Exception is never thrown in body of corresponding try statement

Question

I have a problem with exception handling in Java, here's my code. I got compiler error when I try to run this line: throw new MojException("Bledne dane");. The error is:

exception MojException is never thrown in body of corresponding try statement

Here is the code:

public class Test {
  public static void main(String[] args) throws MojException {
    // TODO Auto-generated method stub

    for(int i=1;i<args.length;i++){
      try{
        Integer.parseInt(args[i-1]);
      }
      catch(MojException e){
        throw new MojException("Bledne dane");
      }
      try{
        WierszTrojkataPascala a = new WierszTrojkataPascala(Integer.parseInt(args[0]));
        System.out.println(args[i]+" : "+a.wspolczynnik(Integer.parseInt(args[i])));
      }
      catch(MojException e){
        throw new MojException(args[i]+" "+e.getMessage());
      }
    }
  }
}

And here is a code of MojException:

public class MojException extends Exception{
    MojException(String s){
        super(s);
    }
}

Can anyone help me with this?

Answer 1

A catch-block in a try statement needs to catch exactly the exception that the code inside the try {}-block can throw (or a super class of that).

try {     //do something that throws ExceptionA, e.g.     throw new ExceptionA("I am Exception Alpha!"); } catch(ExceptionA e) {     //do something to handle the exception, e.g.     System.out.println("Message: " + e.getMessage()); } 

What you are trying to do is this:

try {     throw new ExceptionB("I am Exception Bravo!"); } catch(ExceptionA e) {     System.out.println("Message: " + e.getMessage()); } 

This will lead to an compiler error, because your java knows that you are trying to catch an exception that will NEVER EVER EVER occur. Thus you would get: exception ExceptionA is never thrown in body of corresponding try statement.

Answer 2

As pointed out in the comments, you cannot catch an exception that's not thrown by the code within your try block. Try changing your code to:

try{
    Integer.parseInt(args[i-1]); // this only throws a NumberFormatException
}
catch(NumberFormatException e){
    throw new MojException("Bledne dane");
}

Always check the documentation to see what exceptions are thrown by each method. You may also wish to read up on the subject of checked vs unchecked exceptions before that causes you any confusion in the future.