Order of constructor calls

The order of constructor calls was briefly discussed in Chapter 4 and again in Chapter 6, but that was before polymorphism was introduced.

A constructor for the base class is always called during the construction process for a derived class, chaining up the inheritance hierarchy so that a constructor for every base class is called. This makes sense because the constructor has a special job: to see that the object is built properly. A derived class has access to its own members only, and not to those of the base class (whose members are typically private). Only the base-class constructor has the proper knowledge and access to initialize its own elements. Therefore, it’s essential that all constructors get called, otherwise the entire object wouldn’t be constructed. That’s why the compiler enforces a constructor call for every portion of a derived class. It will silently call the default constructor if you don’t explicitly call a base-class constructor in the derived-class constructor body. If there is no default constructor, the compiler will complain. (In the case where a class has no constructors, the compiler will automatically synthesize a default constructor.)

Let’s take a look at an example that shows the effects of composition, inheritance, and polymorphism on the order of construction:

//: c07:Sandwich.java
// Order of constructor calls.
package c07;
import com.bruceeckel.simpletest.*;

class Meal {
  Meal() { System.out.println("Meal()"); }
}

class Bread {
  Bread() { System.out.println("Bread()"); }
}

class Cheese {
  Cheese() { System.out.println("Cheese()"); }
}

class Lettuce {
  Lettuce() { System.out.println("Lettuce()"); }
}

class Lunch extends Meal {
  Lunch() { System.out.println("Lunch()"); }
}

class PortableLunch extends Lunch {
  PortableLunch() { System.out.println("PortableLunch()");}
}

public class Sandwich extends PortableLunch {
  private static Test monitor = new Test();
  private Bread b = new Bread();
  private Cheese c = new Cheese();
  private Lettuce l = new Lettuce();
  public Sandwich() {
    System.out.println("Sandwich()");
  }
  public static void main(String[] args) {
    new Sandwich();
    monitor.expect(new String[] {
      "Meal()",
      "Lunch()",
      "PortableLunch()",
      "Bread()",
      "Cheese()",
      "Lettuce()",
      "Sandwich()"
    });
  }
} ///:~

This example creates a complex class out of other classes, and each class has a constructor that announces itself. The important class is Sandwich, which reflects three levels of inheritance (four, if you count the implicit inheritance from Object) and three member objects. You can see the output when a Sandwich object is created in main( ). This means that the order of constructor calls for a complex object is as follows:

1. The base-class constructor is called. This step is repeated recursively such that the root of the hierarchy is constructed first, followed by the next-derived class, etc., until the most-derived class is reached.
2. Member initializers are called in the order of declaration. The body of the derived-class constructor is called.

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