md5 = MessageDigest.getInstance(hashType);
    int numRead = 0;
    while ((numRead = fis.read(buffer)) > 0) {
        md5.update(buffer, 0, numRead);
    }
    fis.close();
    return toHexString(md5.digest());
}

public static String toHexString(byte[] b) {
    StringBuilder sb = new StringBuilder(b.length * 2);
    for (int i = 0; i < b.length; i++) {
        sb.append(hexChar[(b[i] & 0xf0) >>> 4]);
        sb.append(hexChar[b[i] & 0x0f]);
    }
    return sb.toString();
}

}[/code]
请高手解释以下toHexString（byte[] b）方法，主要是这两句：
[code="java"]sb.append(hexChar[(b[i] & 0xf0) >>> 4]);
sb.append(hexChar[b[i] & 0x0f]);[/code]
hexChar[(b[i] & 0xf0>>> 4，hexChar[b[i] & 0x0f]这到底是什么意识？
谢谢！
[b]问题补充：[/b]
谢谢楼上的！大致明白了，这里应该是拿字节byte和十六进制的做and的比较！但这里为什么要用0xf0呢？

Answer 1

&这个是位运算“且”

sb.append(hexChar[(b[i] & 0xf0) >>> 4]);
//(b[i] & 0xf0)这个意思就是取b[i]的后4位
“>>> 无符号右移”

sb.append(hexChar[b[i] & 0x0f]);
//这个意思就是取b[i]的前4位