Constant: During the program run, a fixed amount unchanged.
Constant Category:
1. Character Constants: Any double quotes portion ,, called the string constants. For example: "abc", "Hello", "123"
2. Integer constants: number on the direct write, no decimal point. For example: 100,200,0, -250
3. Float constant: to write directly on the numbers, decimals. For example: 2.5, -3.14,0.0
4. character constant: all caused by a single character in single quotes, to do a character constant. For example: 'A', 'b', '9', 'medium'
5. Boolean constants: Only the amount of value. true, false
6. Empty constants: null. No data on behalf of