为什么不能在if语句中声明变量？ [英] Why can't variables be declared in an if statement?

问题描述

以下Java代码无法编译。

The following Java code does not compile.

int a = 0;

if(a == 1) {
    int b = 0;
}

if(a == 1) {
    b = 1;
}

为什么？没有代码路径导致程序将1分配给 b 而不首先声明它。

Why? There can be no code path leading to the program assigning 1 to b without declaring it first.

它发生在我， b 的变量范围可能仅限于第一个 if 语句，但后来我不明白为什么。如果我真的不想不必要地声明 b ，以提高性能怎么办？我不喜欢在声明后让变量保持未使用状态。

It occurred to me that b's variable scope might be limited to the first if statement, but then I wouldn't understand why. What if I really don't want to declare b needlessly, in order to improve performance? I don't like having variables left unused after declaration.

（你可能想争辩，我只能在第二个中声明b如果语句，在这种情况下，想象它可能在其他地方的循环中。）

(You may want to argue than I could simply declare b in the second if statement, in that case, just imagine that it could be in a loop somewhere else.)

推荐答案

为什么？没有代码路径可以导致程序在没有首先声明的情况下将1分配给b。

Why? There can be no code path leading to the program assigning 1 to b without declaring it first.

你是对的，但是编译器没有不知道。编译器不执行代码。编译器只转换为字节码而不评估表达式。

You are right, but the compiler doesn't know that. The compiler does not execute the code. The compiler only translates to bytecode without evaluating expressions.

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