Why can't java variables start with a number?

The analysis of programming language is divided into two parts: word segmentation and grammar. Lexical analysis mainly uses formal grammar, that is, type three grammar. This type of grammar mainly uses regular expression analysis. The characteristic of the regular grammar analyzer is that it does not backtrack, so the implementation is very simple. If a variable starts with a number, then the parser must backtrack when it encounters the first or second English character to determine whether it is a number, a variable name, or a lexical error. At this time, it becomes a type 2 grammar. The advantage of the type 2 grammar analyzer is that it supports backtracking and recursive grammar (so grammar analysis relies on it), but the disadvantage is that the state machine greatly increases the number of states compared with regular grammar, and the code is more difficult to write. Considering that the lexical analysis part is only used for word hyphenation, we really do not have to write the entire lexical analysis part in type 2 grammar in order to support a small function such as variable names starting with numbers.
Therefore, in the end everyone agreed that variables should avoid starting with a number.

java variable naming rules

Environment variables usually refer to the operating system, used to specify some parameters required when the operating system is running

Variables The name starts with a letter, underscore, or dollar sign (￥ above 4). It cannot start with a number, followed by letters, underscores, dollar signs, and numbers. Variable names are case-sensitive and have no length limit.

Camel case naming Method:

1. The variable name should use meaningful English words

2. If the variable name has only one word, all letters should be lowercase

3. If the variable name If it consists of multiple English words, the first letter will be capitalized starting from the second word

4. In Java, the bool value is represented by boolean. There are only two types: true and false. 0, non-0, or empty cannot be used. and non-empty representation, as shown below, what this means is that there is an error in the third line, and the int type variable cannot be assigned to the boolean type variable

5.java uses unicode character code, which means that it can be used in the world in java In the character set of any language, a char type variable can be used to store a Chinese character, that is, the storage space used for a Chinese character and an English character is the same.

6. The occurrence of garbled characters: use a character set to Chinese characters are converted to codes, but when the system uses the b character set to convert the codes to Chinese characters, the code will not be found and garbled characters

will appear.

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