What's wrong with overridable method calls in constructors?

Calling an overridable method (a method that can be overridden in a subclass) from within a constructor is considered problematic because it can lead to unexpected behavior, especially when the constructor of a base class is called before the subclass is fully initialized. Here's why this is a potential issue:

1. Subclass State Is Not Yet Initialized

When a base class constructor calls an overridable method, the subclass's constructor has not yet run, and its fields may not be initialized. If the overridden method in the subclass relies on these fields, it may operate on uninitialized or incorrect data, leading to runtime errors or undefined behavior.

Example in Java:

class Base {
    Base() {
        System.out.println("Base constructor");
        display(); // Overridable method call
    }

    void display() { // This method can be overridden
        System.out.println("Base display");
    }
}

class Derived extends Base {
    private String message = "Derived";

    @Override
    void display() { // Overriding method
        System.out.println("Message: " + message); // `message` is not initialized yet
    }
}

public class Main {
    public static void main(String[] args) {
        new Derived(); 
    }
}

Output:

Base constructor
Message: null

What Happened:

• The Base constructor calls display().
• The Derived class's overridden display() method is executed, but the message field is not yet initialized. This results in a null output instead of the expected "Derived."

2. Violation of Constructor Contract

The purpose of a constructor is to initialize the current class's state. By calling an overridable method, you introduce behavior that depends on the subclass, which violates the principle of encapsulation and the expectation that the constructor should only manage the state of the class being constructed.

3. Hard-to-Debug Issues

Overridable method calls in constructors can cause subtle and hard-to-trace bugs because the behavior of the constructor depends on the subclass implementation. This can lead to:

• Unexpected side effects.
• Issues when modifying the subclass later.
• Bugs that only appear when certain subclasses are instantiated.

4. Best Practices

To avoid these issues:

1. Avoid calling overridable methods in constructors. Use private, final, or static methods within constructors instead, as these cannot be overridden.

Example:

class Base {
    Base() {
        System.out.println("Base constructor");
        init(); // Use a private or final method instead
    }

    private void init() {
        System.out.println("Base initialization");
    }
}

2. Defer Method Calls Until After Initialization. If subclass-specific logic is needed, consider using a factory method or an initialization method called after the constructor finishes.

Example:

class Base {
    Base() {
        System.out.println("Base constructor");
    }

    void initialize() {
        // Subclass can override this safely
    }
}

class Derived extends Base {
    private String message = "Derived";

    @Override
    void initialize() {
        System.out.println("Message: " + message); // Safe to use; already initialized
    }
}

public class Main {
    public static void main(String[] args) {
        Derived derived = new Derived();
        derived.initialize(); // Explicitly call initialization
    }
}

Summary

Calling overridable methods in constructors can lead to unexpected behavior because the subclass is not fully initialized when the base class constructor runs. Avoid this by:

• Using private, final, or static methods in constructors.
• Deferring subclass-specific initialization until after construction.

For mastering best practices and design principles, explore Grokking Advanced Coding Patterns for Interviews or Grokking System Design Fundamentals on DesignGurus.io! These resources offer insights into robust and maintainable software development.