所有异常都必须是检查异常。

Unchecked exceptions are unrestricted gotos. And unrestricted gotos are considered a bad thing. Unchecked exceptions break encapsulation. To process them correctly, all the functions in the call tree between the thrower and the catcher must be known to avoid bugs. Exceptions are errors in the function that throws them but not errors in the function that processes them. The purpose of exceptions is to give the program a second chance by deferring the decision of whether it's an error or not to another context. It's only in the other context can the correct decision be made.

1)不，NumberFormatException是一个未检查的异常。即使你发现了它(你不需要)，因为它是未检查的。这是因为它是IllegalArgumentException的子类，而IllegalArgumentException是RuntimeException的子类。

2) RuntimeException是所有未检查异常的根。RuntimeException的每个子类都未选中。所有其他异常和Throwable都被检查，除了错误(属于Throwable)。

3/4)你可以提醒用户他们选择了一个不存在的文件，并要求一个新的文件。或者停止通知用户他们输入了无效的内容。

5)抛出并捕获“异常”是不好的做法。但更一般地，您可能会抛出其他异常，以便调用者可以决定如何处理它。例如，如果您编写了一个库来处理读取一些文件输入，而您的方法被传递给一个不存在的文件，您不知道如何处理它。打电话的人是想再问一次还是想退出?因此，您将异常沿链向上抛出给调用者。

在许多情况下，出现未检查的异常是因为程序员没有验证输入(例如您的第一个问题中的NumberFormatException)。这就是为什么捕获它们是可选的，因为有更优雅的方法来避免生成这些异常。

Java distinguishes between two categories of exceptions (checked & unchecked). Java enforces a catch or declared requirement for checked exceptions. An exception's type determines whether an exception is checked or unchecked. All exception types that are direct or indirect subclasses of class RuntimeException are unchecked exception. All classes that inherit from class Exception but not RuntimeException are considered to be checked exceptions. Classes that inherit from class Error are considered to be unchecked. Compiler checks each method call and deceleration to determine whether the method throws checked exception. If so the compiler ensures the exception is caught or is declared in a throws clause. To satisfy the declare part of the catch-or-declare requirement, the method that generates the exception must provide a throws clause containing the checked-exception. Exception classes are defined to be checked when they are considered important enough to catch or declare.

检查异常:

The exceptions which are checked by the compiler for smooth execution of the program at runtime are called Checked Exception. These occur at compile time. If these are not handled properly, they will give compile time error (Not Exception). All subclasses of Exception class except RuntimeException are Checked Exception. Hypothetical Example - Suppose you are leaving your house for the exam, but if you check whether you took your Hall Ticket at home(compile time) then there won't be any problem at Exam Hall(runtime).

未检查异常:

The exceptions which are not checked by the compiler are called Unchecked Exceptions. These occur at runtime. If these exceptions are not handled properly, they don’t give compile time error. But the program will be terminated prematurely at runtime. All subclasses of RunTimeException and Error are unchecked exceptions. Hypothetical Example - Suppose you are in your exam hall but somehow your school had a fire accident (means at runtime) where you can't do anything at that time but precautions can be made before (compile time).

某个异常是否为“受控异常”与是否捕获它或在捕获块中做了什么无关。它是异常类的属性。Exception的子类，除了RuntimeException及其子类，都是受控异常。

Java编译器迫使您要么捕获已检查的异常，要么在方法签名中声明它们。它本应提高程序的安全性，但大多数人的意见似乎是，它不值得它带来的设计问题。

为什么他们让异常冒泡 起来吗?不是处理错误越快 更好吗?为什么要冒出来?

因为这就是例外的意义所在。如果没有这种可能性，就不需要异常。它们使您能够在您选择的级别上处理错误，而不是强迫您在错误最初发生的低级方法中处理它们。

我认为受控异常对于使用外部库的开发人员是一个很好的提醒，在异常情况下，该库中的代码可能会出错。