Why can't I put hex value to byte array in java?

My code looks like this:

public byte s[] = {
        0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76,
        0xCA, 0x82, 0xC9, 0x7D, 0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 0x72, 0xC0,
        0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC, 0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15,
        0x04, 0xC7, 0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2, 0xEB, 0x27, 0xB2, 0x75,
        0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84,
        0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF,
        0xD0, 0xEF, 0xAA, 0xFB, 0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C, 0x9F, 0xA8,
        0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5, 0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2,
        0xCD, 0x0C, 0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 0x64, 0x5D, 0x19, 0x73,
        0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB,
        0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3, 0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79,
        0xE7, 0xC8, 0x37, 0x6D, 0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 0xAE, 0x08,
        0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A,
        0x70, 0x3E, 0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9, 0x86, 0xC1, 0x1D, 0x9E,
        0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9, 0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF,
        0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16
    };

Netbeans tells me:

possible loss of precision - required byte, found int

What am I doing wrong? If I use a short instead of the int it works correctly.

Answers


Byte is a signed datatype in Java. It can take values from -128 (-0x80) to 127 (0x7f). Some of your constants won't fit in this range and are not, therefore, valid byte's. That's unlike C++, where BYTE is usually defined to be unsigned char. Also, Java is much stricter about datatype conversions with loss of precision.

So 0xF2 is not a valid byte literal. It's a valid short and int and long. Java employs the crazy notion of negative hex literals for values like those. A negative hex literal for 0xF2 would be -0x0e.

Depending on your situation, either use a larger datatype, or use inline conversion (like (byte)0xF2), or use something like Excel to convert your literals of value 0x80 and more to negative-hex representation.


Bytes in Java (as all primitive number types) are signed.

The valid range of byte values is -128 to 127.

Thus, there is no byte that has the same value as the integer 0xFF = 255, but you need a cast, which is what your compiler is warning you of. Note that 255 will actually become -1.

Try the following code yourself:

byte test = (byte) 0xFF;
System.out.println(test == 0xFFFFFFFF); // True: -1 == -1 (as int)
System.out.println((byte)0xFF == 0xFFFFFFFF); // True: -1 == -1 (as int)
System.out.println(0xFF == (byte) 0xFFFFFFFF); // False: +255 != -1 (as int)
System.out.println((byte)0xFF == (byte) 0xFFFFFFFF); // True: -1 == -1 (as byte)
byte warn = (byte) 0xFF; // "error: possible loss of precision" unless cast
byte nowarn = -0x1; // No error. -0x1 is a *valid* byte in Java!
System.out.println(warn == nowarn); // True: -1 == -1 (as bytes)
System.out.println((int)warn == (int)nowarn); // True: -1 == -1 (as int, too)

This should print true, true, false, true, true, true as indicated.

You must realize that == when applied to byte == int, converts the (signed) byte to a (signed) integer, and byte -1 should of course become int -1.


As an alternative, in Java 7 you write byte literals in binary notation. eg. 0b101 is 5.


Simply cast every byte:

byte a[]={(byte)0x01,(byte)0x01,(byte)0x01,}

Instead of taking 0XFF we could use 0X7F and follow up the code as below

byte[] ba = new byte[charArray.length*2];
int j = 0;
byte mask = (byte) 0x7f;
System.out.println("mask value is:" + mask);

for (int i = 0; i < charArray.length; ++i, j+=2) {
    byte upper8bits = (byte) ((byte)(charArray[i] >> (1<<3)) & mask);
    byte lower8bits = (byte) ((byte) charArray[i] & mask);
    ba[j] = upper8bits;
    ba[j+1] = lower8bits;
    System.out.println("byte[]:" + Integer.toHexString(ba[j+1]));
}

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