有没有更好的方法将字节数组转换为 int？

Question

The first 3 bytes of a byte array are just integers, is there a better way to convert them? So far I have this but it just feels like a bad way of doing it.

public int parse_code(byte[] bs) {
    char[] array = new char[3];
    for(int i = 0; i < 3; i++) {
        array[i] = (char) bs[i];
    }

    // Dirty way of doing it
    return Integer.parseInt(new String(array));
}

Answer 1

If you know that there will always be 3 decimal digits at the beginning of the byte array you can just convert them into a integer directly:

public int parse_code( byte[] bs )
{
    int intval = 0;
    for( int i = 0; i < 3; i++ )
        intval = intval * 10 + ( bs[ i ] - '0' );
    return intval;
}

Answer 2

Your example code seems to associate the byte s with their ASCII value. The proper way to do that would be to use the String constructor that takes the byte array and a character set:

// note: throws charset exception that will never be thrown on a valid JVM
//  as all JVMs must support US-ASCII
Integer.parseInt(new String(byteArray, "US-ASCII"));

---

Note : If the byte s were not ASCII character values representing integers (eg, it's just an int encoded as four byte s), then you would want to look at the ByteBuffer class. It has helpers that can convert from the ByteBuffer into other buffers (eg,IntBuffer ) to enable simple allow looping if it's all one type (as opposed to a mixed message, such as an incoming C-struct or something). It also has the added perk of enabling endianness to be changed.

int bytesValue = ByteBuffer.wrap(byteArray).getInt();

It's also worth noting, as x4u pointed out, that ByteBuffer does require the proper number of byte s for each value it gets. So, the above getInt() method will use the next 4 byte s and fail (with an exception) if there are [0, 3].

Answer 3

If you want to convert it in to int, the size of the array should be max 4 and the int not to exceed 214783647. Then:

int i = (bs[3] << 24) + (bs[2] << 16) + (bs[1] << 8) + bs[0];

Other things you have to know: which position in the byte array where it fits in the int (big-endian vs little-endian): (instead the previous expression) maybe you need:
int i = (bs[0] << 24) + (bs[1] << 16) + (bs[2] << 8) + bs[3];

if not all 4 bytes present you have to check the length.

Answer 4

It looks like you are trying to convert three byte of ASCII text like 123 as [49, 50, 51] to 123.

public static long parse_code(byte... bs) {
    long value = 0;
    for(byte b: bs)
       value = value * 10 + b - '0';
    return value;
}

if you know it will always be 3 bytes.

public static int parse_code(byte... bs) {
    return bs[0]*100 + bs[1]*10 + bs[2] - '0' * 111;
}