Java中的已检查和未检查的异常，这些断言是真的吗？

Question

I have some doubts about the difference between checked exception and unchecked exception .

I know that checked exception typically represent invalid conditions in areas outside the immediate control of the program such as invalid user input, database problems, network outages, absent files, etc.

I also know that checked exception are subclass of Exception abstract class and that a method is obliged to establish a policy for all checked exceptions thrown by its implementation (either pass the checked exception further up the stack, or handle it somehow).

So, in practice, what exactly means this last assertion?

Can I simply say that checked exceptions are checked at compile-time. It means if a method is throwing a checked exception then it should handle the exception using try-catch block or it should declare the exception using throws keyword, otherwise the program will give a compilation error. It is named as checked exception because these exceptions are checked at Compile time.

So, if a method throws a checked exception I can handle it in 2 differents way:

1. I have to handle it into a try-catch block, something like this:

    try{ //statements that may cause an exception } catch (exception(type) e(object))‏ { //error handling code } 

2. Using the throws keyword used in method declaration, in order to explicitly specify the exceptions that a particular method might throw. When a method declaration has one or more exceptions defined using throws clause then the method-call must handle all the defined exceptions.

   So, correct me if I am sayng a wrong assertion, the throws keyword is used to throw the retrieved checked exception to the upper level in the stack (into the caller) . So if a method call another method that throw a checked excepion, if it is thrown the caller method have to handle it (for example by a try catch block). Is it this reasoning correct?

   So is it true that checked exceptions have the disadvantage that introduce a form of tight-coupling because if I have a chain of methods and the last one throws a checked exception or I handle it into the code of its father or all the intermediate methods must declare the exception by the throws keyword on the method declaration? Is this assertion correct?

At the contrary the unchecked exception are not checked at compile time. It means if your program is throwing an unchecked exception and even if you didn't handle/declare that exception, the program won't give a compilation error. Most of the times these exception occurs due to the bad data provided by user during the user-program interaction. It is up to the programmer to judge the conditions in advance, that can cause such exceptions and handle them appropriately. All Unchecked exceptions are direct sub classes of RuntimeException class.

For example I can have a situation like this:

class Example {  
   public static void main(String args[])
   {
    int num1=10;
    int num2=0;
    /*Since I'm dividing an integer with 0
     * it should throw ArithmeticException*/
    int res=num1/num2;
    System.out.println(res);
   }
}

Compiling this class the compiler give me no error but running it I will obtain an ArithmeticException tath is an unchecked exception because it happens at runtime.

So I think that it is the developer that have to handle this case doing something like:

class Example {  
   public static void main(String args[])
   {
    int num1=10;
    int num2=0;
    /*Since I'm dividing an integer with 0
     * it should throw ArithmeticException*/
    try {
        int res=num1/num2;
    } catch(ArithmeticException e) {
         System.out.println("Division for 0, this is not a good idea !!!");
    }

    System.out.println(res);
   }
}

So my doubts are:

1. It seems to me that I can use the try catch block also for the unchecked exception . So what is the difference in the use of try catch block that exist between checked exception and unchecked exception ? Is it only that with checked exception I have to use it (or use the throws keyword on the method declaration) otherwise I will obtain a compile time error message?

2. Also the unchecked exception are automatically propagated to the superior level into the stack of the methods call?

Tnx

Answer 1

In regard to the coupling introduced by the throws , you are absolutely right: exceptions declared in the throws clause become part of a method's public interface (in general sense of the word; without regard to Java's interfaces and public access). However, this coupling is not different from that introduced by declaring a return type or parameter types of a method. You can think of it as a third attribute of method's signature.

It seems to me that I can use the try catch block also for the unchecked exception. So what is the difference in the use of try catch block that exist between checked exception and unchecked exception?

You can certainly catch unchecked exceptions, but you should do it at the very top level of your program in order to prevent complete crashes. Unchecked exceptions represent programming errors, so you cannot handle them in a meaningful way. The best you can do is to log them, and move on.

Also the unchecked exception are automatically propagated to the superior level into the stack of the methods call?

Yes, they do so in the same way the checked exceptions do when an exception is declared in the throws clause and not caught.

Answer 2

1. Yes. You can use try catch block to catch unchecked exceptions as well as checked exceptions.

2. All exceptions, whether checked or unchecked are propagated up to the next level if not caught.

The difference is that if a method can throw a checked exception E (either because it calls a method that throws E or because it directly throws it itself with the statement throw E ) the method declaration must include this information using throws E .

Therefore if you want to call a method that can throw the checked exception E you must either put the method call in a try block or declare throws E . These things are optional for unchecked exceptions.

Most of the time you should not catch an unchecked exception.

Answer 3

1. There are no differences between try catch of unchecked and checked exceptions. As you told.

   -If a method uses another method that may throw a checked exception, one of the two following things should be true: The method should be enclosed within a try-catch block or The method should specify this exception to be thrown in its method signature.

   -A unchecked exception may not be a part of the method signature, even if a method may throw it.

2. When a piece of code hits an obstacle in the form of an exceptional condition, it creates an objects of class java.lang.Throwable. Checked and Unchecked Exceptions propagates through method calls because both are subclass java.lang.Throwable.