Instantiating a generic type

Question

I m trying to convert an existing class to use generics  and am getting stumped while converting the constructors   The original class was a POJO that contained logic for moving from one room to another in a text-based console game  Literally  it was a class that held some string triggers that would fire the action  eg  the user types  walk right    a description  and a pointer to the new Location   The current  non generic version of the class looks like this   public class Navigation implements Serializable       private static final long serialVersionUID   1L      private String trigger      private String description      private Location target       public Navigation             this         new Location                public Navigation String trigger  String description  Location target            this trigger   trigger          this description   description          this target   target                plus getters  setters  etc       The Location class is another POJO that describes a location  It is irrelevant    I want to extend the Navigation class to be able to handle targets that are not Locations  I thought that the best way to do this would be to convert the Navigation class to use generics  so I tried this   public class Navigation lt T gt  implements Serializable       private static final long serialVersionUID   2L      private String trigger      private String description      private T target       public Navigation             this         new T                public Navigation String trigger  String description  T target            this trigger   trigger          this description   description          this target   target                plus getters  setters  etc      However  this doesn t compile at the line this         new T     because T cannot be instantiated  Is is possible to instantiate the generic type object in this context

User · Accepted Answer

You basically have two choices   1 Require an instance   public Navigation T t        this         t       2 Require a class instance   public Navigation Class lt T gt  c        this         c newInstance         You could use a factory pattern  but ultimately you ll face this same issue  but just push it elsewhere in the code

User · Answer

You cannot do new T   due to type erasure  The default constructor can only be   public Navigation         this         null           You can create other constructors to provide default values for trigger and description  You need an concrete object of T

User · Answer

No  and the fact that you want to seems like a bad idea  Do you really need a default constructor like this

[java] Instantiating a generic type

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