I am mainly a C# developer and I was teaching Data Structures to my friend and they use Java in their University and I saw such an expression in Java: void printCollection(Collection<?> c) {...

the below method compiles without problems: static Stream<Optional<? extends Number>> getNumbers(Stream<Number> numbers) { return numbers.map(Optional::of); } yet if I add a sim...

Based on the information provided in the link, it says that: It's important to note that List<Object> and List<?> are not the same. You can insert an Object, or any subtype of Object...

Consider this case: class A {} class B<T extends A, E extends T> { B<?, A> b; B<?, ? extends A> b2; } As I understand type bounds, in this case effective upper bounds of bot...

I was reading about generics and I did not understand the need for unbound wildcards and how it differs from raw type. I read this question but still did not get it clearly. In the Java tutorial pa...

I have a getter returning a List with a wildcard: import java.util.List; public interface Foo { List<? extends Bar> getList(); } Where Bar is an other interface. When I write an asserti...

The Oracle doc about Wildcards in generics says, The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type (though i...

I believe that the type ? in generics is a specific unknown type. Which means, declaring let's say a list of that type would prevent us from adding any type of object into it. List<?> unknow...

I was playing around in the Scala REPL when I got error: unbound wildcard type. I tried to declare this (useless) function: def ignoreParam(n: _) = println("Ignored") Why am I getting this error...

I am new to Java. In this document they give this as a use case for using wildcard: static void printCollection(Collection c) { Iterator i = c.iterator(); for (int k = 0; k < c.size(); k++) {...

I did not get this code to compile either way: List<List> a = new ArrayList(); List<List<?>> b = new ArrayList(); a = b; // incompatible types b = a; // incompatible types...

A raw list converts to List<?> just fine. Why can't a list of raw lists convert to a list of List<?>? { // works List raw = null; List<?> wild = raw; } { // Type mismatch: cann...

How come one must use the generic type Map<?, ? extends List<?>> instead of a simpler Map<?, List<?>> for the following test() method? public static voi...

I was reading this article, , about subclassing a builder class. I understood the article but there was one small bit that bothered me. There was this method, public static Builder<?> builde...

Following on from this question, which provides a solution but doesn't explain it (unfortunately, the links in the answers are now dead): Take the following method: void method(Map<?, ?&...