Java代碼執行順序-範例

 

class Insect...{ 
    private int i = 9; 
    int j; 
    Insect()...{ 
        System.out.println("i=" + i + ",j=" + j); 
        j=37; 
    } 
    private static int x1 = print("static Insect.x1.initialized"); 
    public static int print(String s)...{ 
        System.out.println(s);
        return 47;
    }
} 

class Bettle extends Insect...{ 
    private int k = print("Bettle.k initialized"); 
    public Bettle()...{
        System.out.println("k=" + k); 
        System.out.println("j=" + j); 
    } 
    private static int x2=print("static Insect.x2 initialized"); 
    public static void main(String [] args) ...{ 
        System.out.println("Bettle constructor"); 
        Bettle b = new Bettle();
    }
}

運行結果：
static Insect.x1.initialized
static Insect.x2 initialized
Bettle constructor
i=9,j=0
Bettle.k initialized
k=47
j=37

解釋：

Bettle運行需要先載入父類Insect,在載入Insect時先運行所有的Static代碼,print()函數建立,但不運行.
這一句會運行：private static int x1 = print("static Insect.x1.initialized");
父類載入完成後載入子類Bettle,同樣先初始化所有static代碼,於是先運行：private static int x2=print("static Insect.x2 initialized");
然後進入main(),運行System.out.println("Bettle constructor");
再運行Bettle b=new Bettle();建立對象時先建立父類,此時先把所有變數設為初始值(i=9,j=0),然後觸發父類constructor.
載入完父類再載入子類,同上先設定義值:private int k=print("Bettle.k initialized");然後運行建構函式