Define an interface using Multiple bound Types of Java Generics

Question

I am relatively scaled at beginner level of knowledge in Advanced Java Generics. I wanted to define a interface something like this

public interface Transformer {
 <T extends String & List<String>> T transform(String input) throws            
  IOException;
}

My Implementation Class A is shown as below:

public Class A implements Transformer{
   ....
   ....

  @Override
  public <T extends String & List<String>> T transform(String input)     throws IOException {
     String response = "a";
    return response; // compilation error "Incompatible Types: Required T but found java.lang.String"
   }

}

What I want to have : The implementation class should be able to pass a String input and return type can be String or a List. The implementation class is totally given freedom to choose either of the return types.

Question: 1. Why is the compilation error "Incompatible Types: Required T but found java.lang.String" is showing up?

the return value of the transform method needs to extend a String (which it cant since String is final) and implement the List<String> interface. I highly doubt thats something you wanted... you should rephrase your question so that your target is clear... — Ofek Ron
– Ofek Ron, Commented May 23, 2016 at 19:37

Sleiman Jneidi · Accepted Answer · 2016-05-23 19:38:49Z

1

I think you misunderstood intersection types.

The definition you have means: T is a String and List<String>, and not String OR List<String>, which is not possible because String is not a List<String> and nothing can extend a String and List<String> because String is final.

Such a definition makes sense if you have an intersection between two interfaces because Java support multiple interface inheritance, or one implementation and multiple interfaces since a class can extend another implementation and implement multiple interfaces.

Because a List<String> is capable of having zero or more Strings, you can make your method just return List<String>.

edited May 23, 2016 at 19:38

answered May 23, 2016 at 19:31

Sleiman Jneidi

23.4k14 gold badges60 silver badges80 bronze badges

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

2 Comments

saripallivskp Over a year ago

Appreciate for your response. I understood your answer.I was actually looking to see if Java Generics has an option to support like T extends String or List<String>. It might be dumb but looking around the docs and tutorials. How to work around to define a interface which accepts String input and returns a response of type String or Collection ?

Sleiman Jneidi Over a year ago

@saripallivskp As I said, a Collection is capable of having zero or more values.

Collectives™ on Stack Overflow

Define an interface using Multiple bound Types of Java Generics

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related