The absolute quantity of information that you can store in 64 bit is of course the same.

What changes is the meaning you assign to the bits.

In an integer or long variable, the codification used is the same you use for decimal numbers in your normal life, with the exception of the fact that number two complement is used, but this doesn't change that much, since it's only a trick to gain an additional number (while storing just one zero instead that a positive and a negative).

In a float or double variable, bits are split in two kinds: the mantissa and the exponent. This means that every double number is shaped like XXXXYYYYY where it's numerical value is something like XXXX*2^YYYY. Basically you decide to encode them in a different way, what you obtain is that you have the same amount of values but they are distribuited in a different way over the whole set of real numbers.

The fact that the largest/smallest value of a floating number is larger/smaller of the largest/smalles value of a integer number doesn't imply anything on the amount of data effectively stored.

I don't understand how an int 63823, takes up less space than a double 1.0. Is there not more information stored in the int, in this particular instance?

Good question. What you're seeing when you see 63823 and 1.0 is a representation of the underlying data, you are not seeing the underlying data. It is specially formatted so that you can read it, but it is not how the machine sees it.

Java uses very special formats for representing int and double. You need to look at those representations to understand why 63823 takes thirty-two bits when represented as a Java int and 1.0 takes sixty-four bits when represented as a Java double.

In particular, 63823 as an int in Java is represented as:

00000000000000001111100101001111

and 1.0 as a double is represented in Java as:

0011111111110000000000000000000000000000000000000000000000000000

If you want to explore more, I recommend Two's Complement and What Every Computer Scientist Should Know About Floating-Point Arithmetic.

See @Frank Kusters' answer, below!

(My original answer here was for Java versions < 8.)

Or even simpler,

import java.nio.ByteBuffer;

public static byte[] toByteArray(double value) {
    byte[] bytes = new byte[8];
    ByteBuffer.wrap(bytes).putDouble(value);
    return bytes;
}

public static double toDouble(byte[] bytes) {
    return ByteBuffer.wrap(bytes).getDouble();
}

UTF 8 characters byte length can be between 1 to 4 bytes. So your code is printing whatever is the correct byte length for the input japanese character.

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.