Using generic method parameters in Java

Question

I'm trying to use generic methods in java for the first time. It isn't working, and it appears to me that Java generics are simply not meant to be used this way - that I will need to implement a common interface rather then using a generic method. Please confirm (or deny) that conclusion.

This is what I'm trying to do:

public <T> void updateItem( T item ) {
  String itemKey = item.getKey();
  ...

This gives me the error 'The method getKey() is undefined for the type T'.

Looking into why this doesn't work I see that type erasure "replaces all type parameters in generic types with their bounds or Object if the type parameters are unbounded".

The only way I can 'bound' my type T is if I create a common subclass or interface for all the types I was planning to use, and if I do that then I can use polymorphism rather then generics.

What is T supposed to be? You could use <T extends SomeClassWithGetKey> — Supericy
– Supericy, Commented Apr 4, 2013 at 17:06
I have this idea (from Python or C++) that I don't have to specify what T is. T will be type checked when it is invoked with a type. — Tom
– Tom, Commented Apr 4, 2013 at 17:07
@Tom generics are a compile time feature in Java. It is type checked at compile time, not when it is invoked, hence it has to be resolvable at compile time. — Affe
– Affe, Commented Apr 4, 2013 at 17:11

AlexR · Accepted Answer · 2013-04-04 17:07:39Z

5

This is right. Method getKey() is not defined in class T. However if you define your method as following:

public <T extends Entry> void updateItem( T item ) {
  String itemKey = item.getKey();

the compiler can find getKey() method. Please pay attention that I used <T extends Entry> (I mean java.util.Map.Entry). This class has method getKey() and compiler can resolve it.

answered Apr 4, 2013 at 17:07

AlexR

116k16 gold badges137 silver badges216 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Tom Over a year ago

Yeah, I had this idea (from Python and C++) that the compiler wouldn't try to resolve the call until invocation (or runtime in Python) so as long as it worked as used it was fine.

Tom Over a year ago

The odd thing (to me) is that, if T has a common base class (Entry above) then I don't need generics at all.

Paul Bellora Over a year ago

@Tom That's corrent - just have the method take Entry for example. Sometimes polymorphism is enough and you don't need generics.

Collectives™ on Stack Overflow

Using generic method parameters in Java

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related