Generics

The main purpose of generics is to generate compiler errors.... this is a good thing.

Why Generics

First, a very brief history... Generics were not part of the original design of the Java language. Generics were added to the Java language in 2004 with the release of J2SE 5.0.

Generics are said to provide static type safety for collections and eliminate the need for most typecasts. Essentially, they provide a mechanism for the developer to express her intent and then allows the compiler to verify that she has not violated that intent. Consider the following example:

Collection words = new ArrayList();
words.add("first");
words.add("second");
words.add("third");
for(Object word : words) {
  System.out.println(word + " is " + ((String)word).length() + " in length.");
}

Here we're not making use of generics, so the compiler has no idea what we intend to store in the collection. As a result, we had to cast the word Object reference to a String reference before we are able to call the length() method on the string in the collection. That's not the big deal though. The big deal is that I can add anything to this collection without the compiler objecting. For example, the last words.add("third") could be replaced with: words.add(3) and the compiler is still happy.¹

With generics, we can be much more explicit in declaring our intented use for the collection:

Collection<String> words = new ArrayList<>();
words.add("first");
words.add("second");
words.add("third");
for(String word : words) {
  System.out.println(word + " is " + word.length() + " in length.");
}

Here we have declared our intent to only store references to Strings within our collection. We no longer need to get Object references and then cast them to String references, but, more importantly, the compiler will be sad, perhaps even angry, if we try to do something stupid like: words.add(3);. When do you want an angry compiler? Anytime you are doing something stupid. The compiler has now become your friend. Instead of letting you do something stupid that you won't discover until later (perhaps much later), the compiler now cares so much about you that it won't let you proceed with your idiocy.

Generics allow the compiler to become your good friend. With generics you can tell the compiler about your dreams for the software you are developing, and the compiler can tell you when you are doing things that will turn your dream into a nightmare.²

Define Your Own Class with Generic Support

We can define our own classes that use generics. In the declaration of the class, we need to add <SomeLabel> immediately after the class name. For example, in our simple ArrayList<E> implementation, we had:

public class ArrayList<E> implements List<E>, RandomAccess {

• The <E> specified that E will be the identifier for our generic type.
• When an ArrayList is declared, we can specify the type of data that will be stored within it. E.g., ArrayList<String>.
• The E in the definition of the class is replace with the actual type used in the declaration of the ArrayList (in this case, String).
• Our attribute within our class was declared as:

  private E[] elements = null;

• This says that we have a reference to an array of references and that each reference within the array is of type E.
• In reality, the array is really an array of Object references; however, we cast this to an array of E references when the array is created³

    E[] temp = (E[])new Object[size()+1];

• As a result, the compiler can ensure that whenever we place an element in the array, it is compatible with the type of references intended to be stored in the array (E, or in our example, String).
• By making use of generics we equip the compiler with the ability to ensure that we never add something to the ArrayList that isn't of type E (or a subclass).

Compiler errors are good if they keep you from making mistakes. It is much easier to find your mistakes if the compiler finds them for you.

Specifying Constraints on Generics

Suppose we have a Shape hierarchy that consists of an abstract Shape class that includes an abstract calculateArea method. In addition, there are a number of concrete subclasses: Circle, Square, Triangle, etc... that provide concrete implementations of the calculateArea() method.

Consider the following implementation of a totalArea() method:

public static double totalArea(Collection<Shape> shapes) {
    double totalArea = 0;
    for(Shape shape : shapes) {
        if(shape!=null) {
            totalArea += shape.calculateArea();
        }
    }
    return totalArea;
}

• By specifying a collection of Shapes we can loop through each shape without needing to cast each reference to a Shape reference before calling calculateArea.
• The following code will work just fine with our totalArea method:

   List<Shape> shapes = new LinkedList<>();
   for(int i=0; i < 10; ++i) {
       shapes.add(new Circle(Math.random()));
   }
   System.out.println(totalArea(shapes));

where the constructor for the Circle class takes the radius of the circle as an argument.

• We could have added Squares instead of Circles or some combination of any concrete shapes in the hierarchy and things will continue function properly; however, the following will make a good day bad, or a bad day worse:

   List<Circle> shapes = new LinkedList<>();
   for(int i=0; i < 10; ++i) {
       shapes.add(new Circle(Math.random()));
   }
   System.out.println(totalArea(shapes));

• Adding Circles to a collection of Circles is still a happy affair, but things turn gloomy when we try to call totalArea().
• The compiler won't let us make the call because totalArea() needs a collection of Shape references and we are passing it a collection of Circle references.
  • Not so fast, you object. A Circle is a Shape, you say?
  • Right you are, but this still isn't allowed... and for good reason.
  • Suppose we did allow this, once in the totalArea method, we have a collection of Shapes, so from within the method, I should be able to add Squares, Triangles, and maybe even a Rhombus! Once the method returns, I'm back to my collection of Circles, but to my horror it's filled with shapes for which I don't know the plural name. Is it rhombuses or rhombi?
  • Whether it is rombuses or rhombi, it's just bad for them to be in my collection of circles, and so the compiler has graciously prevented me from making such a grave mistake.
• Okay, fine. But in this case, it sure would be nice if we could pass a collection of Circles or whatever and not be required to have a collection of Shapes.
• Bully for you for continuing to read this because now you are going to find out how.
• Our current implementation over specifies what we can pass to totalArea. It's as if the porridge is too hot.
• We could underspecify it like this:

public static double totalArea(Collection shapes) {

but here the porridge is too cold...

• Here anything could be passed in.
• We'd also need to cast the references to Shape references before we called calculateArea and this could result in a ClassCastException at runtime (if somebody passed in something that wasn't a shape).
• To make Goldilocks happy, we need to get the porridge just right:

public static double totalArea(Collection<? extends Shape> shapes) {

• Here we make use of a generic wildcard.
• The wildcard says anything goes, but then we constrain it by adding the extends Shape.
• We are saying that totalArea should accept any collection as long as the collection is holding Shapes or references from any type that extends from Shape (like, Circle, Square, Rhombus, etc...).
• We're now ready to let Goldilocks go break a chair.⁴