An interface is not a distinct program component. However, the set of methods belonging to a data type can be viewed by running either the dir or the help functions with that data type as an argument.
dir returns a list of the names of the type’s attributes, including method names.
help returns a string consisting of the docstrings of the type and all of its methods.
An interface consists of a name and a set method headers. It specifies the set of methods that an implementing class must include. The interfaces of the built-in classes can be viewed in the JavaDoc.
A single class can implement several different interfaces.
An interface guarantees a common, abstract behavior of all implementing classes. For example, the java.util.List interface specifies methods for all classes of lists, including java.util.ArrayList and java.util.LinkedList.
An interface can extend another, more general interface. For example, the List interface extends the Collection interface. This means that all of the methods required by the Collection interface will also be required for all lists.
Interfaces are like the interstitial glue that holds program components together. Whenever possible, use interface names for the types of variables, parameters, and method return types.
Uses of interfaces:
List<String> aList = new ArrayList<String>();
The ArrayList class implements the List interface. Note the use of List rather than ArrayList and LinkedList to type the list1 and list2 variables.
The method Collections.sort expects a Collection of Comparables as an argument. Because the String class implements the Comparable interface and the List interface extends the Collection interface, the compiler does not complain and all goes well.
Finally, note that the LinkedList constructor can accept a Collection as an argument. This allows a new list to be built from the elements contained in another list, regardless of implementation.