Generic Methods and Classes

It is very useful to be able to write and use methods and classes which are generic, meaning that they will work with instances of any type of object.

For example, last time we used the Arrays.sort static method, which can sort an array of any kind of object, as long as the objects stored in the array implement the Comparable interface (so Array.sort can use the compareTo method to determine the correct sorted order).

Because it works with any kind of objects, Arrays.sort is a generic method.

It is also useful to have generic classes. A generic class is one which can store references to any kind of object. The ability to make a class generic is especially useful for container classes, also known as collections. A container class is one which stores references to a collection of other objects. Containers are very useful when the program will need to keep track of an arbitrary (and unpredictable) amount of data when it runs.

It is worth mentioning that an array is a kind of container, and potentially even a generic container. The main limitation of arrays as containers is that once an array object is created, its size is fixed, and cannot be expanded dynamically.

Generic Types

To be able to implement generic methods and containers, we need the abililty to define generic types in the program. A generic type is one that could point to objects of many different classes.

There are two approaches to generic types in Java: java.lang.Object, and type parameters.

java.lang.Object is a possible generic type, because Object is the ultimate supertype of all objects and arrays in Java. Using Object as a generic type works, but has some problems, as we will see in a moment.

Type parameters are a better approach to generic types. When a method or class has a type parameter, you can think of it as a “placeholder” for some actual type (class or array type). When an instance of a generic class is created, or when a generic method is called, actual types are passed as “arguments”.

Object as a generic type

Consider the generic java.util.ArrayList class. This class works much like an array, except that it can grow as necessary to accomodate any number of element values.

Using Object as a generic type, it would be defined something like the following:

public class ArrayList {
    // fields

    public ArrayList() {
        ...
    }

    public void add(Object elt) {
        // add elt to the end of the collection
    }

    public int size() {
        // return number of objects in the collection
    }

    public Object get(int i) {
        // return element i of the collection
    }

    // other methods...
}

Here’s how we might use this version of ArrayList to store strings:

BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

ArrayList names = new ArrayList();

System.out.println("Type some names:");
while(true) {
    String name = reader.readLine();
    if (name == null) {
        break;
    }
    names.add(name);
}

int count =
    countStringsContaining(names, 'J') +
    countStringsContaining(names, 'j');

System.out.println("You entered "+ count + " names containing J");

Here is the countStringsContaining method:

public int countStringsContaining(ArrayList strings, char ch) {
    int count = 0;

    for (int i = 0; i < strings.size(); i++) {
        String s = (String) strings.get(i); // (!)
        if (s.indexOf(ch) >= 0) {
            count++;
        }
    }

    return count;
}

Note that because the get method returns Object, we need a type cast in order to get a String reference back out of the collection.

This is the weakness of using Object as a generic type: these casts can fail!

For example:

ArrayList deck = new ArrayList();
deck.add(new Card(Suit.DIAMONDS, Rank.QUEEN));
deck.add(new Card(Suit.SPADES, Rank.TWO));

int count = countStringsContaining(deck, 'J'); // (!)

If we accidentally pass an ArrayList containing Card references to countStringsContaining, then we’ll get a ClassCastException when that method tries to covert a Card reference into a String reference.

The real problem here is that the type Object does not document what kind of objects are being stored in a collection. If the programmer makes a mistake, a ClassCastException occurs when the program runs.

In practice, most collections will store a single kind of object. Type parameters document what kind of object a collection will store.

Generics using type parameters

Here’s (almost) what ArrayList really looks like. It is defined to use a type parameter to represent the type of element stored in the collection:

public class ArrayList<E> {
    // fields

    public ArrayList() {
        ...
    }

    public void add(E elt) {
        // add elt to the end of the collection
    }

    public int size() {
        // return number of objects in the collection
    }

    public E get(int i) {
        // return element i of the collection
    }

    // other methods...
}

Note that where previously Object was used as the generic type, now the type parameter E is used.

We can see the advantage by rewriting the same code example as above:

BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

ArrayList<String> names = new ArrayList<String>();

System.out.println("Type some names:");
while(true) {
    String name = reader.readLine();
    if (name == null) {
        break;
    }
    names.add(name);
}

int count =
    countStringsContaining(names, 'J') +
    countStringsContaining(names, 'j');

System.out.println("You entered "+ count + " names containing J");

...

public int countStringsContaining(ArrayList<String> strings, char ch) {
    int count = 0;

    for (int i = 0; i < strings.size(); i++) {
        String s = strings.get(i); // No cast is needed!
        if (s.indexOf(ch) >= 0) {
            count++;
        }
    }

    return count;
}

Now instead of creating and using an ArrayList, we’re creating and using an ArrayList<String>. You can read this as “ArrayList of String” elements.

The way to understand this works is by imagining that, in the case of declaring an ArrayList of String elements, the type parameter E is replaced with String everywhere in the text of the ArrayList class.

Type parameters are a superior approach to generics because the Java compiler enforces the type parameter. So, for example:

Because the type argument of a collection is enforced when objects are added to the collection, casts are not necessary when getting a reference back out of the collection.

Type erasure

Behind the scenes, Java type parameters work by type erasure. The “real” type of a type parameter is actually Object: the compiler inserts type casts in the program where necessary to convert Object references to whatever argument type is appropriate.