Checked vs Unchecked Exceptions in Java

In Java, there are two types of exceptions:

1. Checked Exception

2. Unchecked Exception

1. Checked Exception

These exceptions are check at compile time.  If a method throws a checked exception, then the caller have to either caught by throws or try/catch block.

Example 1:

The java.io.IOException is a checked exception. The method1() throws this exception.

void method1() throws IOException 

{
         

}

The following  method2() gives compile error because it calls method1() but doesn't throws  IOException or doesn't caught it within try/catch block.

void method2()
    {
        method1();
    }

Error when compile:    

 unreported exception IOException; must be caught or declared to be thrown method1();

Example 2:

If some code throws a checked exception within a method, then it must be caught within try/catch or throws keyword.

The following 2 implementations of method2() will compile.

Using try/catch block.

void method2()
    {
        try
        {
            method1();
        }
        catch(IOException e)
        {
           
        }
    }

Declaring throws keyword.

void method2() throws IOException
    {
       
            method1();
       
    }

 

Example 3:

The following will not compile because checked exception not handled in either try/catch block or throws keyword.

public void ioOperation(boolean isResourceAvailable) 

{

       if (!isResourceAvailable) {

                 throw new IOException();

       }

}

 

The following 2 implementations of ioOperation() will compile.

Using try/catch block.

public void ioOperation(boolean isResourceAvailable) 

{
            try {

                        if (!isResourceAvailable) {

                                        throw new IOException();

                       }

            } catch(IOException e) {

             }

}

 

Declaring throws keyword

public void ioOperation(boolean isResourceAvailable) throws IOException
{

      if (!isResourceAvailable) {

            throw new IOException();

      }

}

2. Unchecked Exception

These exceptions are not checked at compile time. In java, Under Error and RunimeException classes are unchecked exceptions, everything else under throwable classes are checked exceptions.

                   +-----------+
                    | Throwable|
                   +----------- +
                    /            \
                  /                \
          +-------+         +-----------  +
          | Error   |         | Exception |
          +-------+          +----------- +
          /     |       \              /     |        \
         \______/           \_____/      \
        unchecked         checked         \                             
                                                    +----------------------  +

                                                     | RuntimeException |                                        
                                                    +----------------------  + 

                                                                /   |    |      \                                               
                                                     \_________________/                                                                                    

                                                              unchecked

 




Example 4:

Consider the following Java program. It compiles fine, but it throws ArithmeticException when run. The compiler allows it to compile, because ArithmeticException is an unchecked exception.

class Main {
   public static void main(String args[]) {
      int x = 0;
      int y = 10;
      int z = y/x;
  }
}

 

 Example 5:

 The method3() throw RuntimeException. But doesn't require to caught within try/catch or throws keyword. It'll compile fine.

void method3()
    {
       
            throw new RuntimeException();
       
    }

 


 

As for the particular questions:

1. Is the NumberFormatException consider a checked exception?
   No. NumberFormatException is unchecked (= is subclass of RuntimeException). Why? I don't know. (but there should have been a method isValidInteger(..))
2. Is RuntimeException an unchecked exception?
   Yes, exactly.
3. What should I do here?
   It depends on where this code is and what you want to happen. If it is in the UI layer - catch it and show a warning; if it's in the service layer - don't catch it at all - let it bubble. Just don't swallow the exception. If an exception occurs in most of the cases you should choose one of these:
   
   • log it and return
   • rethrow it (declare it to be thrown by the method)
   • construct a new exception by passing the current one in constructor
4. Now, couldn't the above code also be a checked exception? I can try to recover the situation like this? Can I?
   It could've been. But nothing stops you from catching the unchecked exception as well
5. Why do people add class Exception in the throws clause?
   Most often because people are lazy to consider what to catch and what to rethrow. Throwing Exception is a bad practice and should be avoided.