[英]Java casting a list of <object> to its interface
I have: 我有:
class Hammer Implements Part {
public String getName() { return name; }
public int getId() { return id; }
...
}
class Wrench implements Part {
public String getName() { return name; }
public int getId() { return id; }
...
}
interface Part {
String getName();
int getId();
}
I am using a prebuilt database manager written for Android SQLite that returns a list of Objects based on what I am retrieving: 我正在使用为Android SQLite编写的预构建数据库管理器,它根据我检索的内容返回一个对象列表:
dataManager().getWrenchDao().getAll(); // returns List<Wrench>
I cannot change how this function operates. 我无法改变这个功能的运作方式。
Now I am getting both lists like: 现在我得到两个列表:
List<Wrench> wrenches = dataManager().getWrenchDao().getAll();
List<Hammer> hammers = dataManager().getHammerDao().getAll();
However, I want to populate spinners with this data (Spinners are the drop down lists in android). 但是,我想用这些数据填充微调器(Spinners是android中的下拉列表)。
loadSpinner(Spinner s, List<Part> data) {
...
data.ElementAt(i).getName();
data.ElementAt(i).getId();
...
}
loadSpinner(wrenchSpinner, wrenches);
But it gives me a casting error that you cannot change Wrench to Part. 但它给了我一个铸造错误,你无法改变扳手到零件。 Why doesn't Java let me do this?
为什么Java不允许我这样做? Wrenches have all methods that Parts do so why cant I cast it into something it implements and use it?
扳手有各种方法,部件这样做,为什么我不能把它投入它实现和使用它的东西?
Error: 错误:
The method loadSpinner(Spinner, List<Part>) in the type NewActivity is not applicable for the arguments (Spinner, List<Wrench>)
You have to replace the second parameter of loadSpinner
to: 您必须将
loadSpinner
的第二个参数loadSpinner
为:
loadSpinner(Spinner s, List<? extends Part> data)
See also: When do Java generics require <? 另请参阅: Java泛型何时需要<? extends T> instead of <T> and is there any downside of switching?
扩展T>而不是<T>并且是否存在切换的缺点?
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