静态和非静态成员数据的初始化和构造函数的顺序

Question

I am beginner in Java and stuck with simple concept of initialization of data and constructor calls when there is static member available in definition of class.

Code:

class Bowl {
    Bowl(int marker) {
        System.out.println("Bowl(" + marker + ")");
    }
    void f1(int marker) {
        System.out.println("f1(" + marker + ")");
    }
}
class Table {
    static Bowl bowl1 = new Bowl(1);
    Table() {
        System.out.println("Table()");
        bowl2.f1(1);
    }
    void f2(int marker) {
        System.out.println("f2(" + marker + ")");
    }
    static Bowl bowl2 = new Bowl(2);
}
class Cupboard {
    Bowl bowl3 = new Bowl(3);
    static Bowl bowl4 = new Bowl(4);
    Cupboard() {
        System.out.println("Cupboard()");
        bowl4.f1(2);
    }
    void f3(int marker) {
        System.out.println("f3(" + marker + ")");
    }
    static Bowl bowl5 = new Bowl(5);
}
class StaticInitialization {
    public static void main(String[] args) {
        System.out.println("Creating new Cupboard() in main");
        new Cupboard();
        System.out.println("Creating new Cupboard() in main");
        new Cupboard();
        table.f2(1);
        cupboard.f3(1);
    }
    static Table table = new Table();
    static Cupboard cupboard = new Cupboard();
}

Output:

Bowl(1)
Bowl(2)
Table()
f1(1)
Bowl(4)
Bowl(5)
Bowl(3)
Cupboard()
f1(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f1(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f1(2)
f2(1)
f3(1)

I am unable to understand the flow of code and there is not much explanation in the book where this code appeared except that static members get initialized when they have not been initialized before and when it is necessary. Any leads?

Answer 1

When your program starts,

1) StaticInitialization class is loaded into memory and its static fields are initialized first in the order they are described

2) Static field table is initialized, that's why Table class is loaded into memory and all of its static fields are initialized.

3) Static field bowl1 is initialized, that's why Bowl class is loaded into memory but it has no static fields, so the constructor is called and prints Bowl(1)

4) Static field bowl2 is initialized, but Bowl class is already loaded, so its constructor is simply called and prints Bowl(2)

5) Table constructor is called, prints Table() and calls method f1(1)

6) Static field cupboard in StaticInitialization class is initalized, that's why Cupboard class is loaded into memory and all of its static fields are initialized.

7) Static field bowl4 is initialized, but Bowl class is already loaded, so Bowl constructor is called and prints Bowl(4)

8) Same with bowl5 static field.

9) Then, after all static fields of Cupboard are initialized, class members are initialized, that is bowl3 field. So, Bowl constructor is called and prints Bowl(3)

10) Cupboard constructor is called, prints Cupboard() and calls f1(2) method

11) Main method starts to work. Prints Creating... and creates new Cupboard object, so class members of Cupboard are initialized. That is, bowl3 field is initialized within this new object and its constructor is called, so Bowl(3) is printed.

12) Constructor of this new Cupboard object is called, it prints Cupboard() and calls f1(2) method of bowl3 object

13) Steps 11 and 12 are repeated. (Main prints Creating... and creates new Cupboard object)

14) f2(1) and f3(1) methods are called one by one on different objects

Just remember that static fields are initialized once when the class loads into memory and class members are initialized by default when the object is created.