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I want to make tests in a constructor to find out if it is currently a good idea to instantiate the object or not, with the given parameters. But how could I abort and return a warning from a constructor to the new statement? Must such tests instead be done by the caller before each "new" statement? I thought that the constructor would be a good place for it.

Tombola
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5 Answers5

11

You could use a factory object instead. This could then run your checks and return the instansiated object, or null. This would probably be more efficient than an exception. 

MyObject myObject = MyObjectFactory.createMyObject();
GavinCattell
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7

Yes, you throw an exception in the constructor.

In java you usually throw an IllegalArgumentException if one of the arguments were wrong which is a common thing to do really as a guard statement:

public class Roman {

    public Roman(int arabic) {

        // "Guard statement" in the beginning of the constructor that
        // checks if the input is legal
        if (arabic < 0) {
            throw new IllegalArgumentException("There are no negative roman numerals");
        }

        // Continue your constructor code here

    }

}

If you don't want exceptions you can do as GavinCatelli's answer and create a factory method that returns null if the object won't be "correct".

public class RomanFactory {

    public static Roman getSafeRoman(int a) {
        Roman r;
        try {
            r = new Roman(a);
        } catch(IllegalArgumentException e) {
            r = null;
        }
        return r;
    }

}

You do have to check for null's though, or else the program might crash with NullPointerException.

Community
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Spoike
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3

The only sure way to abort object construction is to throw an Exception before completion of the constructor

John Vint
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1

You can have the constructor throw an exception if the parameters are invalid.

If it's just a question of input validity that a caller should be able to check itself, you should throw a RuntimeException. If it's something that a caller won't necessarily be able to control, the constructor should throw a checked exception; note that this will require all code which calls the constructor to handle or declare the exception.

Taymon
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0

Make your own class of exception and accordingly pass the message based on the parameters that are passed to the constructor. Thus throw this exception from the constructor.

Noroi
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