What's wrong with overridable method calls in constructors?

user_238134 0
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I have a Wicket page class that sets the page title depending on the result of an abstract method.

public abstract class BasicPage extends WebPage {

    public BasicPage() {
        add(new Label("title", getTitle()));
    }

    protected abstract String getTitle();

}

NetBeans warns me with the message "Overridable method call in constructor", but what should be wrong with it? The only alternative I can imagine is to pass the results of otherwise abstract methods to the super constructor in subclasses. But that could be hard to read with many parameters.

overriding inheritance constructor oop java
8 Answers

I certainly agree that there are cases where it is better not to call some methods from a constructor.

Making them private takes away all doubt: "You shall not pass".

However, what if you DO want to keep things open.

It's not just the access modifier that is the real problem, as I tried to explain here. To be completely honest, private is a clear showstopper where protected usually will still allow a (harmful) workaround.

A more general advice:

  • don't start threads from your constructor
  • don't read files from your constructor
  • don't call APIs or services from your constructor
  • don't load data from a database from your constructor
  • don't parse json or xml documents from your constructor

Don't do so (in)directly from your constructor. That includes doing any of these actions from a private/protected function which is called by the constructor.

Calling an start() method from your constructor could certainly be a red flag.

Instead, you should provide a public init(), start() or connect() method. And leave the responsibility to the consumer.

Simply put, you want to separate the moment of "preparation" from the "ignition".

  • if a constructor can be extended then it shouldn't self-ignite.
  • If it self-ignites then it risks being launched before being fully constructed.
  • After all, some day more preparation could be added in the constructor of a subclass. And you don't have any control over the order of execution of the constructor of a super class.

PS: consider implementing the Closeable interface along with it.

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