java - 我可以定义 Map> 以便我可以从 map 中 get() 列表而无需未经检查的转换？

Question

我有以下 Java 代码来获取与多个位置的对象相关的多个集合:

Map<String, List<? extends BaseView>> additionalCollections = deployEnvironmentService.getInputPageInfo(getProjectOid());
List<MachineView> machineViews = (List<MachineView>) additionalCollections.get("machineViewCollection");
List<LevelView> levelViews = (List<LevelView>) additionalCollections.get("levelViewCollection");
List<ToolView> toolViews = (List<ToolView>) additionalCollections.get("toolViewCollection");
List<BuildEnvironmentView> buildEnvironmentViews = (List<BuildEnvironmentView>) additionalCollections
                .get("buildEnvironmentViewCollection");

集合的名称和数量根据我使用的服务而有所不同，但集合始终继承自 BaseView。集合的检索方法与此类似:

public Map<String,List<? extends BaseView>> getInputPageInfo(Integer projectOid) {
    Map<String,List<? extends BaseView>> allInfoMap = new HashMap<String,List<? extends BaseView>>();
    PersistenceBroker broker = PersistenceBrokerFactory.createPersistenceBroker();
    try {
        allInfoMap.put("levelViewCollection", getLevelViewCollection(broker, projectOid));
        allInfoMap.put("machineViewCollection", getMachineViewCollection(broker));
        allInfoMap.put("toolViewCollection", getToolViewCollection(broker, projectOid));

        allInfoMap.put("buildEnvironmentViewCollection", getBuildEnvironments(broker, projectOid));

    } finally {
        broker.close();
    }
    return allInfoMap;
}

我遇到的问题是，我总是在第一个片段中的 Map.get() 方法上收到警告，显示 Type safety: Unchecked cast from List<capture#1-of ? extends BaseView> to Collection<LevelView> 。有办法解决这个警告吗？我尝试定义类型 <T extends BaseView> ，但是我找不到在函数调用本身中使用该类型 T 的方法。

Answer 1

将 map 包装在“查找”对象中，然后输入键:

public final class PageInfoKey<T extends BaseView> {
    public static final PageInfoKey<MachineView> MACHINE_VIEWS
            = new PageInfoKey<>("machineViews", MachineView.class);
    /*... more static final keys defined here ...*/

    /* Adding the 'name' and 'type' fields,
     * and equals() and hashCode() as below,
     * allow us to have multiple instances of keys
     * defined elsewhere if desired, which should still
     * work with the lookup.
     *
     * If all keys are defined statically inside this class
     * (i.e. constructor is made private), then these fields
     * and methods are unnecessary.
     */

     private final String name;
     private final Class<T> type;

     public PageInfoKey(String name, Class<T> type) {
         this.name = name;
         this.type = type;
     }

     @Override
     public int hashCode() {
         return Objects.hash(name, type);
     }

     @Override
     public boolean equals(Object other) {
         if(other==this) return true;
         if(other instanceof PageInfoKey) {
             PageInfoKey<?> o = (PageInfoKey<?>)other;
             return Objects.equals(this.name, o.name) &&
                     Objects.equals(this.type, o.type);
         }
         return false;
     }
}

public class PageInfoLookup {

    private final Map<PageInfoKey<?>, List<? extends BaseView>> data = new HashMap<>();

    public <T extends BaseView> void put(PageInfoKey<T> key, List<T> value) {
        data.put(key, value);
    }

    @SuppressWarnings("unchecked")
    public <T extends BaseView> List<T> get(PageInfoKey<T> key) {
        return (List<T>) data.get(key);
    }

}

现在您可以通过传入静态定义的键来获取 View :

List<MachineView> machineViews = lookup.get(PageInfoKey.MACHINE_VIEWS);

一切都是类型安全的，并在编译时进行检查，前提是您仅通过查找类中的公共(public)方法修改/访问映射。也不需要只在 PageInfoKey 类中定义键 - 只要该类上有合理定义的 equals 和 hashCode 方法即可，您可以在任何地方定义新实例，包括在依赖库中。

如Aleksander指出，如果您知道映射中最多只会有一个特定类型的列表，那么您可以使用 Class 本身作为键。