overridden method does not throw exception

Question

I have a problem when compiling my code, I'm trying to make a method of a class throw an personalized exception, given some conditions. But at the time of compiling I get the message:

Overridden method does not throw exception

Here's the class and exception declaration:

public class UNGraph implements Graph

Graph is an interface with all the methods of UNGraph in it (the method getId() doesn't have the throws declaration on that script)

After the constructor I create the exception (inside the class UNGraph):

public class NoSuchElementException extends Exception {
    public NoSuchElementException(String message){
        super(message);
    }
}

Here is the method with the exception

public int getId(....) throws NoSuchElementException {
    if (condition is met) {
        //Do method
        return variable;
    }
    else{
       throw new NoSuchElementException (message);
    }
}

Obviously I don't want the method to throw an exception every time, just when the condition is not met; and when it's met, I want to return a variable.

Solution

The compiler is issuing an error because Java does not allow you to override a method and add a checked Exception (any user-defined custom exception that extends the Exception class). Because it is clear that you want to handle the scenario where some condition is not met as an unexpected occurrence (a bug), your best option is to throw a RuntimeException. A RuntimeException, such as: IllegalArgumentException or NullPointerException, does not have to be included in a method signature, so you will alleviate your compiler error.

I suggest the following changes to your code:

//First: Change the base class exception to RuntimeException:
public class NoSuchElementException extends RuntimeException {
    public NoSuchElementException(String message){
        super(message);
    }
}

//Second: Remove the exception clause of the getId signature
//(and remove the unnecessary else structure):
public int getId(....) {
    if ( condition is met) { return variable; }
    //Exception will only be thrown if condition is not met:
    throw new NoSuchElementException (message);
}