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在Java中，我们有时会使用final对变量进行修饰，被修饰的变量只能够被赋值一次。如果final变量是引用类型，则它将永远持有相同的对象的引用；如果是简单类型，则它会拥有着不变的值。

那么，final变量就是我们所说的常量吗？事实上不是的，因为一个被final修饰的变量，虽然只能被赋值一次，但是如果它为引用类型的变量，那么它所指向的对象则可能改变，所以称其为'可变变量'更好，而常量的范围相对可变变量来说，更加狭窄。在Java语言规范(§4.12.4)中，对于常量有着明确的定义：A constant variable is a final variable of primitive type or type String that is initialized with a constant expression (§15.28). Whether a variable is a constant variable or not may have implications with respect to class initialization (§12.4.1), binary compatibility (§13.1, §13.4.9), and definite assignment (§16 (Definite Assignment)).

这说明，常量(或者说是常量变量)仅可能为简单类型或者String类型，是被一个常量表达式初始化，并且必须为final的，而常量表达式的定义在Java语言规范(§15.28)中有着定义：A constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the following:

Literals of primitive type and literals of type String (§3.10.1, §3.10.2, §3.10.3, §3.10.4, §3.10.5)

Casts to primitive types and casts to type String (§15.16)

The unary operators +, -, ~, and ! (but not ++ or --) (§15.15.3, §15.15.4, §15.15.5, §15.15.6)

The multiplicative operators *, /, and % (§15.17)

The additive operators + and - (§15.18)

The shift operators <>, and >>> (§15.19)

The relational operators , and >= (but not instanceof) (§15.20)

The equality operators == and != (§15.21)

The bitwise and logical operators &, ^, and | (§15.22)

The conditional-and operator && and the conditional-or operator || (§15.23, §15.24)

The ternary conditional operator ? : (§15.25)

Parenthesized expressions (§15.8.5) whose contained expression is a constant expression.

Simple names (§6.5.6.1) that refer to constant variables (§4.12.4).

Qualified names (§6.5.6.2) of the form TypeName . Identifier that refer to constant variables (§4.12.4).

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