Java equivalent of Python's struct.pack?

Question

Is there any function equivalent to Python's struct.pack in Java that allows me to pack and unpack values like this?

pump_on = struct.pack("IIHHI", 0, 0, 21, 96, 512)

Epaminondas

Is there an accepted answer for this question? – Houghton

It's not the exact equivalent of Python's pack but maybe it helps you anyway: github.com/marc-christian-schulze/structs4java – Inextinguishable

Answer 1

I think what you may be after is a ByteBuffer:

ByteBuffer pump_on_buf = ...
pump_on_buf.putInt(0);
pump_on_buf.putInt(0);
pump_on_buf.putShort(21);
pump_on_buf.putShort(96);
pump_on_buf.putInt(512);
byte[] pump_on = pump_on_buf.array();

Uvular

ByteBuffer doesn't allow you to write Strings, and the contractor requires a buffer size limitation - you need to pre-decide how long your input would be. – Turbary

@Yonatan, it does allow you to write strings: buf.put(s.getBytes()). ByteBuffer requires you to specify the capacity when initially allocating the buffer. Are you saying this is a good thing or a bad thing? – Uvular

@Uvular ByteBuffer does allow you two write strings in the manner you have shown. However, s.getBytes() will return a UTF-8 encoded byte array from the string (still variable length). It will not have a trailing zero. Beyond that buf.put() uses the current position in the ByteBuffer. This may not be exactly what you want. – Suetonius

Answer 2

Something like this:

final ByteArrayOutputStream data = new ByteArrayOutputStream();
final DataOutputStream stream = new DataOutputStream(data);
stream.writeUTF(name);
stream.writeUTF(password);
final byte[] bytes = stream.toByteArray(); // there you go

Later, you can read that data:

final DataInputStream stream = new DataInputStream(
  new ByteArrayInputStream(bytes)
);
final String user = stream.readUTF();
final String password = stream.readUTF();

Yoho

DataOutputStream is good for java primitives only. ObjectOutputStream (which also implements DataOutput interface) gives you ability to write primitives and serializable objects. – Turbary

in this code example it should be data.toByteArray() there is no toByteArray on DataOutputStream but instead is available on the ByteArrayOutputStream – Loupe

Answer 3

I started development of project which is very close to Python Struct: java-binary-block-parser in JBBP it will look like

JBBPOut.BeginBin().Int(0,0).Short(21,96).Int(512).End().toByteArray();

Zootechnics

Answer 4

Closest feature in core Java is Serialization. It converts object into byte sequence and back.

Kimsey

CAn't believe you. Even as I dislike Java, there should be something closer to being able to do things like this than serialization: Serialization gives you no control on which actual bytes are being created - it just allows you to recreate the same object with those bytes. – Cystoid

By default it gives you no control, but you can always customize it. Hint: search for readObject/writeObject on the page I gave you a link to. If that is not enough, check out download.oracle.com/docs/cd/E17409_01/javase/6/docs/api/java/io/… – Kimsey