Enter Generics

Generics are an enhancement to the syntax of classes that allow us to specialize the class for a given type or set of types. A generic class requires one or more type parameters wherever we refer to the class type and uses them to customize itself.

If you look at the source or Javadoc for the List class, for example, you’ll see it defined something like this:

    public class List< E > {
       public void add( E element ) { ... }
       public E get( int i ) { ... }

The identifier E between the angle brackets (<>) is a type variable. It indicates that the class List is generic and requires a Java type as an argument to make it complete. The name E is arbitrary, but there are conventions that we’ll see as we go on. In this case, the type variable E represents the type of elements we want to store in the list. The List class refers to the type variable within its body and methods as if it were a real type, to be substituted later. The type variable may be used to declare instance variables, arguments to methods, and the return type of methods. In this case, E is used as the type for the elements we’ll be adding via the add() method and the return type of the get() method. Let’s see how to use it.

The same angle bracket syntax supplies the type parameter when we want to use the List type:

    List<String> listOfStrings;

In this snippet, we declared a variable called listOfStrings using the generic type List with a type parameter of String. String refers to the String class, but we could have specialized List with any Java class type. For example:

    List<Date> dates;
    List<java.math.BigDecimal> decimals;
    List<Foo> foos;

Completing the type by supplying its type parameter is called instantiating the type. It is also sometimes called invoking the type, by analogy with invoking a method and supplying its arguments. Whereas with a regular Java type, we simply refer to the type by name, a generic type must be instantiated with parameters wherever it is used.[21] Specifically, this means that we must instantiate the type everywhere types can appear as the declared type of a variable (as shown in this code snippet), as the type of a method argument, as the return type of a method, or in an object allocation expression using the new keyword.

Returning to our listOfStrings, what we have now is effectively a List in which the type String has been substituted for the type variable E in the class body:

    public class List< String > {
       public void add( String element ) { ... }
       public String get( int i ) { ... }

We have specialized the List class to work with elements of type String and only elements of type String. This method signature is no longer capable of accepting an arbitrary Object type.

List is just an interface. To use the variable, we’ll need to create an instance of some actual implementation of List. As we did in our introduction, we’ll use ArrayList. As before, ArrayList is a class that implements the List interface, but in this case, both List and ArrayList are generic classes. As such, they require type parameters to instantiate them where they are used. Of course, we’ll create our ArrayList to hold String elements to match our List of Strings:

    List<String> listOfStrings = new ArrayList<String>
    List<String> listOfStrings = new ArrayList<>(); // Or shorthand in Java 7.0
                                                    //  and later

As always, the new keyword takes a Java type and parentheses with possible arguments for the class’s constructor. In this case, the type is ArrayList<String>—the generic ArrayList type instantiated with the String type.

Declaring variables as shown in the first line of the preceding example is a bit cumbersome because it requires us to type the generic parameter type twice (once on the left side in the variable type and once on the right in the initialing expression). And in complicated cases, the generic types can get very lengthy and nested within one another. In Java 7, the compiler is smart enough to infer the type of the initializing expression from the type of the variable to which you are assigning it. This is called generic type inference and boils down to the fact that you can shorthand the right side of your variable declarations by leaving out the contents of the <> notation, as shown in the example’s second line.

We can now use our specialized List with strings. The compiler prevents us from even trying to put anything other than a String object (or a subtype of String if there were any) into the list and allows us to fetch them with the get() method without requiring any cast:

    List<String> listOfStrings = new ArrayList<String>();
    listOfStrings.add("eureka! ");
    String s = listOfStrings.get(0); // "eureka! "

    listOfStrings.add( new Date() ); // Compile-time Error!

Let’s take another example from the Collections API. The Map interface provides a dictionary-like mapping that associates key objects with value objects. Keys and values do not have to be of the same type. The generic Map interface requires two type parameters: one for the key type and one for the value type. The Javadoc looks like this:

    public class Map< K, V > {
        public V put( K key, V value ) { ... } // returns any old value
        public V get( K key ) { ... }

We can make a Map that stores Employee objects by Integer “employee ID” numbers like this:

    Map< Integer, Employee > employees = new HashMap< Integer, Employee >();
    Integer bobsId = ...;
    Employee bob = ...;

    employees.put( bobsId, bob );
    Employee employee = employees.get( bobsId );

Here, we used HashMap, which is a generic class that implements the Map interface, and instantiated both types with the type parameters Integer and Employee. The Map now works only with keys of type Integer and holds values of type Employee.

The reason we used Integer here to hold our number is that the type parameters to a generic class must be class types. We can’t parameterize a generic class with a primitive type, such as int or boolean. Fortunately, autoboxing of primitives in Java (see Chapter 5) makes it almost appear as if we can by allowing us to use primitive types as though they were wrapper types:

    employees.put( 42, bob );
    Employee bob = employees.get( 42 );

Here, autoboxing converted the integer 42 to an Integer wrapper for us twice.

In Chapter 11, we’ll see that all of the Java collection classes and interfaces are generic. Furthermore, dozens of other APIs use generics to let you adapt them to specific types. We’ll talk about them as they occur throughout the book.

Talking About Types

Before we move on to more important things, we should say a few words about the way we describe a particular parameterization of a generic class. Because the most common and compelling case for generics is for container-like objects, it’s common to think in terms of a generic type “holding” a parameter type. In our example, we called our List<String> a “list of strings” because, sure enough, that’s what it was. Similarly, we might have called our employee map a “Map of employee IDs to Employee objects.” However, these descriptions focus a little more on what the classes do than on the type itself. Take instead a single object container called Trap< E > that could be instantiated on an object of type Mouse or of type Bear; that is, Trap<Mouse> or Trap<Bear>. Our instinct is to call the new type a “mouse trap” or “bear trap.” Similarly, we could have thought of our list of strings as a new type: “string list” or our employee map as a new “integer employee object map” type. You may use whatever verbiage you prefer, but these latter descriptions focus more on the notion of the generic as a type and may help a little bit later when we discuss how generic types are related in the type system. There we’ll see that the container terminology turns out to be a little counterintuitive.

In the following section, we’ll continue our discussion of generic types in Java from a different perspective. We’ve seen a little of what they can do; now we need to talk about how they do it.

[21] That is, unless you want to use a generic type in a nongeneric way. We’ll talk about “raw” types later in this chapter.

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