类型Map <String中的方法，捕获＃1-of？ extends Object>不适用

Question

I have the following JAVA method implemented due a interface:

public String importDocument(ImportSource source, Map<String, ? extends Object> paramMap);

When i try to do following, i get an compile warning. Snippet:

paramMap.put("Key", "Value");

Error:

The method put(String, capture#1-of ? extends Object) in the type Map is not applicable for the arguments (String, String)

Why?

Answer 1

? 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).

For more details on using wildcard you can refer oracle docs .

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 .