Considering the two functions:
public static void myFunction(List<?> list); public static <T> void myFunction(List<T> list); Please explain why we have included <T> before myFuction, and what does it signify?
1 Answer
When you use
<?>you are telling the virtual machine that you don't know what type of object will be in the List, could one type could be more than one type.List<?>is the equivalent ofListfrom previous 1.4 Java version when you could add to a list any type of object and there was no constraint on that.When you use
<T>you are defining a generic method. You are telling the compiler that theList<T>is a list of objet of typeT, which will determined from the call to the method, but you enforce that all object from List are of the same type,T. You include<T>before the the function name to specify that you are defining a generic method, and the compiler to know thatTshould be treated as a type and not as variable.
5 Comments
yshavit
You should use backticks ( ` ) when writing out generics, or else the site will treat them as unrecognized/unallowed HTML tags and strip them out.
yshavit
Np! And while I'm here, a small nit: in neither case do you tell the virtual machine anything -- all of that info is erased by the time the JVM runs. You're giving the information to the Java-to-bytecode compiler (javac or equivalent).
iullianr
you are right, I intended to refer to the compiler :). thanks again.
Prateek Joshi
What is the meaning of writing <T> before void ?
yshavit
@PrateekJoshi It just declares that
T is the name of a generic parameter. You can think of it as "for some type T". If you haven't read the generics tutorial, I would recommend it: docs.oracle.com/javase/tutorial/java/generics
public static <T> void myFunction(List<T> list);(<T>was placed before return type).