Custom Checked Exception Java

Question

I'm trying to understand exceptions and I'm getting errors when I try to implement a custom one I made. I was given directions to make a no constructor that passes the string below to the super constructor. I keep getting compiler errors and I'm not sure how to fix them. The custom file looks like this

import java.io.*;

public class UnknownRegionException extends FileNotFoundException {

    public UnknownRegionException() {
        super("Could not find your region!");
    }
}

and this block of code isn't running properly

public Adventure(String file) {
    try {
        File inFile = new File(file);
        Scanner scan = new Scanner(inFile);
        int i = 0;
        while (scan.hasNext()) {
            String name = scan.nextLine();
            pokemon[i] = name;
            i++;
        }
    } catch (UnknownRegionException e) {
        System.err.println(e);
    }
}

The errors i'm getting are below

D:\Documents\Google Drive\Homework\1331\HW8>javac Adventure.java
Adventure.java:23: error: unreported exception FileNotFoundException; must be ca
ught or declared to be thrown
            Scanner scan = new Scanner(inFile);
                           ^
Adventure.java:63: error: unreported exception PokemonAlreadyExistsException; mu
st be caught or declared to be thrown
            throw new PokemonAlreadyExistsException(message);
            ^
Adventure.java:78: error: unreported exception PokemonAlreadyExistsException; mu
st be caught or declared to be thrown
            throw new PokemonAlreadyExistsException();
            ^
Adventure.java:84: error: unreported exception PartyIsFullException; must be cau
ght or declared to be thrown
                throw new PartyIsFullException();
                ^
Adventure.java:99: error: unreported exception FileNotFoundException; must be ca
ught or declared to be thrown
        PrintWriter outWriter = new PrintWriter(outFile);
                                ^
5 errors

Answer 1

Checked exceptions need to be either caught or declared in the method signature. You're catching UnknownRegionException but the errors you're seeing indicate that the code may potentially throw several other exceptions such as FileNotFoundException and PokemonAlreadyExistsException and PartyIsFullException. So, those either need to be caught in your catch block or declared in the method signature like so:

public Adventure(String file) throws FileNotFoundException, PokemonAlreadyExistsException, PartyIsFullException {

It's common for checked exceptions to be seen in I/O related methods (such as operating on files). Unchecked exceptions which extend RuntimeException are more common and don't have the necessary verbose syntax.

EDIT

Even though your code knows about your UnknownRegionException, Scanner does not know about it and therefore cannot throw it. Scanner only declares that it throws FileNotFoundException. If you want it to behave as if it throws UnknownRegionException you need to catch FileNotFoundException and wrap the message in UnknownRegionException

public Adventure(String file) throws UnknownRegionException {
    try {
        File inFile = new File(file);
        Scanner scan = new Scanner(inFile);
        int i = 0;
        while (scan.hasNext()) {
            String name = scan.nextLine();
            pokemon[i] = name;
            i++;
        }
    } catch (FileNotFoundException e) {
        throw new UnknownRegionException(e.getMessage());
    }
}

Aside from that, there's no way for Scanner to throw an exception that is a subclass of the exception it declares that it throws. Essentially, FileNotFoundException knows about IOException because it's a subclass of IOException, but it doesn't know about UnknownRegionException because UnknownRegionException is a subclass of it. Your only option is to throw your exception yourself.