Interfaces in the Wild

The first situations where you’ll want to use interfaces involve applying pre-defined interfaces and classes that are part of Java. Here are a few examples.

Comparable<T>

Purpose: A class implements Comparable<T> in order to allow comparison—in a “greater than” and “less than” sense—to another instance of the class. This is a “parameterized” interface, which means that you need to specify the class that it will be comparing. For example, Comparable<Job> would compare Job objects.

Important Methods: compareTo(T)

Comparable Documentation

Comparator<T>

Purpose: Compares two objects of a given class. To allow comparisons and ordering based on the different fields, you can create several different Comparator classes. The class for the compared objects does NOT implement the interface itself.

Important Methods: compare(T, T)

Comparator Documentation

This interface can be used to determine, given two objects of the given type, which one is “greater” than the other. It is also used by collections such as an ArrayList to sort its contents with the sort() method.

Iterable<T>

Purpose: Enables iteration over a collection of objects using a for-each loop

Important Methods: iterator()

Iterable Documentation

This interface is implemented by the ArrayList<T> class, which we’ve been using throughout this course.

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Iterable<String> collection = new ArrayList<>();

// Add items to the collection

for (String item : collection) {
  // do something with each item
}

List<E>

Purpose: Enables access to objects in a collection by index. In other words, it enables ordered collections.

Important Methods: add(int, T), get(int), indexOf(T)

List Documentation

This interface is also implemented by the ArrayList<T> class, which we’ve been using throughout this course. In fact, List<T> extends Iterable<T>. An interface may extend another interface, in the same way that classes may extend each other.

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List<String> collection = new ArrayList<>();

// Add items to the collection

// Get the first item
String firstItem = collection.get(0);

Map<K,V>

Purpose: Represents a collection of key/value pairs.

Important Methods: get(K), containsKey(K), put(K, V)

Map Documentation This interface is implemented by theHashMap<K, V>` class, which we’ve been using throughout this course.

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Iterable<String> collection = new ArrayList<>();

// Add items to the collection

for (String item : collection) {
  // do something with each item
}

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