What's wrong with overridable method calls in constructors?

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.

Solution 1:

On invoking overridable method from constructors

Simply put, this is wrong because it unnecessarily opens up possibilities to MANY bugs. When the @Override is invoked, the state of the object may be inconsistent and/or incomplete.

A quote from Effective Java 2nd Edition, Item 17: Design and document for inheritance, or else prohibit it:

There are a few more restrictions that a class must obey to allow inheritance. Constructors must not invoke overridable methods, directly or indirectly. If you violate this rule, program failure will result. The superclass constructor runs before the subclass constructor, so the overriding method in the subclass will be invoked before the subclass constructor has run. If the overriding method depends on any initialization performed by the subclass constructor, the method will not behave as expected.

Here's an example to illustrate:

public class ConstructorCallsOverride {
    public static void main(String[] args) {

        abstract class Base {
            Base() {
                overrideMe();
            }
            abstract void overrideMe(); 
        }

        class Child extends Base {

            final int x;

            Child(int x) {
                this.x = x;
            }

            @Override
            void overrideMe() {
                System.out.println(x);
            }
        }
        new Child(42); // prints "0"
    }
}

Here, when Base constructor calls overrideMe, Child has not finished initializing the final int x, and the method gets the wrong value. This will almost certainly lead to bugs and errors.

On object construction with many parameters

Constructors with many parameters can lead to poor readability, and better alternatives exist.

Here's a quote from Effective Java 2nd Edition, Item 2: Consider a builder pattern when faced with many constructor parameters:

Traditionally, programmers have used the telescoping constructor pattern, in which you provide a constructor with only the required parameters, another with a single optional parameters, a third with two optional parameters, and so on...

The telescoping constructor pattern is essentially something like this:

public class Telescope {
    final String name;
    final int levels;
    final boolean isAdjustable;

    public Telescope(String name) {
        this(name, 5);
    }
    public Telescope(String name, int levels) {
        this(name, levels, false);
    }
    public Telescope(String name, int levels, boolean isAdjustable) {       
        this.name = name;
        this.levels = levels;
        this.isAdjustable = isAdjustable;
    }
}

And now you can do any of the following:

new Telescope("X/1999");
new Telescope("X/1999", 13);
new Telescope("X/1999", 13, true);

You can't, however, currently set only the name and isAdjustable, and leaving levels at default. You can provide more constructor overloads, but obviously the number would explode as the number of parameters grow, and you may even have multiple boolean and int arguments, which would really make a mess out of things.

As you can see, this isn't a pleasant pattern to write, and even less pleasant to use (What does "true" mean here? What's 13?).

Bloch recommends using a builder pattern, which would allow you to write something like this instead:

Telescope telly = new Telescope.Builder("X/1999").setAdjustable(true).build();

Note that now the parameters are named, and you can set them in any order you want, and you can skip the ones that you want to keep at default values. This is certainly much better than telescoping constructors, especially when there's a huge number of parameters that belong to many of the same types.

Solution 2:

Here's an example which helps to understand this:

public class Main {
    static abstract class A {
        abstract void foo();
        A() {
            System.out.println("Constructing A");
            foo();
        }
    }

    static class C extends A {
        C() { 
            System.out.println("Constructing C");
        }
        void foo() { 
            System.out.println("Using C"); 
        }
    }

    public static void main(String[] args) {
        C c = new C(); 
    }
}

If you run this code, you get the following output:

Constructing A
Using C
Constructing C

You see? foo() makes use of C before C's constructor has been run. If foo() requires C to have a defined state (i.e. the constructor has finished), then it will encounter an undefined state in C and things might break. And since you can't know in A what the overwritten foo() expects, you get a warning.

Solution 3:

Invoking an overridable method in the constructor allows subclasses to subvert the code, so you can't guarantee that it works anymore. That's why you get a warning.

In your example, what happens if a subclass overrides getTitle() and returns null ?

To "fix" this, you can use a factory method instead of a constructor, it's a common pattern of objects instanciation.

Solution 4:

Here is an example that reveals the logical problems that can occur when calling an overridable method in the super constructor.

class A {

    protected int minWeeklySalary;
    protected int maxWeeklySalary;

    protected static final int MIN = 1000;
    protected static final int MAX = 2000;

    public A() {
        setSalaryRange();
    }

    protected void setSalaryRange() {
        throw new RuntimeException("not implemented");
    }

    public void pr() {
        System.out.println("minWeeklySalary: " + minWeeklySalary);
        System.out.println("maxWeeklySalary: " + maxWeeklySalary);
    }
}

class B extends A {

    private int factor = 1;

    public B(int _factor) {
        this.factor = _factor;
    }

    @Override
    protected void setSalaryRange() {
        this.minWeeklySalary = MIN * this.factor;
        this.maxWeeklySalary = MAX * this.factor;
    }
}

public static void main(String[] args) {
    B b = new B(2);
    b.pr();
}

The result would actually be:

minWeeklySalary: 0

maxWeeklySalary: 0

This is because the constructor of class B first calls the constructor of class A, where the overridable method inside B gets executed. But inside the method we are using the instance variable factor which has not yet been initialized (because the constructor of A has not yet finished), thus factor is 0 and not 1 and definitely not 2 (the thing that the programmer might think it will be). Imagine how hard would be to track an error if the calculation logic was ten times more twisted.

I hope that would help someone.

Solution 5:

If you call methods in your constructor that subclasses override, it means you are less likely to be referencing variables that don’t exist yet if you divide your initialization logically between the constructor and the method.

Have a look on this sample link http://www.javapractices.com/topic/TopicAction.do?Id=215