Final variable assignment with try/catch

Question

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.

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
}

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

Answer 1

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

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;
  }
}

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

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 .

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;
  }
}

Answer 2

不，它不是正确的位置，想象一下您的try and catch块中有1条以上的语句，第一个语句是：x =42。在其他语句之后，try块失败了，然后转到catch块，在此处假设x =30。现在您定义了x两次。