[問題] 繼承與多型的新手問題

[wtchen (沒有存在感的人)]

最近在自學data structure的分數運算(真的是自學，非作業)
我c++已有很久經驗，可是到了java(剛開始碰)很多轉不過來，
(應版主要求把程式貼全，並重新發問)

class Rational
{
  protected long numerator;
  protected long denominator;

  public Rational()
  {
    // assume the rational number is 0
    numerator = 0;
    denominator = 1;
  }

  public Rational(long i)
  {
    numerator = i;
    denominator = 1;
  }

  public Rational(long num,long denom)
  {
    numerator = num;
    denominator = denom;
  }

  public void setRational(long i,long j)
  {
    numerator = i;
    denominator = j;
  }

  public void print()
  {
    System.out.println("Rational is :" + numerator + "/" + denominator);
  }


  public void error(String message)
  {
    System.out.println(message);
  }

  public void reduce()
  {
    long a, b, rem, sign;

    sign = 1;
    if ( numerator == 0 )
    {
      denominator = 1;
    }else if ( numerator == denominator )
    {
      denominator = 1;
      numerator = 1;
    }else if ( numerator < 0 && denominator < 0 )
    {
      numerator = -numerator;
      denominator = -denominator;
    }else if ( numerator < 0 )
    {
      numerator = -numerator;
      sign = -1;
    }else if ( denominator < 0)
    {
      denominator = -denominator;
      sign = -1;
    }

    if (numerator>denominator)
    {
      a = numerator;
      b = denominator;
    }else
    {
      a = denominator;
      b = numerator;
    }
    while (b != 0)
    {
      rem = a%b ;
      a = b ;
      b = rem ;
    }
    numerator = sign * numerator / a ;
    denominator = denominator / a ;
  }

  public Rational add(Rational r)
  {
    long k, denom, num;
    Rational rnl = new Rational() ;

    reduce();
    r.reduce();

    rnl.setRational(denominator, r.denominator);
    rnl.reduce();
    k = rnl.denominator;
    denom = denominator * k;
    num = numerator * k + r.numerator*denom/k;

    rnl.setRational(num, denom);
    rnl.reduce();

    return rnl;
  }

  public Rational add(long i)
  {
    Rational r = new Rational() ;
    r.setRational(i,1);
    return add(r);
  }

  public Rational sub(Rational r)
  {
    long k, denom, num;
    Rational rnl = new Rational() ;

    reduce();
    r.reduce();

    rnl.setRational(denominator, r.denominator);
    rnl.reduce();
    k = rnl.denominator;
    denom = denominator * k;
    num = numerator * k - r.numerator*denom/k;

    rnl.setRational(num, denom);
    rnl.reduce();

    return rnl;
  }

  public Rational mult(Rational r)
  {
    reduce();
    r.reduce();

    Rational rnl1 = new Rational(numerator, r.denominator);
    Rational rnl2 = new Rational(r.numerator, denominator);
    Rational rnl3 = new Rational();

    rnl1.reduce();
    rnl2.reduce();

    rnl3.setRational(rnl1.numerator*rnl2.numerator,
rnl1.denominator*rnl2.denominator);

    return rnl3;
  }

  public Rational divide(Rational r)
  {
    reduce();
    r.reduce();
    Rational rnl = new Rational(r.denominator, r.numerator);

    return mult(rnl);
  }

  public int equal(Rational r)
  {
    reduce();
    r.reduce();
    if (numerator == r.numerator && denominator == r.denominator)
    {
      return 1;
    }else
    {
      return 0;
    }
  }
}

class Rational2 extends Rational
//這邊是想用Rational2繼承Rational後增加一個setRational
{
  public void setRational(long num)
  {
    numerator = num;
    denominator = 1;
  }


  public Rational2() //純粹繼承
  {
    super();
  }

  public Rational2(long i)
  {
    super(i);
  }

  public Rational2(long num,long denom)
  {
    super(num, denom);
  }

  public Rational2 divide(Rational2 r) //這邊會出問題，不寫一樣不能編譯
  {
    super(r);
  }

}

public class Integer
{
  public static void main(String []args)
  {
    Rational2 a = new Rational2() ;
    a.setRational(18,69);
    Rational2 b = new Rational2(22,8);
//    Rational2 c = a.divide(b) ;

    b.print( );
    System.out.println(a.equal(b));
    System.out.println(System.getProperty("os.arch"));
//    c.error("test");
  }
}

編譯錯誤訊息：
Integer.java:211: error: incompatible types
    Rational2 c = a.divide(b) ;
                          ^
  required: Rational2
  found:    Rational
1 error

請問這種情況下要怎麼改才對？
我只是在練習c++/java之間的對換，謝謝
當 Rational2 繼承 Rational以後
原來的Rational divide(Rational)要怎麼讓Rational2也能使用？
(就是Rational2 divide(Rational2) 跟 Rational2 divide(Rational)都能用？)


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◆ From: 86.200.55.194
推 haha02:單就這個編譯錯誤: public Rational2 divide(Rational r)
→ haha02:算了當我沒說好了 剛剛沒仔細看 癥結點貌似不在這邊
→ wtchen:爬文爬很久沒找到該怎麼做
推 haha02:單就這邊貼出來的程式碼其實還有兩三個問題會發生
→ wtchen:當Class ABC裏面有 ABC xxx(ABC yyy)的時候要怎麼做繼承
※ 編輯: wtchen          來自: 86.200.55.194        (01/22 18:34)
推 haha02:直接會繼承啊不用特別寫 看起來你是想做override?
→ wtchen:我是想搞清楚繼承的概念
→ wtchen:class Rational2 extends Rational 後在Rational2增加一個
※ 編輯: wtchen          來自: 86.200.55.194        (01/22 20:13)
※ 編輯: wtchen          來自: 86.200.55.194        (01/22 20:15)
推 haha02:用 Rational [變數] = new Rational2() 就可以了吧?
推 haha02:另外java不允許arg與名稱一樣但是回傳值不一樣的method共存
→ wtchen:原來是這樣，感謝說明。
推 psliurt:C++也有繼承觀念阿，還有所謂的virtual table的樣子