Update User.java

Remove constants and add empty constructor. 

Jackson fails otherwise

libraries-http/src/main/java/com/baeldung/javalin/User/User.java

@@ -1,8 +1,11 @@
 package com.baeldung.javalin.User;
+import com.fasterxml.jackson.annotation.JsonProperty;
 
 public class User {
-    public final int id;
-    public final String name;
+    public int id;
+    public String name;
+    
+    public User(){}
 
     public User(int id, String name) {
         this.id = id;