使用try with catch时未初始化局部变量，但在最终与catch一起使用时初始化。为什么？ [英] Local variable not initialized when using try with catch but is initialized when finally is used with catch . Why ?

问题描述

public static void main (String args[] )
{
    int a ;
    try 
    {
        a=4 ;
    }
    finally {
    }
    System.out.println (a ) ;
}

为什么在最终使用时这不会产生编译时错误？我无法理解这背后的逻辑。我已经理解了当我们使用catch而不是finally时的情况并且它给出了编译时错误。但为什么不在这里？



我尝试过的事情：



我在Netbeans IDE上运行它。

Why this does not give a compile time error when finally is used ? I am not able to understand the logic behind this . I have understood the case when we use catch instead of finally and it gives a compile time error . But why not here ?

What I have tried:

I have run this on Netbeans IDE .

推荐答案

想一想：什么是try ... catch块意味着什么？

它捕获异常，并将执行直接转移到catch块代码。

因此，如果您的代码如下所示：

Think about it: what is a try...catch block meant to do?
It catches exceptions, and diverts execution directly to the catch block code.
So if your code looks like this:

int a ;
try 
   {
   a=4 ;
   }
catch (Exception ex)
   {
   Console.WriteLine(ex.Message);
   }

然后系统可以看到未初始化a的执行路径：当分配中出现问题时。在你的例子中不太可能发生，但在更复杂的情况下它可能会发生：

Then the system can "see" an execution path where a is not initialised: when a problem occurs in the assignment. It's not likely to happen in your example, but it very much could in a more complex case:

int a ;
try 
   {
   a=GetValue();
   }
catch (Exception ex)
   {
   ...
   }

如果GetValue中发生异常，则永远不会设置a的值。

使用finally是不同的：它不会捕获异常，因此要么执行赋值，要么执行后的代码尝试... finally块不会（因为异常未处理，一旦最终代码完成，它由更高级别的方法处理）。



这与昨天的问题非常相似：为什么在使用inputstreamreader类时必须初始化变量x？ [ ^ ]

If an exception occurs in GetValue, the value of a is never set.
Using finally is different: it doesn't catch an exception, so either the assignment will be executed or the code after the try...finally block will not (as the exception isn't handled, it it dealt with by a higher level method once the finally code is complete).

This is very much the same problem you had yesterday: Why the variable x has to be initialized when using inputstreamreader class ?[^]

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