No instance(s) of type variable(s) U exist so that void conforms to U

Optional.map() expects a Function<? super T, ? extends U> as parameter. Since your method returns void, it cannot be used like this in the lambda.

I see three options here:

• make that method return Void/Object/whatever instead – semantically not ideal but it would work
• make that method return the exception, and change the lambda definition to

  .map(v -> {
      throw printAndReturnException();
  });
  

• use ifPresent(), and move the orElseThrow() outside of the call chain:

  values.stream()
      .filter(value -> getDetails(value).getName.equals("TestVal"))
      .findFirst()
      .ifPresent(value -> printAndThrowException(value))
  throw new Exception2("xyz");