Why is a default constructor created even though my abstract class has a protected constructor?

Question

I have an abstract class MousableActor that extends a concrete class Actor:

public abstract class MousableActor extends Actor
{   
    /**
     * Constructs a MousableActor.
     */
    protected void MousableActor()
    {
    }
}

When I look at the javadoc generated for the class, I see a public no-args constructor:

According to Section 8.8.9 of the JLS:

If a class contains no constructor declarations, then a default constructor with no formal parameters and no throws clause is implicitly declared.

I always considered that an if-and-only-if. Why is a public default constructor being created even though I explicitly declared a protected constructor? Does it have something to do with the superclass having a public no-args constructor?

I am using Greenfoot version 2.4.2 (which shouldn't matter) on top of Java 1.8.0.

If you had used a good IDE, it would have told you there was a problem, e.g. Eclipse shows the warning "This method has a constructor name". — Andreas, Nov 21 '15 at 00:55
Nice & clear question, lots of effort, though the solution is a little embarrassing :) — zapl, Nov 21 '15 at 01:18
@zapl Lessons in humility are always good for a CS professor like me. My students love to hear about my mistakes, which make them feel better about theirs. — Ellen Spertus, Nov 21 '15 at 01:23

score 5 · Accepted Answer · answered Nov 21 '15 at 00:52

5

A constructor is not a void method.

protected void MousableActor()

should be

protected MousableActor()

answered Nov 21 '15 at 00:52

Elliott Frisch

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D'oh! I hadn't realized that a method could have the same name as a constructor, not that I was conscious of having typed "void". – Ellen Spertus Nov 21 '15 at 01:21

score 2 · Answer 2 · answered Nov 21 '15 at 00:53

2

Because it's returning void, it's not a constructor; it's a method, so there are no constructors, and the default constructor is created.

If you intend for that to be a constructor, then remove void.

answered Nov 21 '15 at 00:53

rgettman

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Why is a default constructor created even though my abstract class has a protected constructor?

2 Answers2