Java Try Catch块

问题描述

我最初在大学开始编程，学习vb.net。现在我已经决定移动到Java并有一些查询。在vb中，try catch语句的布局如下

  异常。 
 
 
 
如果其中一些与继承有关，则始终先抓住子类（即更具体的异常 s），以免你的代码不会编译：
  try {
 // throws FileNotFoundException AND IOException 
} 
 //好：FileNotFoundException是IOException的子类 - 检查
 catch（FileNotFoundException fnfe）{
 // TODO 
} 
 catch （IOException ioe）{
 // TODO 
} 
  
另外看看在Java 7的 ，其中不相关的异常可以在每个异常之间一个 |  类型：
  try（可选的资源）{
 //抛出FileNotFoundException的东西和MyOwnCheckedException 
} 
 //下面异常是不相关的
 catch（FileNotFoundException | MyOwnCheckedException e）{
 // TODO 
} 
  
 注意 
 
 
 
在链接的例子，下面的第一个代码片段将它们整合在一起可能被认为是次优的：它 捕获潜在的抛出异常，但其中之一是一个 IndexOutOfBoundsException ，这是一个 RuntimeException （未经检查），理论上不应该处理。 
 
 
 
相反， SIZE 变量（或可能的常量）应替换为对列表被迭代，即 list.size（），以防止 IndexOutOfBoundsException 被抛出。 
 
 
 
我想在这种情况下只是提供一个例子。
 
I initially started programming in college and learnt vb.net. Now I have decided to make the move to Java and have some queries. In vb, the try catch statement is laid out as follows 
try
Catch ex as exception
finally
End catch
but from the java website (https://docs.oracle.com/javase/tutorial/essential/exceptions/putItTogether.html)i found that in java you use two catches like so:
    try {

} catch (ExceptionType name) {

} catch (ExceptionType name) {

}
i was hoping someone could explain why you need two catches in java and what do the respective catches do/catch.
Thanks.
 解决方案 
In Java, you can use multiple catch blocks. 
It doesn't necessarily means you have to. 
It depends on the code your have in the try block, and how many checked Exceptions it may potentially throw (or even unchecked Exceptions if you really want to catch that, typically you don't and you don't have to). 
One bad practice is to use a single handler for general Exception (or worse, Throwable, which would also catch RuntimeExceptions and Errors):
try {
    // stuff that throws multiple exceptions
}
// bad
catch (Exception e) {
    // TODO
}
The good practice is to catch all potentially thrown checked Exceptions. 
If some of them are related in terms of inheritance, always catch the child classes first (i.e. the more specific Exceptions), lest your code won't compile:
try {
    // stuff that throws FileNotFoundException AND IOException
}
// good: FileNotFoundException is a child class of IOException - both checked
catch (FileNotFoundException fnfe) {
    // TODO
}
catch (IOException ioe) {
    // TODO
}
Also take a look at Java 7's multi-catch blocks, where unrelated Exceptions can be caught all at once with a | separator between each Exception type:
try (optionally with resources) {
    // stuff that throws FileNotFoundException and MyOwnCheckedException
}
// below exceptions are unrelated
catch (FileNotFoundException | MyOwnCheckedException e) {
    // TODO
}
Note
In this example you linked to, the first code snippet below Putting it all together may arguably be considered as sub-optimal: it does catch the potentially thrown Exceptions, but one of them is an IndexOutOfBoundsException, which is a RuntimeException (unchecked) and should not be handled in theory. 
Instead, the SIZE variable (or likely constant) should be replaced by a reference to the size of the List being iterated, i.e. list.size(), in order to prevent IndexOutOfBoundsException from being thrown. 
I guess in this case it's just to provide an example though.
                        
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