成員方法的初始化

• 在類的內部，變量的定義順序決定了初始化的順序。創建一個對象時，會先初始化對象中的成員變量，然后才會初始化方法（包括構造器）。
• 成員變量 ==> 方法
  x

class Window {
    Window (int marker) {
        System.out.println("Window: " + marker);
    }
}
class House {
    Window w1 = new Window(1);
    House () {
        System.out.println("init house");
        w3 = new Window(4);
    }
    Window w2 = new Window(2);
    Window w3 = new Window(3);
}
public class OrderOfInitialization {
    public static void main (String[] args) {
        House house = new House();
    }
}

$ javac OrderOfInitialization.java 
$ java OrderOfInitialization
Window: 1
Window: 2
Window: 3
init house
Window: 4

靜態成員的初始化

• 靜態成員變量的初始化要優先于成員變量，但是靜態成員變量只有在必要的時候才會初始化。
• 如果不創建對象，或者不通過類名直接調用，靜態成員永遠也不會初始化。
• 靜態成員變量只會初始化一次，就是在第一次創建對象時，后面無論創建多少次該對象，都不會再次初始化。
• 靜態成員變量 ==> 成員變量 ==> 方法

class Bowl {
    Bowl(int marker) {
        System.out.println("Bowl class marker: " + marker);
    }
    void method(int marker) {
        System.out.println("Bowl method: " + marker);
    }
}

class Table {
    static Bowl bowl1 = new Bowl(1);
    Table() {
        System.out.println("Table class marker");
        bowl2.method(1);
    }
    void method(int marker) {
        System.out.println("Table method: " + 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 class marker");
        bowl4.method(1);
    }
    void method(int marker) {
        System.out.println("Cupboard method: " + marker);
    }
    static Bowl bowl5 = new Bowl(5);
}

public 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.method(1);
        cupboard.method(1);
    }
    static Table table = new Table();
    static Cupboard cupboard = new Cupboard();
}

$ javac StaticInitialization.java
$ java StaticInitialization
Bowl class marker: 1
Bowl class marker: 2
Table class marker
Bowl method: 1
Bowl class marker: 4
Bowl class marker: 5
Bowl class marker: 3
Cupboard class marker
Bowl method: 1
Creating new Cupboard in main
Bowl class marker: 3
Cupboard class marker
Bowl method: 1
Creating new Cupboard in main
Bowl class marker: 3
Cupboard class marker
Bowl method: 1
Table method: 1
Cupboard method: 1

顯式的靜態初始化

• 靜態代碼塊初始化的優先級要高于靜態變量，只要對該類發送了消息。
• 無論是通過new關鍵字對該類進行了初始化，還是通過Cups.cup1直接調用，靜態代碼塊都會進行初始化。

class Cup {
    Cup(int marker) {
        System.out.println("Cup class marker: " + marker);
    }
    void method(int marker){
        System.out.println("Cup method: " + marker);
    }
}
class Cups {
    static Cup cup1;
    static Cup cup2;
    static {
        cup1 = new Cup(1);
        cup2 = new Cup(2);
    }
    Cups (){
        System.out.println("Cup class");
    }
}
public class ExplicitStatic {
    public static void main(String[] args) {
        System.out.println("Inside main");
        Cups.cup1.method(1);
    }
    //static Cups cups1 = new Cups();
    //static Cups cups2 = new Cups();
}

$ javac ExplicitStatic.java
$ java ExplicitStatic
Inside main
Cup class marker: 1
Cup class marker: 2
Cup method: 1