Java TreeMap故障

Question

我有一台TreeMap

final Map<UserSetting, ItemsIntersectionAndComplement> _usersSimilar = new TreeMap<UserSetting, ItemsIntersectionAndComplement>(new Comparator<UserSetting>() {

@Override
public int compare(UserSetting lhs, UserSetting rhs) {

int prec = (lhs.id()).compareTo(rhs.id());
return lhs.totalMatch > rhs.totalMatch ? -1 : (lhs.totalMatch < rhs.totalMatch ) ? 1 : prec;
}
});

当检查key是否存在时，它会返回false，但在特定的第n个位置，我看到了该项。这怎么可能呢？并且在上一次迭代中添加了项。

if (!_usersSimilar.containsKey(rating2.userSetting())) { // <-- CHECKING HERE EXISTENCE

    ItemsIntersectionAndComplement iiac = new ItemsIntersectionAndComplement();
    iiac.intersection.add(rating2.item());
    rating2.userSetting().totalMatch = 1;
    _usersSimilar.put(rating2.userSetting(), iiac);
} else {
  //.. SOME CODE, BUT CALL DOES NOT REACH IT
}

public class UserSetting extends _UserSetting {
    public UserSetting() {
    }

    int totalMatch;
}

Answer 1

if (!_usersSimilar.containsKey(rating2.userSetting())) { // <-- CHECKING HERE EXISTENCE

    ItemsIntersectionAndComplement iiac = new ItemsIntersectionAndComplement();
    iiac.intersection.add(rating2.item());
    rating2.userSetting().totalMatch = 1;
    _usersSimilar.put(rating2.userSetting(), iiac);
}

您在映射中搜索的键和您放入映射中的键具有不同的状态，因为您要在if子句中更新rating2.userSetting().totalMatch。

因此，您正在搜索的键很可能不存在于Map中，但是您要添加的键存在于Map中。

Answer 2

在TreeMap的Javadoc中，您可以读到：

请注意，与任何排序的映射一样，树映射维护的排序，无论是否提供显式比较器，如果该排序的映射要正确实现映射接口，则必须与equals一致。

由于您没有覆盖equals，因此在您的示例中，比较器与equals不一致。您必须为UserSetting类配备一个equals方法，例如：

public boolean equals(Object o) {
    if (this == o) {
        return true;
    }
    if (!(o instanceof UserSetting)) {
        return false;
    }
    UserSetting s = (UserSetting)o;
    return id == s.id && totalMatch = s.totalMatch;
}

Answer 3

Eran在上面的回答中是正确的。您的TreeMap已损坏，因为您正在更新totalMatch。这段代码再现了这个问题。

import java.util.Comparator;
import java.util.Map;
import java.util.TreeMap;

public class HashMapWithComparatorTest {
    public static void main(String[] args) {
        final Map<UserSetting, String> _usersSimilar = new TreeMap<UserSetting, String>(new Comparator<UserSetting>() {

            @Override
            public int compare(UserSetting lhs, UserSetting rhs) {

                int prec = (lhs.id()).compareTo(rhs.id());
                return lhs.totalMatch > rhs.totalMatch ? -1 : (lhs.totalMatch < rhs.totalMatch ) ? 1 : prec;
            }
        });

        UserSetting setting1 = new UserSetting(2, 1);
        UserSetting setting2 = new UserSetting(3, 1);
        UserSetting setting3 = new UserSetting(1, 1);
        UserSetting setting4 = new UserSetting(1, 1);
        UserSetting setting5 = new UserSetting(2, 2);

        _usersSimilar.put(setting1, "test");
        _usersSimilar.put(setting2, "test");
        _usersSimilar.put(setting3, "test");
        _usersSimilar.put(setting4, "test");


        System.out.println("Before touching totalMatch:");
        System.out.println(_usersSimilar.containsKey(setting1));
        System.out.println(_usersSimilar.containsKey(setting2));
        System.out.println(_usersSimilar.containsKey(setting3));
        System.out.println(_usersSimilar.containsKey(setting4));
        System.out.println(_usersSimilar.containsKey(setting5));

        setting1.totalMatch++;
        setting1.totalMatch++;

        setting2.totalMatch--;
        setting2.totalMatch--;

        setting3.totalMatch++;
        setting3.totalMatch++;

        setting4.totalMatch--;
        setting4.totalMatch--;

        setting5.totalMatch++;
        setting5.totalMatch++;

        System.out.println("After changing totalMatch:");
        System.out.println(_usersSimilar.containsKey(setting1));
        System.out.println(_usersSimilar.containsKey(setting2));
        System.out.println(_usersSimilar.containsKey(setting3));
        System.out.println(_usersSimilar.containsKey(setting4));
        System.out.println(_usersSimilar.containsKey(setting5));
    }

    private static class UserSetting {
        private int id;
        public int totalMatch;

        public Integer id() {
            return id;
        }

        public UserSetting(int id, int totalMatch) {
            this.id = id;
            this.totalMatch = totalMatch;
        }
    }
}