Java Map<String,capture#1-of 类型中的方法？extends Object> 不适用
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Method in the type Map<String,capture#1-of ? extends Object> is not applicable
提问by Emaborsa
I have the following JAVAmethod implemented due a interface:
JAVA由于接口，我实现了以下方法：
public String importDocument(ImportSource source, Map<String, ? extends Object> paramMap);
When i try to do following, i get an compile warning. Snippet:
当我尝试执行以下操作时，会收到编译警告。片段：
paramMap.put("Key", "Value");
Error:
错误：
The method put(String, capture#1-of ? extends Object) in the type Map is not applicable for the arguments (String, String)
类型 Map 中的 put(String, capture#1-of ? extends Object) 方法不适用于参数 (String, String)
Why?
为什么？
采纳答案by Aniket Thakur
? extends Object
You are using generic wildcard. You cannot perform add operation as class type is not determinate. You cannot add/put anything(except null).
您正在使用通用通配符。您不能执行添加操作，因为类类型不确定。您不能添加/放置任何内容（空值除外）。
For more details on using wildcard you can refer oracle docs.
有关使用通配符的更多详细信息，您可以参考 oracle文档。
Collection<?> c = new ArrayList<String>();
c.add(new Object()); // Compile time error
Since we don't know what the element type of c stands for, we cannot add objects to it. The add()method takes arguments of type E, the element type of the collection. When the actual type parameter is ?, it stands for some unknown type. Any parameter we pass to add would have to be a subtype of this unknown type. Since we don't know what type that is, we cannot pass anything in. The sole exception is null, which is a member of every type.
因为我们不知道 c 的元素类型代表什么，所以我们不能向它添加对象。该add()方法采用 的参数type E，即集合的元素类型。当实际类型参数为 时?，它代表 some unknown type。我们传递给 add 的任何参数都必须是这种未知类型的子类型。因为我们不知道那是什么类型，所以我们不能传入任何东西The sole exception is null, which is a member of every type。。
