Difference between Class and Class<?>

Question

What is the difference between a Class and a Class<?> declaration.

• Class a;
• Class<?> b;

Answer 1

Class javadoc:

Type Parameters: T - the type of the class modeled by this Class object. For example, the type of String.class is Class<String>. Use Class<?> if the class being modeled is unknown.

Use of Class without the type parameter is similar to using any generic class (Map, List, etc.) without the type parameter - either it's a pre-1.5 legacy usage or it's just a segment of code that does not care about unchecked type casting.

Answer 2

Class<?> shows that you're intentionally writing Java 5-level code that doesn't know or care what class you are dealing with. Leaving out the makes it look like old code or code written by someone who hasn't learned generics yet.

Answer 3

It's the same as with all generic and raw types:

Class          // An unknown class (raw type)
Class<?>       // An unknown class (generic version)
Class<String>  // The String class

In this special case there's no much practical difference between Class and Class<?> because they both denote an unknown class. Depending on the existing declarations the compiler can demand a generic type instead of a raw type.

But: Since Java 1.5 you should use the generic form wherever possible. Class<?> clearly states that you mean "an unknown class", Class<String> cleary states that you mean the String class. A raw Class could mean both.

In the end it makes not much of a difference to the compiler but it makes a huge difference in making the intentions of your code more understandable and maintainable.