Java的变量如何实际存储在memory中？

Question

I'm learning about Text I/O and Binary I/O in java right now. I read that each value that you write to a file is initially stored in binary. For text I/O, the individual digits are converted to it's corresponding Unicode values and then encoded to the file-specific encoding such as ASCII. For binary I/O, the binary value is directly represented in the file. For example, 199 would be represented as 0xC7 which in binary is 11000111. Now I'm confused on one part. If a variable is initially stored as a binary format, does each digit represent a separate byte that is stored or is the entirety of the number stored as a single byte. For example, is 199 originally stored as 0xc7 which would be 11000111 in binary? Or would it be stored in 3 bytes with each byte representing the binary value for the digit. If it was stored in 3 separate bytes, does binary I/O convert that 3 byte number to a single byte? If it's stored in a single byte, how does text I/O translate that single byte into 3 separate byte values. I'm just confused on how to word this. Hope you can understand what I'm getting at. Thanks

Answer 1

The only thing which a computer is capable of dealing with are sets of 0/1 bits which are stored in memory or, if you wish on a storage device. Those bits can be streamed to monitors and converted to characters by graphical hardware. Sams story with keyboards, you type a key and a few bits of data will be send to the computer.

Bits are stored in memory and are accessible by memory addresses. The addresses are also sets of bits.

For practical reasons the bits are grouped into bytes, words, long words, ... A byte used to be the smallest addressable unit of bits and historically ended up as a group of 8 bits, which is currently used in most of the hardware. Modern memory can store data in multiple byte addressable chunks. Same for the disk, you store data there, using specific addressing mechanisms. But in any case those are just sets of bits.

What you are confused about is the interpretation of those bits. They can represent integer numbers, floating point numbers, characters, addresses, ... The way they are interpreted only depends on the program which uses them.

Characters do not exist in the computer. They are just an abstraction which is provided by programming languages. The programs interpret the bits stored on the computer. There are standards. For example the ASCII encoding maps English characters plus a few special characters into numbers from 0 to 127. Those fit into a single byte (leaving number 128 to 255 for special use). A print command will read those bytes one by one and send them to graphics to form letters on the screen as specified in the encoding standard. Different encoding scheme will display the same bytes differently.

If you write a program wit the "hello world" sting in it, the program will convert the symbols between quotes into a set of 11 ascii bytes. (In 'c' it will add yet another byte which is equal to '0' and ends the string this way). Unicode is yet another way to represent characters. Every unicode character is represented by multiple bytes of data. There are other schemes as well. One thing to pay attention to. If you write strings on the disk using certain encoding, you should read them with the same encoding, or your prints will give you garbage. But you can always read and copy then as binary data without interpretation.

So, any variable of any type is just an abstraction and always consists of bytes of data which your program knows how to interpret based on the data type and/or operations it wants to perform. Variables of type int, double, any java object, including String, are just sets of bytes of different sizes. Only the program (and java interpreter is a program) knows what to do with them, use them in calculations or display as characters.