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 而不先声明它.

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

我突然想到 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.如果我真的不想为了提高性能而不必要地声明 b 怎么办?我不喜欢变量在声明后未被使用.

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.

(你可能想争辩说我可以在第二个 if 语句中简单地声明 b，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?没有代码路径导致程序将 1 分配给 b 而不先声明它.

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

你说得对，但编译器不知道.编译器不执行代码.编译器只转换为字节码而不计算表达式.

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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