Question

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Convert 4 bytes to int

I’m trying to pack 4 bytes into an int using some of the solutions found here, but it doesn’t seem to work for one of my tests.

This is the code I’m using:

public static int pack(int c1, int c2, int c3, int c4)
{
    return (c1 << 24) | (c2 << 16) | (c3 << 8) | (c4);
}

Now when I use it on something simple like 0x34, 0x68, 0x77, and 0x23 I get what I expect: 0x34687723. But when I use it on 0xBA, 0xAD, 0xBE, and 0xEF I get something way off. Does anyone see what the problem might be?

EDIT

The above code was able to give me what I wanted, and the “wrong value” I mention below is just another way of representing 0xBAADBEEF in a decimal form.

Answer 1

A number stored in a Java int can only represent positive values up to 0x7fffffff (2147483647, Integer.MAX_VALUE), as it’s a signed type (like all Java number types), and in the internal representation the most significant bit is used as a sign bit.

To hold positive numeric values larger than that you need to use a long instead:

public static long pack(int c1, int c2, int c3, int c4)
{
        return ((c1 << 24) | (c2 << 16) | (c3 << 8) | (c4)) & 0xffffffffL;
}

Note the explicit long mask operation at the end which is necessary to ensure that sign extension doesn’t cause a negative integer result from being turned into a negative long.