字面量是你临时给定的 一个没有任何引用的值（包括基本数据类型和字符串）
常量是一个带引用的不可更改的值 你可以在程序的任何地方（当然只要是可访问到的地方）引用它
字面量不行 在你出现字面量的改行语句结束以后 这个字面量就无法被再次使用了
In programming, a value written exactly as it’s meant to be interpreted. In contrast, a variable is a name that can represent different values during the execution of the program. And a constant is a name that represents the same value throughout a program. But a literal is not a name — it is the value itself.
A literal can be a number, a character, or a string. For example, in the expression,
x = 3
x is a variable, and 3 is a literal.
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维基百科的解释是：In computer science, a literal is a notation for representing a fixed value in source code.
字面量就是用来描述源代码中固定值的表示法。
x = 3; //x是变量,3这个值就是字面量。