What is the difference between 'E', 'T', and '?' for Java generics?

I come across Java code like this:

public interface Foo<E> {}

public interface Bar<T> {}

public interface Zar<?> {}

What is the difference among all three of the above and what do they call this type of class or interface declarations in Java?

Solution 1:

Well there's no difference between the first two - they're just using different names for the type parameter (E or T).

The third isn't a valid declaration - ? is used as a wildcard which is used when providing a type argument, e.g. List<?> foo = ... means that foo refers to a list of some type, but we don't know what.

All of this is generics, which is a pretty huge topic. You may wish to learn about it through the following resources, although there are more available of course:

• Java Tutorial on Generics
• Language guide to generics
• Generics in the Java programming language
• Angelika Langer's Java Generics FAQ (massive and comprehensive; more for reference though)

Solution 2:

It's more convention than anything else.

• T is meant to be a Type
• E is meant to be an Element (List<E>: a list of Elements)
• K is Key (in a Map<K,V>)
• V is Value (as a return value or mapped value)

They are fully interchangeable (conflicts in the same declaration notwithstanding).