Java中的执行顺序

Question

I am trying to understand this example from Thinking in Java :

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()"
        });
    }
}

As I understand from Java Language Specification, the order of execution begins by loading the class containing the main method. Then all the statics and member variables of this class must be initialized (before which all the member variables of the superclasses must be intialized, although there are none of those in this case).

So I thought b , c , l would be initialized before main starts executing. That does not seem to be the case from the output though. Am I missing something?

Answer 1

No, b and c are instance variables.

There's no automatic instantiation of the class containing main . Only static variables are initialized. It's just as if some outside caller wrote:

Sandwich.main(args);

So when you wrote:

Then all the statics and member variables of this class must be initialized

... that was wrong. Only the static variables are initialized - just as normal.

Answer 2

The example output is correct. Here are the important rules:

• when the class is created, the constructor of the super class has to be called first. This bubbles up to Object class

• before the constructor is called, member variable initialization is called.

No static s are involved in this example, except the technical monitor .

Answer 3

JLS # 12.4.1. When Initialization Occurs

A class or interface type T will be initialized immediately before the first occurrence of any one of the following:

• T is a class and an instance of T is created.
• T is a class and a static method declared by T is invoked.
• A static field declared by T is assigned.
• A static field declared by T is used and the field is not a constant variable (§4.12.4).
• T is a top level class (§7.6), and an assert statement (§14.10) lexically nested within T (§8.1.3) is executed.

JLS # 12.5. Creation of New Class Instances

Whenever a new class instance is created, memory space is allocated for it with room for all the instance variables declared in the class type and all the instance variables declared in each superclass of the class type, including all the instance variables that may be hidden (§8.3).