將String，ArrayList等對象添加到列表中<Object>

Question

我有一個數據結構Map < String, Map < ConfigKey, ConfigValue<Object > > >。這就像是一個Map的「名字空間」到Map的「鍵」到「值」。我的解析器讀取配置文件並動態決定是否將該值視爲String或ArrayList。

我看到錯誤添加ArrayList以上的數據結構。請告訴我如何解決這個問題。

private Map<String, Map<ConfigKey, ConfigValue<Object>>> configuration = new HashMap<String, Map<ConfigKey, ConfigValue<Object>>>(); 
: 
configuration.get(groupName).put(new ConfigKey(key), new ConfigValue<Object>(override, value)); // works 
: 
configuration.get(groupName).put(new ConfigKey(key), new ConfigValue<List>(override, Arrays.asList(values))); // does not work 

Answer 1

ConfigValue<List> is not a ConfigValue<Object>. 

Read more about covariance and contravariance in Java或programming in general.

這將修復它

private Map<String, Map<ConfigKey, ConfigValue<? extends Object>>> configuration = new HashMap<String, Map<ConfigKey, ConfigValue<? extends Object>>>(); 

或

Map<String, Map<ConfigKey, ConfigValue<?>>> 

請避免raw-types在你的c頌。