从java中的多字节变量中获取单个字节

Question

How can I split a variable into single bytes in java? I have for example following snippet in C++:

    unsigned long someVar;
    byte *p = (byte*)(void*) someVar; // byte being typedef unsigned char (from 0 to 255)

    byte *bytes = new byte[sizeof(someVar)];

    for(byte i = 0;i<sizeof(someVar);i++)
    {
        bytes[i] = *p++;
    }

    .... //do something with bytes

I want to accomplish the same under java, but I can't seem to find an obvious workaround.

Answer 1

There are two ways to do it with the ByteBuffer class. One is to create a new byte array dynamically.

long   value = 123;
byte[] bytes = ByteBuffer.allocate(8).putLong(value).array();

Another is to write to an existing array.

long   value = 123;
byte[] bytes = new byte[8];

ByteBuffer.wrap(bytes).putLong(value);

// bytes now contains the byte representation of 123.

Answer 2

If you use Guava, there is a convenience Longs.toByteArray . It is simply a wrapper for John's ByteBuffer answer above, but if you already use Guava, it's slightly "nicer" to read.