What does the question mark in java mean?

Question

I have encounter a code snippt

putAll(Map<? extends K, ? extends V> map)

on the Android developer site, I know that the K and V are placeholders, but what does the question mark ? mean? Does it mean that the param must be a reference type or something else?

Answer 1

In Java ? is known as Wildcard, you can use it to respresent an unknown type.

The upper bounded wildcard, , where Foo is any type, matches Foo and any subtype of Foo. The process method can access the list elements as type Foo:

public static void process(Map<? extends A> list) {
  /* code */
}

In your case it is known as Upper bounded wildcard.

http://docs.oracle.com/javase/tutorial/java/generics/upperBounded.html

putAll(Map<? extends K, ? extends V> map)

It means, that any object, that can extend the A class is applicable in this conditionn.

Answer 2

It's a wild card for class type, in your case

putAll(Map<? extends K, ? extends V> map)

the first question mark indicates that any type which extends K is applicable, and so on.

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