排序程序的比较方法中出现“找不到符号”错误

Question

我有一个类任务，我们必须对包含 Integer、Double、Book 和 PlayingCard 对象的 ArrayList

代码如下：

import java.util.ArrayList;

public class Sorter {
    /**
     *  Checks two objects in the list to see what type they are,
     *  if they are the same type, use compareTo method to sort,
     *  otherwise, sort in order of Integer and Double > Book > PlayingCard.
     *
     * @param a  first object
     * @param b  second object
     * @return  a negative number if a < b, a positive number if a > b,
     *          0 if a = b
     */
    public static int compare(Object a, Object b) {
        if ((a instanceof Integer || a instanceof Double) && (b instanceof Integer || b instanceof Double)) {
            return a.compareTo(b);
        }
        else if ((a instanceof Integer || a instanceof Double) && (b instanceof Integer == false || b instanceof Double ==  false)) {
            return -1;
        }
        else if ((a instanceof Integer == false || a instanceof Double == false) && (b instanceof Integer || b instanceof Double)) {
            return 1;
        }
        else if ((a instanceof Book) && (b instanceof PlayingCard)) {
            return -1;
        }
        else if ((a instanceof PlayingCard) && (b instanceof Book)) {
            return 1;
        }
        else if ((a instanceof Book) && (b instanceof Book)) {
         return a.compareTo(b);
        }
        else {
         return a.compareTo(b);
        }
    }

/**
 * Sort a list of objects. Uses the selection sort algorithm.
 *
 * @param stuff  list of objects
 */
public static void sort(ArrayList<Object> stuff) {
    // selection sort
    for (int i = 0; i < stuff.size() - 1; i++) {
        int lowest = i;
        for (int j = 1; j < stuff.size(); j++) {
            if (compare(stuff.get(j), stuff.get(lowest)) < 0) {
                lowest = j;
            }
        }

        // swap to front
        if (lowest != i) {
            Object temp = stuff.get(i);
            stuff.set(i, stuff.get(lowest));
            stuff.set(lowest, temp);
        }
    }
}

/**
 * Main method. Populates an arraylist of stuff and sorts it.
 *
 * @param args  command-line arguments
 */
public static void main(String[] args) {
     ArrayList<Object> list = new ArrayList<>();

     list.add(8);
     list.add(new PlayingCard(PlayingCard.HEARTS, PlayingCard.TWO));
     list.add(3.5);
     list.add(new Book("Mark Twain", "The Adventures of Huckleberry Finn"));
     list.add(new Book("F. Scott Fitzgerald", "The Great Gatsby"));
     list.add(5.65);
     list.add(new PlayingCard(PlayingCard.CLUBS, PlayingCard.SEVEN));
     list.add(new PlayingCard(PlayingCard.SPADES, PlayingCard.ACE));

     System.out.println("Original List: \n" + list); //debugging help
     sort(list);
     System.out.println("Sorted List: \n" + list);
}

}

这是编译器错误：

  Sorter.java:16: error: cannot find symbol
                    return a.compareTo(b);
                            ^
  symbol:   method compareTo(Object)
  location: variable a of type Object
  Sorter.java:31: error: cannot find symbol
                     return a.compareTo(b);
                             ^
  symbol:   method compareTo(Object)
  location: variable a of type Object
  Sorter.java:34: error: cannot find symbol
                     return a.compareTo(b);
                             ^
  symbol:   method compareTo(Object)
  location: variable a of type Object
  3 errors

PlayingCard 的 compareTo 方法：

public int compareTo(PlayingCard other) {
      if (getSuit() < other.getSuit()) {
          return -1;
      }
      else if (getSuit() > other.getSuit()) {
          return 1;
      }
      else {
          if (getRank() < other.getRank()) {
              return -1;
          }
          else if (getRank() > other.getRank()) {
              return 1;
          }
          else {
              return 0;
          }
      }
  }

图书的 compareTo 方法：

public int compareTo(Book other) {
      if (getAuthor().compareTo(other.getAuthor()) < 0) {
          return -1;
      }
      else if (getAuthor().compareTo(other.getAuthor()) > 0) {
          return 1;
      }
      else {
          if (getTitle().compareTo(other.getTitle()) < 0) {
              return -1;
          }
          else if (getTitle().compareTo(other.getTitle()) > 0) {
              return 1;
          }
          else {
              return 0;
          }
      }
  }

Answer 1

您正在对 Object 类的实例调用 compareTo。如果你查看类 Object (https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html)，你会发现它没有这样的方法。这就是编译器告诉你的，尽管它使用“符号”这个词来表示方法或字段。

在调用 compareTo 之前，您需要将对象转换为 Integer 或 Double 的实例，如以下代码，然后您可以使用 Integer 的 compareTo 方法或 Double 的 compareTo 方法，具体取决于您拥有的内容：

if(a instanceof Double && b instanceof Double) {
     Double aDouble = (Double) a;
     Double bDouble = (Double) b;
     return aDouble.compareTo(b);
}

在这种情况下，练习很可能希望您将 Integer 与 Double 进行比较。 为此，您需要选择可以使用的 compareTo 方法，如果您研究转换和 int/double 的表示，那么您应该选择的方法是从 Integer 到 Double，因此您需要这样的方法之一选项：

if(a instanceof Integer && b instanceof Double) {
     Integer aInteger = (Integer) a;
      Double bDouble = (Double) b;
     return -bDouble.compareTo(aInteger.doubleValue()); //note the negative
}

这是另一种选择：

if(a instanceof Double && b instanceof Integer ) {
     Double aDouble = (Double) a;
      Integer  bInteger = (Integer ) b;
     return aDouble.compareTo(bInteger.doubleValue());
}