java - 为什么 Java 语言规范说表达式 (n > 2) 不是常量表达式？

Question

在 Definite Assignment 上的 Java 语言规范一章中, Example 16-2说是

A Java compiler must produce a compile-time error for the code:

{
    int k;
    int n = 5;
    if (n > 2)
        k = 3;
    System.out.println(k);  /* k is not "definitely assigned"
                               before this statement */ 
}

even though the value of n is known at compile time, and in principle it can be known at compile time that the assignment to k will always be executed (more properly, evaluated). A Java compiler must operate according to the rules laid out in this section. The rules recognize only constant expressions; in this example, the expression n > 2 is not a constant expression as defined in §15.28.

但是，如果我们看一下 §15.28，它是这样说的

the relational operators <, <=, >, and >=

可以贡献常量表达式。

表达式 n > 2 是否是常量表达式？我们如何确定这一点？

Answer 1

之所以这么说是因为n不是 constant expression .

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:

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

和

A constant variable is a final variable of primitive type or type String that is initialized with a constant expression (§15.28).

n不是 final因此不是常量变量。因此它不是常量表达式。因此 n < 2不是常量表达式。