Question

我尝试创建具有泛型类的类，其中我创建函数来读取实体列表。我知道我有两种方法可以做到这一点：（＃1）通过循环读取每个实体或（＃2）直接创建实体地图。

第二种方式看起来更有效但不幸的是open不能使用泛型类型 - 当我在代码中使用它时会返回GenericTypeIndicator：

IllegalStateException: Unknown type encountered: T

所以：

有没有办法直接将通用类型的实体读入地图？（＃2）
或者只是在循环中读取实体时，它们在性能上没有任何显着差异（＃1）？

Map<String, T> entityMap =
    dataSnapshot.getValue(new GenericTypeIndicator<Map<String, T>>() {});

例如。数据：

public class SomeClass<T extends KeyEntity> {

    private final Class<T> mGenericClass;
    private final String mRefKey;

    public SomeClass(Class<T> clazz, String refKey) {
        mGenericClass = clazz;
        mRefKey = refKey;
    }

    public class KeyEntity {
        private String mKey;

        public KeyEntity() {}

        public String getKey() {
            return mKey;
        }

        public void setKey(String key) {
            mKey = key;
        }
    }

    public interface EntityListListener<T> {
        abstract public void onEntityListLoaded(List<T> entity);
    }

    public void loadAll(final EntityListListener<T> entityListListener) {
        DatabaseReference dbRef = FireDbUtils.getReference(mRefKey);
        dbRef.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
            List<T> entityList = new ArrayList<>((int) dataSnapshot.getChildrenCount());

            // #1 - read each entity in a loop
            for (DataSnapshot childSnapshot : dataSnapshot.getChildren()) {
                T entity = dataSnapshot.getValue(mGenericClass);
                entity.setKey(childSnapshot.getKey());
                entityList.add(entity);
            }

            // #2 - read all entity into a map
            // GenericTypeIndicator doesn't like generic type
            // if used, it returns "IllegalStateException: Unknown type encountered: T"
            Map<String, T> entityMap 
                = dataSnapshot.getValue(new GenericTypeIndicator<Map<String, T>>() {});
            for (Map.Entry<String, T> pair : entityMap.entrySet()) {
                T entity = pair.getValue();
                entity.setKey(pair.getKey());
                entityList.add(entity);
            }

            entityListListener.onEntityListLoaded(entityList);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {}
    });
}

Answer 1

使用ParameterizedType查找通用类对象

 ParameterizedType type = (ParameterizedType)this.getClass().getGenericSuperclass();
 //all generic classes      
 Type[] args = type.getActualTypeArguments();

 Class<T> genericClass = (Class<T>) args[0];

 T entity = dataSnapshot.getValue(genericClass);

＆＃34;遇到未知类型＆＃34;在Firebase GenericTypeIndicator中使用泛型类型时

1 个答案: