On creating custom exceptions

Question

How are Exceptions in java defined and how can I define my own? As an example, we have ArithmeticException which prohibits us dividing by 0 and does not break the program.

What is try-catch's advantage to catching the same potential error with an if-else logic?

Furthermore, suppose I don't operate in the field of all integers, but specifically the field Z2 formed under addition in which 1+1=0.

Provided I have pre-defined an array of logic of operations, were I to do something like this:

try {
    int a = 1;
    int b = 1;
    int c = a/(a+b);
} catch(myError e) {
//
}

where myError :

public class myError extends Exception {
    public myError(String e) {
        super(e);
    }
}

But then, how does the try-catch clause know it is supposed to catch myError? What makes myError be what it is? In other words: what defines, for example, ArithmeticException, to look for division by 0 among other things?

Alternatively I could throw new myError("something's wrong") , but that would defeat the whole point of defining a "custom" exception to begin with, since I could have thrown any exception like that.

Exceptions are just a container for data. You should put the logic in the custom object context, but you cant do that with primitives. Bank.withdrawMoney(500) withdrawMoney(int n){if(n<0)throw new IllegaArgumentException("You cant withdraw negative number of money");} — Toumash
– Toumash, Commented May 23, 2015 at 6:53

Jesper · Accepted Answer · 2015-05-23 06:57:18Z

1

Exceptions are just classes that extend class Throwable. Defining your own exception is done by creating a class that extends Throwable or one of its subclasses.

You can throw your own exception using throw new myError();.

ArithmeticException is a special exception thrown by the JVM when you divide by zero. It's not the exception that is looking for places where you divide by zero; it's how the / works. The / operator checks if the denominator is zero and will then throw the exception.

There is no way to add for example a check to the + operator so that it will throw whatever exception if the result of adding two numbers is zero. You'd have to write your own method to check and do this.

public int add(int a, int b) {
    int result = a + b;
    if (result == 0) {
        throw new myError();
    }
    return result;
}

// Then use the add() method instead of +
try {
    int a = 1;
    int b = -1;
    int result = add(a, b);
    System.out.println(result);
} catch (myError e) {
    System.out.println("The result was zero!");
}

answered May 23, 2015 at 6:57

Jesper

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5 Comments

AlvinL Over a year ago

What is the purpose for the existence of the myError exception class, then? What kind of logic can the exception class hold? As of right now, the try-catch doesn't seem obvious. An if sentence would do the exact same thing, but if that is the case why are we introduced to exception handling? Were the add method to return what we define to be number X, can the exception class itself hold logic that would re-adjust X to Y?

Jesper Over a year ago

It's just a class. You can put whatever methods and member variables in it that you want. You can also handle errors with if statements but that can quickly clutter up your code if you have to add checks everywhere. Note that you don't have to handle exceptions immediately, you can also have methods throw them up to the next calling method, so that you can catch and handle the exceptions at an appropriate level.

Jesper Over a year ago

See for example Advantages of Exceptions

Jesper Over a year ago

An exception is probably not the right thing to use if all you really want to do is "if the result is X, then return Y instead". Just use an if statement for that instead of an exception.

AlvinL Over a year ago

Understood, thanks for the explanation, I understand a bit more now :)

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