Javaparser: get generic class of a method return type like List<MyClass>

Question

I'm processing a Java source file using the java-parser library.

I have this situation in the analyzed source code:

 List<Allergen> getList() {
        return EnumSet.allOf(Allergen.class).stream()
                .filter(this::isAllergicTo)
                .collect(Collectors.toList());
    }

I need to access the generic class of the List return type ( Allergen in this example)

I'm using an extension of VoidVisitorAdapter class in my analyzer

Here an extract of the code:

public class PlaceholderNormalizer extends VoidVisitorAdapter<String> {

 ...
 ...
 
 
 @Override
    public void visit(MethodDeclaration n, String arg) {
        logger.debug("MethodDeclaration: {}", n.getName().asString());
        logger.debug(" --- " + n.getTypeAsString());  // prints: List<Allergen>
    
    // DO SOME STUFF
        super.visit(n, arg);
    }
  
  ...
  ...
}

I can obtain a Type object of the node MethodDeclaration using n.getType()

But, I don't understand how to retrieve the Allergen class.

Please I need some help

Answer 1

In case of this method the n.getType will return instance of ClassOrInterfaceType. To get to generic parameter you can obtain it as this type and use ClassOrInterfaceType::getTypeArguments method :

@Override
public void visit(MethodDeclaration n, String arg) {
    Type type = n.getType();

    if(type instanceof ClassOrInterfaceType) {
        ClassOrInterfaceType classOrInterfaceType = type.asClassOrInterfaceType();
        Optional<NodeList<Type>> typeArguments = classOrInterfaceType.getTypeArguments();

        //list of arguments might have different length
        typeArguments.ifPresent(types -> System.out.println(types.get(0)));
    }
        // DO SOME STUFF
     super.visit(n, arg);
}

This will print Allergen.