Similar functionality for java to struct for python

Question

I have a program that I made in Python to find specific tags in TIFF IFD's and return the values. It was just a proof of concept thing in python, and now I need to move the functionality to java. I think I can just use the String(byteArray[]) constructor for the ASCII data types, but I still need to get Unsigned short (2 byte) and unsigned long (4 byte) values. I don't need to write them back to the file or modify them, all I need to do is get a Java Integer or Long object from them. This is easy in python with the struct and mmap classes, does any one know of a similar way in java? I looked at the DataInput class, but the readUnsignedLong method reads 8 bytes.

Answer 1

DataInputStream allows you to read shorts and longs. You should mask them with the appropriate bit mask ( 0xFFFF for short, 0xFFFFFFFF for 32 bit) in order to account for the difference between signed/unsigned types.

eg

// omits error handling
FileInputStream fis = ...;
DataInputStream stream = new DataInputStream(fis);
int short_value = 0xFFFF & stream.readShort();
long long_value = 0xFFFFFFFF & stream.readInt();

If you're sure that the data won't be towards the high end of the 2 byte field, or 4 byte field, you can forego the bit masking. Otherwise, you need to use a wider data type to account for the fact that unsigned values hold a larger range of values than their signed counterparts.

Answer 2

I looked at the DataInput class, but the readUnsignedLong method reads 8 bytes.

Java does not have unsigned types. It takes 4 bytes to make an int , and 8 bytes to make a long , unsigned or otherwise.

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If you don't want to use DataInput , you can read the bytes into byte arrays ( byte[] ) and use a ByteBuffer to turn those byte values into int s and long s with left padding. See ByteBuffer#getInt() and ByteBuffer#getLong() .

Answer 3

DataInput would be the preferred method. You can use readUnsignedShort for the two byte values. For the 4 byte values you'll have to use this workaround ...

long l = dis.readInt() & 0xffffffffL;

Answer 4

You could use Javolution's Struct class which provides structure to regions of data. You set up a wrapper and then use the wrapper to access the data. Simples. Java really needs this super-useful class in its default classpath TBQH.

Answer 5

Preon Library is good to create struct in Java. I have tried Javolution's Struct but it was not help full my case. It is open source and very good library.