order of constructor calls in multilevel inheritan

问题:

This question already has an answer here:

• Java Constructors - Order of execution in an inheritance hierarchy 5 answers

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

import java.util.*;

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()"
    });
    // */
  }
} ///:~

The output of this code is

Meal()
Lunch()
PortableLunch()
Bread()
Cheese()
Lettuce()
Sandwich()  

Since the fields in a class are created in the order they are declared, why don't

Bread()
Cheese()
Lettuce()

come at the top of the above list?

Also, what is it trying to do in this code?

   monitor.expect(new String[] {
      "Meal()",
      "Lunch()",
      "PortableLunch()",
      "Bread()",
      "Cheese()",
      "Lettuce()",
      "Sandwich()"
    });  

At first I thought it was an anonymous class, but it doesn't look like it. Is it initializing a String array? Why doesn't it have the name for the String variable? Please tell me the name of the programming construct being used here.

回答1:

The constructor:

public Sandwich() {
    System.out.println("Sandwich()");
}

Is translated by the compiler to:

public Sandwich() {
    super();   // Compiler adds it if it is not explicitly added by programmer
    // All the instance variable initialization is moved here by the compiler.
    b = new Bread();
    c = new Cheese();
    l = new Lettuce();

    System.out.println("Sandwich()");
}

So, the first statement in a constructor is the chaining of super class constructor. In fact, the first statement in any constructor chains to the super class constructor, for that matter. That is why first the super class constructor PortableLunch is invoked, which again chains the call to it's super class constructor, due to the super() added by the compiler (remember?).

This chaining of constructor call is done till the class at the top of the inheritance hierarchy, thereby invoking the Object class constructor at the end.

Now, after each the super class constructor has been executed, and all the super class fields have been initialized, the immediate subclass constructor start the execution after the super() call. And then finally it comes back to the Sandwitch() constructor, which now initializes your 3 fields.

So, basically your fields are initialized at last, and hence they are printed at the end, just before Sandwitch() is printed.

Refer to JLS - §12.5 - Creation of New Class Instance for detailed explanation of the instance creation process.

---

As for your 2nd part of the question:

monitor.expect(new String[] {
      "Meal()",
      "Lunch()",
      "PortableLunch()",
      "Bread()",
      "Cheese()",
      "Lettuce()",
      "Sandwich()"
    });  

This code is creating an unnamed array, and initializing it some string literals at the same time. It is similar to the way you would create a named array:

String[] arr = new String[] { "rohit", "jain" };

回答2:

The objects in your example use inheritance, which causes a chain of constructors to be called. When using inheritance a class inheriting from another (subtype) must call the constructor of the class it extends (super type). When a type hierarchy exists, meaning several classes extend from each other in a chain, the calls to the super constructor propagate to the first class in chain that does not inherit from another class (ignoring Object).

The super class constructors of a subtype must be called prior to the execution of the subtype's constructor since it may rely upon fields or methods in the super types. The constructor calls chain up the type hierarchy and once each constructor has initialized the subtype begins its instantiation.

Once the super type constructors in a classes type hierarchy have been called the fields of the subtype are declared since they may also be needed by the subtype's constructor. After the fields of the subtype are declared the subtype constructor executes.

This order is necessary because the subtype may rely upon fields or methods established in the super type.

Meal() (Top of Class Hierarchy, not including Object)
Lunch() (Extends Meal)
PortableLunch() (Extends Lunch)
Bread() (Field of lunch declared before constructor call)
Cheese() (Field of lunch declared before constructor call)
Lettuce() (Field of lunch declared before constructor call)
Sandwich() (Extends Portable Lunch)

Here is a really good overview of object creation in Java.

回答3:

new String[] {
      "Meal()",
      "Lunch()",
      "PortableLunch()",
      "Bread()",
      "Cheese()",
      "Lettuce()",
      "Sandwich()"
    }

\the above is anonymous array declaration. You don't need to specify size in this case.

All your super class constructors will be invoked first before initializing your instance variables. i.e,

order--- >

  Object(), 
  all your superclass constructors,
  instance variables of this class in that order

回答4:

The constructor is called first, then its instance variables are evaluated, acording to the order specified in the Java Language Specification. That's why you get

Bread()
Cheese()
Lettuce()

after

Meal()
Lunch()
PortableLunch()

about the String[] initialization, as you think is not a anonymous class, is just a String object creation, which doesn't nesesarily have to be assigned to a variable.