类型不匹配：无法从映射转换为映射

Question

我在解决泛型问题时遇到了一些麻烦。我有一个“猫”对象列表和一个“狗”对象列表，我需要将它们传递到同一个方法中。该方法的返回类型是一个“字符串”和“动物列表”的映射，我试图找出一种方法来将带有动物列表的映射转换为带有猫或狗列表的映射。

这工作很好，如果我有一个单独的方法猫和狗，但我正在寻找一个更灵活的解决方案。

标题中出现错误的行：

catMap = PetStore.groupAnimalsByOwner(cats); 
dogMap = PetStore.groupAnimalsByOwner(dogs);

注意：这是一个简化的例子，我必须能够使用地图中的列表作为“猫”或“狗”对象。

    public class Start {

    public static void main(String[] args) {

        Cat cat1 = new Cat("Jerry", "cat1");
        Cat cat2 = new Cat("Jerry", "cat2");
        Cat cat3 = new Cat("Fred", "cat3");

        List<Cat> cats = new LinkedList<Cat>();
        cats.add(cat1);
        cats.add(cat2);
        cats.add(cat3);

        Dog dog1 = new Dog("Frank", "dog1");
        Dog dog2 = new Dog("Jerry", "dog2");
        Dog dog3 = new Dog("Bob", "dog3");

        List<Dog> dogs = new LinkedList<Dog>();
        dogs.add(dog1);
        dogs.add(dog2);
        dogs.add(dog3);

        Map<String, List<Dog>> dogMap = new HashMap<String, List<Dog>>();
        Map<String, List<Cat>> catMap = new HashMap<String, List<Cat>>();

        // catMap should have 2 key/value pairs - key "Jerry" with a list containing cat1 and cat2
        // and a pair - key "Fred" with a list containing only cat3
        catMap = PetStore.groupAnimalsByOwner(cats); 

        // dogMap should have 3 key/value pairs - key "Frank" with a list containing dog1
        // key "Jerry" with a list containing dog2
        // Key "Bob" with a list containing dog3
        dogMap = PetStore.groupAnimalsByOwner(dogs);

    }

}

public class PetStore {

    //Grouping by owner
    public static Map<String, List<Animal>> groupAnimalsByOwner(List<? extends Animal> animals) {
        Map<String, List<Animal>> groupedMap = new HashMap<String, List<Animal>>();
        List<Animal> tempList = null;

        for (Animal summary : animals) {
            String consolidatedInvoiceId = summary.getOwner();
            tempList = groupedMap.get(consolidatedInvoiceId);
            if (tempList == null) {
                tempList = new LinkedList<Animal>();
            }
            tempList.add(summary);
            groupedMap.put(consolidatedInvoiceId, tempList);
        }

        return groupedMap;
    }
}

public interface Animal {

    public String getOwner();
}

  public class Cat implements Animal {

    private String owner;
    private String name;

    public Cat(String owner, String name) {
        this.owner = owner;
        this.name = name;
    }


    @Override
    public String getOwner() {
        return owner;
    }

    public String getName() {
        return name;
    }

    public void doCatStuff() {
        System.out.println("Do cat stuff");
    }

}

提前谢谢你。

Answer 1

正如列表 不是列表 一样，映射 > 也不是映射 。

将GroupAnimalsByOwner方法设置为泛型，并将Animal作为上限，以便T推断为Cat（或Dog)。您需要在方法的正文中将Animal替换为t。

public static <T extends Animal> Map<String, List<T>> 
         groupAnimalsByOwner(List<? extends T> animals)
{
    Map<String, List<T>> groupedMap = new HashMap<String, List<T>>();
    List<T> tempList = null;

    for (T summary : animals) {
        String consolidatedInvoiceId = summary.getOwner();
        tempList = groupedMap.get(consolidatedInvoiceId);
        if (tempList == null) {
            tempList = new LinkedList<T>();
        }
        tempList.add(summary);
        groupedMap.put(consolidatedInvoiceId, tempList);
    }

    return groupedMap;
}