This chapter is from the book
This chapter is from the book
This chapter is from the book
3.3 Examples of Interfaces
At first glance, interfaces don’t seem to do very much. An interface is just a set of methods that a class promises to implement. To make the importance of interfaces more tangible, the following sections show you four examples of commonly used interfaces from the standard Java library.
3.3.1 The Comparable Interface
Suppose you want to sort an array of objects. A sorting algorithm repeatedly compares elements and rearranges them if they are out of order. Of course, the rules for doing the comparison are different for each class, and the sorting algorithm should just call a method supplied by the class. As long as all classes can agree on what that method is called, the sorting algorithm can do its job. That is where interfaces come in.
If a class wants to enable sorting for its objects, it should implement the Comparable interface. There is a technical point about this interface. We want to compare strings against strings, employees against employees, and so on. For that reason, the Comparable interface has a type parameter.
public interface Comparable<T> {
int compareTo(T other);
}
For example, the String class implements Comparable<String> so that its compareTo method has the signature
int compareTo(String other)
NOTE
A type with a type parameter such as Comparable or ArrayList is a generic type. You will learn all about generic types in Chapter 6.
When calling x.compareTo(y), the compareTo method returns an integer value to indicate whether x or y should come first. A positive return value (not necessarily 1) indicates that x should come after y. A negative integer (not necessarily -1) is returned when x should come before y. If x and y are considered equal, the returned value is 0.
Note that the return value can be any integer. That flexibility is useful because it allows you to return a difference of non-negative integers.
public class Employee implements Comparable<Employee> {
...
public int compareTo(Employee other) {
return getId() - other.getId(); // Ok if IDs always ≥ 0
}
}
When comparing floating-point values, you cannot just return the difference. Instead, use the static Double.compare method. It does the right thing, even for ±∞ and NaN.
Here is how the Employee class can implement the Comparable interface, ordering employees by salary:
public class Employee implements Comparable<Employee> {
...
public int compareTo(Employee other) {
return Double.compare(salary, other.salary);
}
}
NOTE
It is perfectly legal for the compare method to access other.salary. In Java, a method can access private features of any object of its class.
The String class, as well as over a hundred other classes in the Java library, implements the Comparable interface. You can use the Arrays.sort method to sort an array of Comparable objects:
String[] friends = { "Peter", "Paul", "Mary" };
Arrays.sort(friends); // friends is now ["Mary", "Paul", "Peter"]
NOTE
Strangely, the Arrays.sort method does not check at compile time whether the argument is an array of Comparable objects. Instead, it throws an exception if it encounters an element of a class that doesn’t implement the Comparable interface.
3.3.2 The Comparator Interface
Now suppose we want to sort strings by increasing length, not in dictionary order. We can’t have the String class implement the compareTo method in two ways—and at any rate, the String class isn’t ours to modify.
To deal with this situation, there is a second version of the Arrays.sort method whose parameters are an array and a comparator—an instance of a class that implements the Comparator interface.
public interface Comparator<T> {
int compare(T first, T second);
}
To compare strings by length, define a class that implements Comparator<String>:
class LengthComparator implements Comparator<String> {
public int compare(String first, String second) {
return first.length() - second.length();
}
}
To actually do the comparison, you need to make an instance:
Comparator<String> comp = new LengthComparator(); if (comp.compare(words[i], words[j]) > 0) ...
Contrast this call with words[i].compareTo(words[j]). The compare method is called on the comparator object, not the string itself.
NOTE
Even though the LengthComparator object has no state, you still need to make an instance of it. You need the instance to call the compare method—it is not a static method.
To sort an array, pass a LengthComparator object to the Arrays.sort method:
String[] friends = { "Peter", "Paul", "Mary" };
Arrays.sort(friends, new LengthComparator());
Now the array is either ["Paul", "Mary", "Peter"] or ["Mary", "Paul", "Peter"].
You will see in Section 3.4.2, “Functional Interfaces,” on p. 109 how to use a Comparator much more easily, using a lambda expression.
3.3.3 The Runnable Interface
At a time when just about every processor has multiple cores, you want to keep those cores busy. You may want to run certain tasks in a separate thread, or give them to a thread pool for execution. To define the task, you implement the Runnable interface. This interface has just one method.
class HelloTask implements Runnable {
public void run() {
for (int i = 0; i < 1000; i++) {
System.out.println("Hello, World!");
}
}
}
If you want to execute this task in a new thread, create the thread from the Runnable and start it.
Runnable task = new HelloTask(); Thread thread = new Thread(task); thread.start();
Now the run method executes in a separate thread, and the current thread can proceed with other work.
NOTE
In Chapter 10, you will see other ways of executing a Runnable.
NOTE
There is also a Callable<T> interface for tasks that return a result of type T.
3.3.4 User Interface Callbacks
In a graphical user interface, you have to specify actions to be carried out when the user clicks a button, selects a menu option, drags a slider, and so on. These actions are often called callbacks because some code gets called back when a user action occurs.
In Java-based GUI libraries, interfaces are used for callbacks. For example, in JavaFX, the following interface is used for reporting events:
public interface EventHandler<T> {
void handle(T event);
}
This too is a generic interface where T is the type of event that is being reported, such as an ActionEvent for a button click.
To specify the action, implement the interface:
class CancelAction implements EventHandler<ActionEvent> {
public void handle(ActionEvent event) {
System.out.println("Oh noes!");
}
}
Then, make an object of that class and add it to the button:
Button cancelButton = new Button("Cancel");
cancelButton.setOnAction(new CancelAction());
NOTE
Since Oracle positions JavaFX as the successor to the Swing GUI toolkit, I use JavaFX in these examples. (Don’t worry—you need not know any more about JavaFX than the couple of statements you just saw.) The details don’t matter; in every user interface toolkit, be it Swing, JavaFX, or Android, you give a button some code that you want to run when the button is clicked.
Of course, this way of defining a button action is rather tedious. In other languages, you just give the button a function to execute, without going through the detour of making a class and instantiating it. The next section shows how you can do the same in Java.