Java int 与 byte的转换 & 0xFF

Java int 与 byte的转换 & 0xFF

int -> byte

采用强制类型转换 byte 类型的取值范围是 -128~127。当把int转换成byte时，超出这个范围，值就不会相等。

int ii = 128;
byte bb = (byte) ii;
System.out.println(ii == bb); //false

int iii = 127;
byte bbb = (byte) iii;
System.out.println(iii == bbb);//true

经过InputStream的read()方法获取的int，可采用强制类型转换将值还原为byte类型，

/**
 * Reads the next byte of data from the input stream. The value byte is
 * returned as an <code>int</code> in the range <code>0</code> to
 * <code>255</code>. If no byte is available because the end of the stream
 * has been reached, the value <code>-1</code> is returned. This method
 * blocks until input data is available, the end of the stream is detected,
 * or an exception is thrown.
 *
 * <p> A subclass must provide an implementation of this method.
 *
 * @return     the next byte of data, or <code>-1</code> if the end of the
 *             stream is reached.
 * @exception IOException  if an I/O error occurs.
 */
public abstract int read() throws IOException;

注意这个int 值的范围是 0～255，转换成byte，不会超出byte的位的长度。

byte -> int

这里有两种状况，一种是要求保持值不变，例如进行数值计算，可采用强制类型转换：int i = (int) aByte; 另外一种是要求保持最低字节中各个位不变，3个高字节所有用0填充，例如进行编解码操做， 则须要采用位操做：int i = b & 0xff;

经过ByteArrayInputStream的read()方法，经过位操做将byte转换为int，

/**
 * Reads the next byte of data from this input stream. The value
 * byte is returned as an <code>int</code> in the range
 * <code>0</code> to <code>255</code>. If no byte is available
 * because the end of the stream has been reached, the value
 * <code>-1</code> is returned.
 * <p>
 * This <code>read</code> method
 * cannot block.
 *
 * @return  the next byte of data, or <code>-1</code> if the end of the
 *          stream has been reached.
 */
public synchronized int read() {
    return (pos < count) ? (buf[pos++] & 0xff) : -1;
}

int <-> byte[]

public class Transfer {

    public static byte[] toByteArray(int iSource, int iArrayLen) {
        byte[] bLocalArr = new byte[iArrayLen];
        for (int i = 0; (i < 4) && (i < iArrayLen); i++) {
            bLocalArr[i] = (byte) (0xFF & (iSource >>> (i << 3)));
        }
        return bLocalArr;
    }

    public static int toInt(byte[] bRefArr) {
        int iOutcome = 0;
        byte bLoop;

        for (int i = 0; i < bRefArr.length; i++) {
            bLoop = bRefArr[i];
            iOutcome += (bLoop & 0xFF) << (i << 3);
        }
        return iOutcome;
    }

    public static void main(String args[]) {
        int a = 100;
        byte[] arr = toByteArray(a, 4);
        int b = toInt(arr);
        System.out.println(b);
    }
}

为何要和 0xFF 进行与操做

以这个为例，

public static byte[] toByteArray(int iSource, int iArrayLen) {
    byte[] bLocalArr = new byte[iArrayLen];
    for (int i = 0; (i < 4) && (i < iArrayLen); i++) {
        bLocalArr[i] = (byte) (0xFF & (iSource >>> (i << 3)));
    }
    return bLocalArr;
}

int类型在java中用8个字节32位来表示，byte类型为一个字节8位。咱们要作的就是将32位分四次，每次获取8位写入到字节数组中。首先是低8位，和0xFF相与后，高24比特就会被置为0了，写入到字节数组中。而后获取后边的8位，首先要移位，无符号右移8位，和0xFF相与后，保留8位，把这八位写入到数组中。后边依次类推。最终完整的把int写入到字节数组中。

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