Message.java

import javax.annotation.Nonnull;
import javax.annotation.meta.When;

public class Message {
    private final String greetingMessage = "Hello ";
    
    @Nonnull(when = When.ALWAYS) //When.ALWAYS is default option and makes the annotated type as non-nullable    
    public String getEchoMessage() {
        return greetingMessage;
    }
}