Dynamic Generic Typing in Java

Question

If I have a class using generic type such as

public class Record<T> {     private T value;      public Record(T value) {         this.value = value;     } } 

it is pretty straight forward to type everything during design time, if I know all types that are used such as it is the case in this example:

// I type explicitly String myStr = "A"; Integer myInt = 1; ArrayList myList = new ArrayList();  Record rec1 = new Record<String>(myStr); Record rec2 = new Record<Integer>(myInt); Record rec3 = new Record<ArrayList>(myList); 

What happens if I get a list of objects from "somewhere" where I don't know the type? How do I assign the type:

// now let's assume that my values come from a list where I only know during runtime what type they have  ArrayList<Object> myObjectList = new ArrayList<Object>();     myObjectList.add(myStr);     myObjectList.add(myInt);     myObjectList.add(myList);      Object object = myObjectList.get(0);      // this fails - how do I do that?     new Record<object.getClass()>(object); 

Answer 1

Java generics are not C++ Templates.

Java generics are a compile time feature, not a run time feature.

Here is a link to the Java generics Tutorial.

This can never work with Java:

new Record<object.getClass()>(object); 

You must either use polymorphism (say, each object implements a known interface) or RTTI (instanceof or Class.isAssignableFrom()).

You might do this:

     class Record      {        public Record(String blah) { ... }        public Record(Integer blah) { ... }        ... other constructors.      } 

or you might use the Builder pattern.