What is the purpose of long, double, byte, char in Java?

With the possible exception of "short", which arguably is a bit of a waste of space-- sometimes literally, they're all horses for courses:

• Use an int when you don't need fractional numbers and you've no reason to use anything else; on most processors/OS configurations, this is the size of number that the machine can deal with most efficiently;
• Use a double when you need fractional numbers and you've no reason to use anything else;
• Use a char when you want to represent a character (or possibly rare cases where you need two-byte unsigned arithmetic);
• Use a byte if either you specifically need to manipulate a signed byte (rare!), or when you need to move around a block of bytes;
• Use a boolean when you need a simple "yes/no" flag;
• Use a long for those occasions where you need a whole number, but where the magnitude could exceed 2 billion (file sizes, time measurements in milliseconds/nanoseconds, in advanced uses for compacting several pieces of data into a single number);
• Use a float for those rare cases where you either (a) are storing a huge number of them and the memory saving is worthwhile, or (b) are performing a massive number of calculations, and can afford the loss in accuracy. For most applications, "float" offers very poor precision, but operations can be twice as fast -- it's worth testing this on your processor, though, to find that it's actually the case! [*]
• Use a short if you really need 2-byte signed arithmetic. There aren't so many cases...

[*] For example, in Hotspot on Pentium architectures, float and double operations generally take exactly the same time, except for division.

Don't get too bogged down in the memory usage of these types unless you really understand it. For example:

• every object size is rounded to 16 bytes in Hotspot, so an object with a single byte field will take up precisely the same space as a single object with a long or double field;
• when passing parameters to a method, every type takes up 4 or 8 bytes on the stack: you won't save anything by changing a method parameter from, say, an int to a short! (I've seen people do this...)

Obviously, there are certain API calls (e.g. various calls for non-CPU intensive tasks that for some reason take floats) where you just have to pass it the type that it asks for...!

Note that String isn't a primitive type, so it doesn't really belong in this list.

A java int is 32 bits, while a long is 64 bits, so when you need to represent integers larger than 2^31, long is your friend. For a typical example of the use of long, see System.currentTimeMillis()

A byte is 8 bits, and the smallest addressable entity on most modern hardware, so it is needed when reading binary data from a file.

A double has twice the size of a float, so you would usually use a double rather than a float, unless you have some restrictions on size or speed and a float has sufficient capacity.

A short is two bytes, 16 bits. In my opinion, this is the least necessary datatype, and I haven't really seen that in actual code, but again, it might be useful for reading binary file formats or doing low level network protocols. For example ip port numbers are 16 bit.

Char represents a single character, which is 16 bits. This is the same size as a short, but a short is signed (-32768 to 32767) while a char is unsigned (0 to 65535). (This means that an ip port number probably is more correctly represented as a char than a short, but this seems to be outside the intended scope for chars...)

For the really authorative source on these details, se the java language specification.

You can have a look here about the primitive types in Java.

The main interest between these types are the memory usage. For example, int uses 32bits while byte only uses 8bits.

Imagine that you work on large structure (arrays, matrices...), then you will better take care of the type you are using in order to reduce the memory usage.

I guess there are several purposes to types of that kind:

1) They enforce restrictions on the size (and sign) of variables that can be stored in them.

2) They can add a bit of clarity to code (e.g. if you use a char, then anyone reading the code knows what you plan to store in it).

3) They can save memory. if you have a large array of numbers, all of which will be unsigned and below 256, you can declare it as an array of bytes, saving some memory compared with if you declared an array of ints.

4) You need long if the numbers you need to store are larger than 2^32 and a double for very large floating point numbers.