java - 一个类可以有一个映射属性，其中 Class 对象作为键，作为该类对象列表的值吗？

Question

我想要编译这样的东西:

public class RepositoryService {
   public Map<Class<? extends T>, List<T>> repository;

   public RepositoryService() {
      repository = new HashMap<>(); // doesn't matter which implementation
      repository.put(Integer.class, Arrays.asList(1, 2, 3));
      repository.put(String.class, Arrays.asList("A", "B", "C"));
   }

   public List<T> get(Class clazz) {
     return repository.get(clazz);
   }
}

这可能吗？

Answer 1

您可以通过 RepositoryService 上确保类型匹配的实例方法仅添加到映射来做得更好。

import static java.util.Arrays.asList;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class RepositoryService {
    private Map<Class<?>, List<?>> repository = new HashMap<>();

    @SuppressWarnings("unchecked")
    public <T> List<T> put(Class<T> key, List<T> instances) {
        return (List<T>) repository.put(key, instances);
    }

    @SuppressWarnings("unchecked")
    public <T> List<T> get(Class<T> key) {
        return (List<T>) repository.get(key);
    }

    public static void main(String[] args) {
        RepositoryService repositoryService = new RepositoryService();
        repositoryService.put(Integer.class, asList(1, 2, 3));
        repositoryService.put(String.class, asList("A", "B", "C"));
        repositoryService.put(String.class, asList(1, 2, 3)); // doesn't compile
        List<Integer> integers = repositoryService.get(Integer.class);
        List<String> strings = repositoryService.get(String.class);
        List<Double> doubles = repositoryService.get(String.class); // doesn't compile
        System.out.println(integers);
        System.out.println(strings);
    }
}