繼承後的成員變數

class Father{
        int money = 500;
        void spendMoney(int n){
                money -= n;
                System.out.println("Father spend " +n+ ",remain " +money);
        }
}
class Son extends Father{ }

public class Money{
        public static void main(String[] args){
                Father father = new Father();
                Son son = new Son();

                System.out.print("Before father spend money, son has ");
                System.out.println(son.money+ ".");
                father.spendMoney(250);
                System.out.print("After father spend money, son has ");
                System.out.println(son.money+ ".");
        }
}

output:

Before father spend money, son has 500.

Father spend 250,remain 250

After father spend money, son has 500.

自父類別繼承而來的實體變數，會變成自己的實體變數，與父類別無關。

========================================================

class Father{
        int money = 500;
        void spendMoney(int n){ money -= n; }
}
class Son extends Father{
        void spendMoney(int n){
                money -= n;
                super.spendMoney(n);    //這裡呼叫父函式來用，並不是用父物件去呼叫函式
        }
}

public class Money2{
        public static void main(String[] args){
                Father father = new Father();
                Son son = new Son();

                System.out.print("Before son spend money, son has ");
                System.out.println(son.money+ ".");
                son.spendMoney(250);
                System.out.print("After son spend money, son has ");
                System.out.println(son.money+ ".");
                System.out.println("Father has " +father.money);
        }

output:

Before son spend money, son has 500.

After son spend money, son has 0.

Father has 500

呼叫super.spendMoney(n)的意思是，Son物件呼叫父函式(spendMoney())來改變本身value的值，並不是叫Father物件呼叫自已的spendMoney()。因此父物件的value的值並不會變成250。

==================================================



class Father{
        int money = 500;
        void spendMoney(int n){ money -= n; }
}      
class Son extends Father{
        void spendMoney(int n){ super.spendMoney(n); }
        int getMoney(){ return super.money; }
}
           
public class Money2{
        public static void main(String[] args){;
                Son son = new Son();
             
                System.out.print("Before son spend money, he has ");
                System.out.println(son.money+ ".");
                son.spendMoney(250);
                System.out.print("After son spend money, he has ");
                System.out.println(son.getMoney()+ ".");
        }
}

output:

Before son spend money, he has 500.
After son spend money, he has 250.

這裡Son類別內的方法全是存取自Father類別內的方法，仍可正常運作，不會因沒有Father實體而不能存取其實體成員；所以應該可以這樣想：Son類別繼承了Father的field：money、spendMoney()，雖然在Son把spendMoney()給覆寫了，但在呼叫super.spendMoney()時，在編譯器眼裡其實是長這樣：spendMoney(250){ money -=250;}

所以我可以大膽的說，在Son繼承父類別時，在Son裡的spendMoney方法其實有兩個：
void spendMoney(int n){ money -= n; } 跟 void spendMoney(int n){ super.spendMoney(n); }
當呼叫super.spendMoney(n)時，就是呼叫自己的spendMoney(int n){ money -= n; }。
至於為什麼可以這樣想呢？因為啊，就Father類別在程式碼內沒有建立實體啊，所以它的spendMoney()根本不存在啊。Son因為繼承了Father，而且又建立了實體，所以當呼叫super.spendMoney()時，其實就是在跟編譯器說：“嘿，幫我跑一下spendMoney函式，不是跑我覆寫的那一個，是我老爸留給我的那一個spendMoney喔！謝謝！“

====================================================


class Father{
        int money = 500;
        void spendMoney(int n){ money -= n; }
}
class Son extends Father{
        int money = 250;
        void spendMoney(int n){ super.spendMoney(n); }
        void getFathersMoney(Father f){ money = f.money; }
}

public class Money3{
        public static void main(String[] args){;
                Son son = new Son();

                System.out.println("Son has " +son.money+ ".");
                son.getFathersMoney(son);
                System.out.println("Son has " +son.money+ ".");
        }
}

output:

Son has 250.
Son has 500.

這段程式碼是說，兒子偽裝成爸爸去銀行提爸爸的錢。