使用try / catch进行最终变量赋值 [英] Final variable assignment with try/catch

问题描述

因为我认为这是一个很好的编程习惯，所以如果打算只编写一次，我会将所有（本地或实例）变量 final 。

Because I believe it is a good programming practice, I make all my (local or instance) variables final if they are intended to be written only once.

但是，我注意到当变量赋值可以抛出异常时，你不能使变量最终成为：

However, I notice that when a variable assignment can throw an exception you cannot make said variable final:

final int x;
try {
    x = Integer.parseInt("someinput");
}
catch(NumberFormatException e) {
    x = 42;  // Compiler error: The final local variable x may already have been assigned
}

是有没有办法在不诉诸临时变量的情况下做到这一点？ （或者这不是最终修饰符的正确位置？）

Is there a way to do this without resorting to a temporary variable? (or is this not the right place for a final modifier?)

推荐答案

这样做的一种方法是引入一个（非 - final ）临时变量，但你说你不想这样做。

One way to do this is by introducing a (non-final) temporary variable, but you said you didn't want to do that.

另一种方法是将代码的两个分支移动到一个函数中：

Another way is to move both branches of the code into a function:

final int x = getValue();

private int getValue() {
  try {
    return Integer.parseInt("someinput");
  }
  catch(NumberFormatException e) {
    return 42;
  }
}

这是否实用取决于具体用途case。

Whether or not this is practical depends on the exact use case.

总而言之，只要 x 是一个适当范围的局部变量，最实用一般方法可能是非 - 最终。

All in all, as long as x is a an appropriately-scoped local variable, the most practical general approach might be to leave it non-final.

如果另一方面， x 是一个成员变量，我的建议是在初始化期间使用非 final 临时：

If, on the other hand, x is a member variable, my advice would be to use a non-final temporary during initialization:

public class C {
  private final int x;
  public C() {
    int x_val;
    try {
      x_val = Integer.parseInt("someinput");
    }
    catch(NumberFormatException e) {
      x_val = 42;
    }
    this.x = x_val;
  }
}

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