{"id":411,"date":"2007-10-21T00:00:00","date_gmt":"2007-10-21T00:00:00","guid":{"rendered":"http:\/\/www.strongd.net\/?p=411"},"modified":"2007-10-21T00:00:00","modified_gmt":"2007-10-21T00:00:00","slug":"Generic Types","status":"publish","type":"post","link":"https:\/\/www.strongd.net\/?p=411","title":{"rendered":"Generic Types"},"content":{"rendered":"

Generic Types<\/DIV>
\n

\n

Let’s update our Box<\/CODE> class to use generics. We’ll first create a generic type declaration<\/I> by changing the code “public class Box<\/CODE>” to “public class Box<T><\/CODE>“; this introduces one type variable<\/I>, named T<\/CODE>, 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<\/CODE> 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<\/CODE> is a formal type parameter<\/I> of the Box<\/CODE> class. <\/P>

\/**
\n * Generic version of the Box class.
\n *\/
\npublic class Box<T><\/B> {<\/p>\n
    private T t<\/B>; \/\/ T stands for “Type”          <\/p>\n
    public void add(T t<\/B>) {
\n        this.t<\/B> = t<\/B>;
\n    }<\/p>\n
    public T<\/B> get() {
\n        return t<\/B>;
\n    }
\n}
\n<\/PRE>
\n
As you can see, we’ve replaced all occurrences of Object<\/CODE> with T<\/CODE>. To reference this generic class from within your own code, you must perform a generic type invocation<\/I>, which replaces T<\/CODE> with some concrete value, such as Integer<\/CODE>: <\/P>
\n
Box<Integer> integerBox;
\n<\/PRE><\/BLOCKQUOTE>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<\/I> \u2014 Integer<\/CODE> in this case \u2014 to the Box<\/CODE> class itself. Like any other variable declaration, this code does not actually create a new Box<\/CODE> object. It simply declares that integerBox<\/CODE> will hold a reference to a “Box<\/CODE> of Integer<\/CODE>“, which is how Box<Integer><\/CODE> is read.
\n
An invocation of a generic type is generally known as a parameterized type<\/I>. <\/P>
\n
<\/P>
\n
To instantiate this class, use the new<\/CODE> keyword, as usual, but place <Integer><\/CODE> between the class name and the parenthesis: <\/P>
\n
integerBox = new Box<Integer>();
\n<\/PRE><\/BLOCKQUOTE>
\n
Or, you can put the entire statement on one line, such as: <\/P>
\n
Box<Integer> integerBox = new Box<Integer>();
\n<\/PRE><\/BLOCKQUOTE>
\n
Once integerBox<\/CODE> is initialized, you’re free to invoke its get<\/CODE> method without providing a cast, as in BoxDemo3<\/CODE><\/A>: <\/P>
\n
public class BoxDemo3 {<\/p>\n
    public static void main(String[] args) {
\n        Box<Integer> integerBox = new Box<Integer>();
\n        integerBox.add(new Integer(10));
\n        Integer someInteger = integerBox.get(); \/\/ no cast!
\n        System.out.println(someInteger);
\n    }
\n}
\n<\/PRE><\/BLOCKQUOTE>Furthermore, if you try adding an incompatible type to the box, such as String<\/CODE>, 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>
\n    cannot be applied to (java.lang.String)
\n        integerBox.add(“10”);
\n                  ^
\n    1 error
\n<\/PRE>
\n
It’s important to understand that type variables are not actually types themselves. In the above examples, you won’t find T.java<\/CODE> or T.class<\/CODE> anywhere on the filesystem. Furthermore, T<\/CODE> is not a part of the Box<\/CODE> class name. In fact during compilation, all generic information will be removed entirely, leaving only Box.class<\/CODE> on the filesystem. We’ll discuss this later in the section on Type Erasure<\/A> <\/P>
\n
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><\/CODE>, for example, would generate an error on the second occurrence of T<\/CODE>, but Box<T,U><\/CODE>, however, would be allowed. <\/P>
\n
<\/P>
\n
Type Parameter Naming Conventions<\/H3>By convention, type parameter names are single, uppercase letters. This stands in sharp contrast to the variable naming<\/A> 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.
\n
The most commonly used type parameter names are: <\/P>
\n