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PATTERNS09 - Generics in .NET and Java
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PATTERNS09 - Generics in .NET and Java


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Generics in .NET and Java. Suitable for intermediate to advanced computing students and those studying software engineering.

Generics in .NET and Java. Suitable for intermediate to advanced computing students and those studying software engineering.

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  • 1. Generics Michael Heron
  • 2. Introduction  There is a common technique available in Java (versions 1.5 and above) and .net (version 2 and above).  The Generic Datatype  Overloading and polymorphism go a long way towards making an object oriented system work ‘properly’  But they only take you to the bridge.
  • 3. The Problem  Let’s say we want to create a basic data structure.  One that works for any object.  It’s something straight forward – a queue, for example.  How do we do that?  Well, in older versions of a language we’d declare the data type as an Object.
  • 4. The Queue  Public class Queue { ArrayList<Object> myObjects; void addToQueue (Object ob) { myObjects.add (ob); } void removeFromQueue () { Object ob = myObjects.get(0); myObjects.remove (0); return ob; } }
  • 5. The Problem  We can use casting to turn whatever is on the queue into whatever class we need: Person p = (Person)queue.removeFromQueue();  This is bad OO.  It’s not type safe  We need to enforce discipline to make sure that we don’t put the wrong things on the wrong queues.  The compiler should be doing as much of that as is feasible.
  • 6. The Solution  Both C# and Java offer the Generic as a solution.  A class which acts as a type-safe template.  The syntax is a little awkward, but it allows us to define the type that should be associated with a data structure.  Like we do with ArrayLists.
  • 7. Queue – Java Generic import java.util.*; public class Queue<T> { ArrayList<T> myList; public Queue() { myList = new ArrayList<T>(); } public void addToQueue(T ob) { myList.add(ob); } public T getFromQueue() { T ob = myList.get(0); myList.remove(0); return ob; } public boolean hasMoreElements() { return (myList.size() != 0); } }
  • 8. Queue – Java Generic public static void main(String args[]) { Queue<String> myQueue = new Queue<String>(); myQueue.addToQueue("Hello!"); myQueue.addToQueue("World!"); while (myQueue.hasMoreElements()) { System.out.println(myQueue.getFromQue ue()); } }
  • 9. Queue – C# Generic class Queue<T> { ArrayList myObjects; public Queue () { myObjects = new ArrayList(); } public void addToStack<T>(T ob) { myObjects.Add(ob); } public T removeFromStack() { T ob = (T)myObjects[0]; myObjects.RemoveAt(0); return ob; } } }
  • 10. Why Use Generics?  Type safe – we can ensure type incompatibilities are dealt with at the earliest possible opportunity.  Simplifies syntax – no need to cast individual objects.  Allows for effective deployment of certain kinds of design patterns.  Avoids the need for excessive specialisation of classes.
  • 11. How do they work?  It is important to know the different ways in which variables are bound during the running of an application.  Traditionally variables are bound to a specific context in one of two ways.  Static binding, which is done at compile time.  Dynamic binding, which is done at runtime.
  • 12. Static Binding  Explicitly indicating the type of a parameter allows for the compiler to link objects and variables when compiled.  They’re not going to change in that respect.  The performance of this is high, and compile-time checking can be rigorous in a way that’s not possible otherwise.
  • 13. Late Binding  Late binding is used extensively in Java and C#.  One key area in which it is used is in polymorphism.  When you use Polymorphism, Java adopts a late binding approach so that it can properly adapt to the object at runtime.  It knows the most specialised method to use when invoked, but only when the object is bound.
  • 14. Strongly Typed Languages  In strongly typed languages, early binding is the norm.  We can tell what the context is going to be by analysing the runtime  However, late binding needs to be dealt with either through polymorphism or compile time casting.  Generics allow for you to defer the binding of a data type until its point of usage arrives.  The <T> parameter is unbound.  When we instantiate the class, we bind it to a specific context.
  • 15. Boxing and Unboxing  In both Java and C# a related mechanism is known as autoboxing.  This is the process of converting a value variable into an object reference, or vice versa.  When a value reference is boxed, it is stored on the ‘managed heap’.  A chunk of memory set aside and tended by the garbage collector.
  • 16. Wrapper Classes  Each primitive data type in Java and C# comes with a corresponding wrapper class.  A class designed to provide a way of dealing with it as a reference.  It used to be impossible to have an ArrayList of ints in Java.  You needed to make them Integer objects first.
  • 17. Wrapper Classes  Autoboxing then is the process at play when a primitive data type is encapsulated within a wrapper.  And vice versa, when it is unwrapped into its primitive form.  Autoboxing is a relatively expensive process.  If you were doing this a lot, it would be worth assessing your specific data manipulation requirements.
  • 18. Generics and Constraints  All of this leads to an obvious problem.  What if we don’t want everything to be on the table for a generic?  Luckily, generics allow us to set constraints on them.  Limitations that restrict what can be a valid specification of our class.  There are six types of these in .NET.
  • 19. Constraints Constraint Description Where T: struct The type argument must be a value type. Where T: class The type argument must be a reference type. Where T: new() The type argument must have a public, parameterless constructor Where T: <class name> The type argument must extend from the indicated class name. Where T: <interface name> The type argument must implement the specified interface, or be the interface itself Where T: U The type argument for T must be or derive from the argument supplied for U.
  • 20. Example class Queue<T> where T : IComparable { ArrayList myObjects; public Queue () { myObjects = new ArrayList(); } public void addToStack<T>(T ob) { myObjects.Add(ob); } public T removeFromStack() { T ob = (T)myObjects[0]; myObjects.RemoveAt(0); return ob; } public Boolean isInQueue(T ob) { T ob2; for (int i = 0; i < myObjects.Count; i++) { ob2 = (T)myObjects[i]; if (ob2.CompareTo(ob) != -1) { return true; } } return false; } }
  • 21. Constraints  Multiple constraints can be applied to the same parameter.  And in turn, they can be generic in and of themselves.  If you are going to be performing operations on a type that are not defined in Object itself, you need to apply a constraint.  That will allow for the method to be made available in a type-safe way.
  • 22. Multiple Parameters  Some classes may provide two types.  For example, Hashtables  T and U are used conventionally to refer to parameter 1 and parameter 2.  You can apply separate constraints to each of these:  Where U : class Where T : iComperable
  • 23. Unconstrained Types  With unconstrained types, we have the following restrictions:  We cannot use simple logical comparators on them, because there is no guarantee the concrete type will support them.  They will need to be formally cast.  You can compare to null, but this will always return false if the type argument is a value type.
  • 24. Conclusion  Generics offer a new and powerful way to deal with type-safe collections.  And other kinds of classes.  Constraints allow us to ensure that we can access useful methods as required.  Polymorphism will ensure that we can reliably access whatever internals we require.  They’re available in both C# and Java.