Java: Syntax and meaning behind "[B@1ef9157"? Binary/Address?

Question

Hey, I'm trying to figure out what the [B@ prefix means in java. They come out when I attempt to print byte arrays. However, byte arrays of size 32 and size 4 are identical in length. Always "[@B1234567".

What is this? Also, they have the property of only printing hex values. I know it can't just be a binary print because random extended ascii chars would appear.

Here is an example of a byte[] to byte[] hashtable mapping print, where mappings are separated by a colon, and these are byte arrays of 4-byte keys and 32-byte elements.

[B@1ef9157:[B@1f82982
[B@181ed9e:[B@16d2633
[B@27e353:[B@e70e30
[B@cb6009:[B@154864a
[B@18aaa1e:[B@3c9217
[B@20be79:[B@9b42e6
[B@16925b0:[B@14520eb
[B@8ee016:[B@1742700
[B@1bfc93a:[B@acb158
[B@107ebe1:[B@1af33d6
[B@156b6b9:[B@17431b9
[B@139b78e:[B@16c79d7
[B@2e7820:[B@b33d0a
[B@82701e:[B@16c9867
[B@1f14ceb:[B@89cc5e
[B@da4b71:[B@c837cd
[B@ab853b:[B@c79809
[B@765a16:[B@1ce784b
[B@1319c:[B@3bc473

Answer 1

You're looking at the object ID, not a dump of the contents.

• The [ means array.
• The B means byte.
• The @ separates the type from the ID.
• The hex digits are an object ID or hashcode.

If the intent is to print the contents of the array, there are many ways. For example:

byte[] in = new byte[] { 1, 2, 3, -1, -2, -3 };
System.out.println(byteArrayToString(in));

String byteArrayToString(byte[] in) {
    char out[] = new char[in.length * 2];
    for (int i = 0; i < in.length; i++) {
        out[i * 2] = "0123456789ABCDEF".charAt((in[i] >> 4) & 15);
        out[i * 2 + 1] = "0123456789ABCDEF".charAt(in[i] & 15);
    }
    return new String(out);
}

A complete list of the type nomenclature can be found in the JNI documentation.

Here is the entire list:

• B - byte
• C - char
• D - double
• F - float
• I - int
• J - long
• L***fully-qualified-class*;** - between an L and a ; is the full class name, using / as the delimiter between packages (for example, Ljava/lang/String;)
• S - short
• Z - boolean
• [ - one [ for every dimension of the array
• (***argument types*)***return-type* - method signature, such as (I)V, with the additional pseudo-type of V for void method