Generic Types

Let’s update our Box class to use generics. We’ll first create a generic type declaration by changing the code “public class Box” to “public class Box<T>“; this introduces one type variable, named T, that can be used anywhere inside the class. This same technique can be applied to interfaces as well. There’s nothing particularly complex about this concept. In fact, it’s quite similar to what you already know about variables in general. Just think of T as a special kind of variable, whose “value” will be whatever type you pass in; this can be any class type, any interface type, or even another type variable. It just can’t be any of the primitive data types. In this context, we also say that T is a formal type parameter of the Box class.

/**

 * Generic version of the Box class.

 */

public class Box<T> {

    private T t; // T stands for “Type”          
    public void add(T t) {

        this.t = t;

    }
    public T get() {

        return t;

    }

}

As you can see, we’ve replaced all occurrences of Object with T. To reference this generic class from within your own code, you must perform a generic type invocation, which replaces T with some concrete value, such as Integer:

Box<Integer> integerBox;

You can think of a generic type invocation as being similar to an ordinary method invocation, but instead of passing an argument to a method, you’re passing a type argument — Integer in this case — to the Box class itself. Like any other variable declaration, this code does not actually create a new Box object. It simply declares that integerBox will hold a reference to a “Box of Integer“, which is how Box<Integer> is read.

An invocation of a generic type is generally known as a parameterized type.

To instantiate this class, use the new keyword, as usual, but place <Integer> between the class name and the parenthesis:

integerBox = new Box<Integer>();

Or, you can put the entire statement on one line, such as:

Box<Integer> integerBox = new Box<Integer>();

Once integerBox is initialized, you’re free to invoke its get method without providing a cast, as in BoxDemo3:

public class BoxDemo3 {

    public static void main(String[] args) {

        Box<Integer> integerBox = new Box<Integer>();

        integerBox.add(new Integer(10));

        Integer someInteger = integerBox.get(); // no cast!

        System.out.println(someInteger);

    }

}

Furthermore, if you try adding an incompatible type to the box, such as String, compilation will fail, alerting you to what previously would have been a runtime bug:

    BoxDemo3.java:5: add(java.lang.Integer) in Box<java.lang.Integer>

    cannot be applied to (java.lang.String)

        integerBox.add(“10”);

                  ^

    1 error

It’s important to understand that type variables are not actually types themselves. In the above examples, you won’t find T.java or T.class anywhere on the filesystem. Furthermore, T is not a part of the Box class name. In fact during compilation, all generic information will be removed entirely, leaving only Box.class on the filesystem. We’ll discuss this later in the section on Type Erasure

Also note that a generic type may have multiple type parameters, but each parameter must be unique within its declaring class or interface. A declaration of Box<T,T>, for example, would generate an error on the second occurrence of T, but Box<T,U>, however, would be allowed.

Type Parameter Naming Conventions

By convention, type parameter names are single, uppercase letters. This stands in sharp contrast to the variable naming conventions that you already know about, and with good reason: Without this convention, it would be difficult to tell the difference between a type variable and an ordinary class or interface name.

The most commonly used type parameter names are:

• E – Element (used extensively by the Java Collections Framework)


• K – Key


• N – Number


• T – Type


• V – Value


• S,U,V etc. – 2nd, 3rd, 4th types

You’ll see these names used throughout the Java SE API and the rest of this tutorial.