CODEWITHSUNDEEP
javagenerics
saripallivskp
saripallivskp

Reputation: 133

Define an interface using Multiple bound Types of Java Generics

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?

Upvotes: 0

Views: 549

Answers (1)

Sleiman Jneidi
Sleiman Jneidi

Reputation: 23349

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>.

Upvotes: 1

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