Interfaces

An interface declares a set of behaviors. A class can chose to advertise that it implements the interface. Any class that makes this claim, must implement all methods declared in the interface.¹

• Storing collections (groups) of similar objects is a common programming task.
  • A text document is a collection of paragraphs. Each paragraph is a collection of words. Each word is a collection of characters.
  • The computer's file system is a collection of files.
  • A browser's bookmark list contains a collection of URLs.
  • etc...
• You have already seen one mechanism for storing collections of objects: the array.
• The Java Collections Framework provides a number of classes that can be used to store collections of data.
• In this page we will:
  • Briefly review arrays, discussing some disadvantages
  • Introduce the concept of an interface in Java
  • Introduce one interface and two classes that are part of the Java Collections Framework:
    • The List<E> interface.
    • The ArrayList<E> class.
    • The LinkedList<E> class.

Interfaces

• The Java language includes a concept known as interfaces.
• It's a fairly simple concept that we'll need in order to better understand collection types that are part of the Java Collections Framework.
• An interface is basically a promise or contract.
• An interface is a list of method signatures (method name, parameters passed to the method, and the type returned by the method, if anything).
  • When a class is declared, it may promise to implement the interface.
  • The compiler then makes sure that the class fulfills its promise by refusing to compile unless all of the methods declared in the interface are implemented in the class.
• In much the same way as we can create a reference to an object, we can create a reference to an interface.
• The reference acts as the gatekeeper for accessing the object to which the reference points. Consider the following code:

String word = new String("nonsense");
CharSequence charSeq = word;

• The first line creates a reference to a String and makes it point to a newly created String object.
• The second line creates a reference to a CharSequence and makes it point to the same String object as word.
• Since the String class implements the CharSequence, the second line is valid code.
• Both word and charSeq refer to the same object, but we can only access methods known to the CharSequence interface when using the charSeq reference.
• The CharSequence consists of four methods:
  • charAt(int)
  • length()
  • subSequence(int, int)
  • toString()
• The following indicates which lines are illegal. All of the legal lines of code function identically regardless of whether the method is called through word or charSeq.

  word.toString();
  word.endsWith("sense");
  word.length();
  word.subSequence(1, 3);
  word.substring(1, 3);
  charSeq.toString();
  charSeq.endsWith("sense");  // ILLEGAL
  charSeq.length();
  charSeq.subSequence(1, 3);
  charSeq.substring(1, 3);    // ILLEGAL
  

Java Collections Framework

• Because manipulating collections of objects is such a common task, Sun provides the Java Collections Framework.
• The framework contains a number of interfaces and classes that provide various ways of storing and interacting with collections of data.
• In data structures you will explore the Java Collections Framework in much more detail.
• In this course we will just touch on a few items.