In java what's the difference between ?, E, T [duplicate]

Question

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Java Generics

Hi,

Can anyone please explain the difference of the three and each proper usage?

Thanks, been googling but I'm still confused on how to use each.

czetsuya

Answer 1

I'm assuming you're talking about Generics. The 'E' and 'T' are placeholders and can be used interchangeably in class definitions. By convention 'E' is an Element and 'T' is a Type. The question mark is a placeholder for an unknown type. You often see things like this:

List<? extends MyObject> x;

This implies that 'x' is a list of objects that are subclasses of MyObject, but we don't know what they are exactly.

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

Answer 2

E, T, K, V, or any other generic type variables are just placeholders - they don't have any intrinsic association. You can even use lowercase letters for generic type variables, but conventionally you use a single capital letter. Read the generics tutorial from Sun.