Right Java exception type for calling method before initialization

Question

I have a class that has an explicit initialization method and some other methods that do the actual work:

public class Worker {

    public void init(Context context) { /* ... */ };

    public void doWork() { /* ... */ };
}

The Init method must be called before any actual work is done, which is documented in class and methods description.

I would like to throw an exception from DoWork method if initialization wasn't performed before the call. What would be the right exception type for this case? UnsupportedOperationException, IllegalStateException or something else?

@BalusC: yes, thanks, I'm new to Java (hence the newbie question and naming problems). Don't worry, they do in actual code! — Dyppl
– Dyppl, Commented Jul 1, 2012 at 15:34
Note that standard Java exceptions usually describe their purpose very well in their javadocs. — BalusC
– BalusC, Commented Jul 1, 2012 at 15:34

T.J. Crowder · Accepted Answer · 2012-07-01 15:32:41Z

3

IllegalStateException. But ideally, try to ensure that the object always has a valid state. :-) In this case, by requiring Context in the constructor rather than a separate "init" function:

public class Worker {

    // Note this is a constructor now, not a method
    public Worker(Context context) {
        /* ... */
    }

    public void doWork() {
        /* ... */
    }
}

answered Jul 1, 2012 at 15:32

T.J. Crowder

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7 Comments

Dyppl Over a year ago

yes, I do try to ensure that. However, I'm reluctant to put code that performs complex computations or other long-running activities in the constructor, which unfortunally is the case.

Louis Wasserman Over a year ago

Then don't expose a constructor. Expose a factory method.

Andrew Aylett Over a year ago

In which case: have the init function return another object, on which you can call doWork(). This looks suspiciously like the factory pattern :).

T.J. Crowder Over a year ago

@Dyppl: In general, it's fine to do anything in a constructor that you do in a method. If by "long running" you mean happening on another thread, and so you can have a semi-constructed instance, you can handle that with a factory that accepts a callback (public static void BuildWorker(Context context, IAcceptWorkers target)) where IAcceptWorkers is an interface with a function you call with the fully-constructed Worker. If by long-running you just mean it takes a while, that's fine, still a constructor. Document it, of course. :-)

Andrew Aylett Over a year ago

@T.J.Crowder: I'm not sure I completely agree; at a VM level, sure, but I think there's benefit in separating actually creating the object from the work required to enable me to create the object.

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