Java int 與 byte的轉換 & 0xFF

Java int 與 byte的轉換 & 0xFF

int -> byte

採用強制類型轉換 byte 類型的取值範圍是 -128~127。當把int轉換成byte時，超出這個範圍，值就不會相等。

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

經過InputStream的read()方法獲取的int，可採用強制類型轉換將值還原爲byte類型，

/**
 * 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;

注意這個int 值的範圍是 0～255，轉換成byte，不會超出byte的位的長度。

byte -> int

這裏有兩種狀況，一種是要求保持值不變，例如進行數值計算，可採用強制類型轉換：int i = (int) aByte; 另外一種是要求保持最低字節中各個位不變，3個高字節所有用0填充，例如進行編解碼操做， 則須要採用位操做：int i = b & 0xff;

經過ByteArrayInputStream的read()方法，經過位操做將byte轉換爲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);
    }
}

爲何要和 0xFF 進行與操做

以這個爲例，

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類型在java中用8個字節32位來表示，byte類型爲一個字節8位。咱們要作的就是將32位分四次，每次獲取8位寫入到字節數組中。首先是低8位，和0xFF相與後，高24比特就會被置爲0了，寫入到字節數組中。而後獲取後邊的8位，首先要移位，無符號右移8位，和0xFF相與後，保留8位，把這八位寫入到數組中。後邊依次類推。最終完整的把int寫入到字節數組中。

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