如何在Java中引发异常？

Question

Hmm.. Here is the instruction given to me:

Supports addition & subtraction with other Money object
Throws exception if currencies are incompatible.

Hmm.. I already got a money class with add() in it. How exactly do I throw an exception in this case? I mean, I know how to do try{} catch but where am I supposed to do it? Do I do it on the same money class? or should the throwing of exceptions be done somewhere like in the main() where all other stuff takes place?

public class Money {

    Currency currency;
    int dollar;
    int cents;

    //constructor
    public Money(Currency currency, int dollar, int cents) {
        super();
        this.currency = currency;
        this.dollar = dollar;
        this.cents = cents;
    }
    .
    .
    .
    .
    public Money add(Money moneyToBeAdded){
        Money result = new Money(moneyToBeAdded.getCurrency(),0,0);
        Money totalInCents;

        if(compareCurrency(moneyToBeAdded)){
            totalInCents = new Money(moneyToBeAdded.getCurrency(),0,(moneyToBeAdded.toCents() + cents));
            result = totalInCents.toDollars();
        }//to do convert currency

        return result;
    }

    public Money subtract()

    public boolean compareCurrency(Money money){
        return money.currency.equals(currency);
    }
}

Answer 1

throw new Exception("Currencies are incompatible");

However, I'd consider creating an app- or Money -specific exception instead, subclassing either Exception or RuntimeException depending on your needs.

Perhaps a MoneyConversionException extending MoneyException if things have to be granular.

Answer 2

I think this is a good place to use an unchecked exception; any attempt to add incompatible monies is due to a programming error. So I'd suggest:

if (!compareCurrency(moneyToBeAdded))
  throw new IllegalArgumentException("Mismatched currencies.");

Since IllegalArgumentException derives from RuntimeException , you don't need to declare that the add() method can throw it.

Do not create a custom subclass of RuntimeException . Custom exception types are useful when an application is trying to recover from a specific error, rather than just using the same logging or alert mechanism to handle every exception. Programs shouldn't try to recover from specific programming errors like this.

Answer 3

As others have mentioned, there are loads of resources at your disposal (eg this ). Anyway, that aside, I'd be wary of throwing a custom exception. Instead, throw an existing one (eg IllegalArgumentException ) so as to minimize complexity.

Answer 4

A method that is supposed to throw any Exception needs a throw clause:

public Money add(Money moneyToBeAdded) throws MoneyException{..}

In the method, you need to filter out the cases where you want the exception to be thrown (with if-clauses for example) and then go:

throw new MoneyException("don't like that currency");

In this example, MoneyException is a class that extends Exception:

class MoneyException extends Exception {..}

Answer 5

Refer the Lesson from Oracle website

http://docs.oracle.com/javase/tutorial/essential/exceptions/

Answer 6

Throwing an exception is about reporting an error, and it has to be declared as part of the method declaration. For example:

class SomeMoneyProblem extends Exception {
      public SomeMoneyProblem() {
            super();
      }

      public SomeMoneyProblem(String msg) {
            super(msg);
      }
}

    public class Money {

       ...

       public Money() { 

       }

       public Money someMoneyMethod(Money moreMoney) throws SomeMoneyProblem {
               ....
               if ( someProblemCondition )
                      throw new SomeMoneyProblem();

               if ( someOtherProblem ) 
                      throw new SomeMoneyProblem("Other Problem");

       }

       public static void main() {

               Money m = new Money();
               try {
                    m.someMoneyMethod();
               } catch (SomeMoneyProblem p) {
                    System.err.println(p);
               }
       }
    }