Difference between Class and Class<?>

Question

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

• Class a;
• Class<?> b;

Answer 1

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.

Answer 2

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.