Creating a Custom Exception

Question

I'm trying to create a method f1(x) that throws an exception when x equals 5. After that I will try to call that method from another method f2() to invoke that exception. Then I have to have f2() recover by calling f1(x+1). I tried coding something, but I'm stuck. Here is the code:

public class FiveException extends Exception {

public void f1(int x) throws FiveException {
    if (x == 5) {
        throw new FiveException();
    }
}

public void f2() {
    int x = 5;
    try {
        f1(x);
    }catch (FiveException e) {
        System.out.println("x is 5");
    }

}


public static void main(String[] args) {
    FiveException x5 = new FiveException();
    x5.f2();
}

}

The print statement works, but I'm not sure how to call f(x+1). Any help on how to fix this and any techniques to write exceptions is appreciated.

Your code that throws the exception should not be in the FiveException class. — Peter Wooster
– Peter Wooster, Commented May 14, 2013 at 23:42
@fhelwanger I tried calling f1(x + 1) under the catch statement in f2() but it was giving me an error. — blutuu
– blutuu, Commented May 14, 2013 at 23:52

fhelwanger · Accepted Answer · 2013-05-15 00:11:38Z

Because f1 throws FiveException, wherever you call f1 you must either catch the exception or throw it to the method calling the method that raises the exception. For example:

public static void main(String[] args) throws FiveException {
    FiveException x5 = new FiveException();
    x5.f1(1);
}

Or:

public static void main(String[] args) {
    FiveException x5 = new FiveException();

    try {
        x5.f1(1);
    } catch (FiveException e) {
        e.printStackTrace();
    }
}

But your code is confusing... normally, it isn't the exception class that throws itself, you have other classes that throw the exception class.

If it's being invoked inside a catch statement, you must surround it with another try-catch, 'cause the code inside catch isn't protected, like this:

public void f2() {
    int x = 5;
    try {
        f1(x);
    }catch (FiveException e) {
       System.out.println("x is 5");
       try {
           f1(x + 1);
       } catch (FiveException e) {
           e.printStackTrace();
       }
    }
}

But this code is ugly, you can write the following:

public void f2() {
    int x = 5;
    fProtected(x);
    fProtected(x + 1);
}

private void fProtected(int x) {
    try {
        f1(x);
    }catch (FiveException e) {
       System.out.println("x is 5");
    }
}

Interesting. I will study this and will see if this is the desired code I'm looking for.

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