[英]Java upcasting and downcasting by interfaces
This is probably a dumb question but I need to know. 这可能是一个愚蠢的问题,但我需要知道。 I have an interface as 我有一个界面作为
import com.amazonaws.services.dynamodbv2.AmazonDynamoDBAsync;
public interface AsyncClient extends AmazonDynamoDBAsync{
}
And I have a ClientCreator class which has the method 我有一个ClientCreator类,它有该方法
import com.company.clients.AsyncClient;
public class ClientCreator {
public static AsyncClient getAsyncClient() throws FileNotFoundException, IllegalArgumentException, IOException{
AmazonDynamoDBAsync client = new AmazonDynamoDBAsyncClient(getCredentials());
client.setRegion(getRegion());
return (AsyncClient)client;
}
.
.
.
Here AmazonDynamoDBAsyncClient implements AmazonDynamoDBAsync and AsyncClient extends AmazonDynamoDBAsync , but this code would not work and throws 这里AmazonDynamoDBAsyncClient实现了AmazonDynamoDBAsync,而AsyncClient扩展了AmazonDynamoDBAsync ,但是这段代码无法正常工作并抛出
com.amazonaws.services.dynamodbv2.AmazonDynamoDBAsyncClient cannot be cast to com.company.clients.AsyncClient com.amazonaws.services.dynamodbv2.AmazonDynamoDBAsyncClient无法强制转换为com.company.clients.AsyncClient
but why ? 但为什么呢?
Basically you've got the hierarchy like this: 基本上你有这样的层次结构:
AmazonDynamoDBAsync
^
|
-----------------------------
| |
AmazonDynamoDBAsyncClient AsyncClient
And you are trying to cast AmazonDynamoDBAsyncClient
instance to AsyncClient
, which isn't possible. 并且您正在尝试将AmazonDynamoDBAsyncClient
实例转换为AsyncClient
,这是不可能的。 Those are siblings. 那些是兄弟姐妹。 Take it like this: "Apple" and "Banana" are both "Fruit", but an "Apple" is not a "Banana". 这样说:“苹果”和“香蕉”都是“水果”,但“苹果”不是“香蕉”。
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