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This chapter is from the book

6.2 Examples of Interfaces

In the next three sections, we give additional examples of interfaces so you can see how they are used in practice.

6.2.1 Interfaces and Callbacks

A common pattern in programming is the callback pattern. In this pattern, you specify the action that should occur whenever a particular event happens. For example, you may want a particular action to occur when a button is clicked or a menu item is selected. However, as you have not yet seen how to implement user interfaces, we will consider a similar but simpler situation.

The javax.swing package contains a Timer class that is useful if you want to be notified whenever a time interval has elapsed. For example, if a part of your program contains a clock, you can ask to be notified every second so that you can update the clock face.

When you construct a timer, you set the time interval and you tell it what it should do whenever the time interval has elapsed.

How do you tell the timer what it should do? In many programming languages, you supply the name of a function that the timer should call periodically. However, the classes in the Java standard library take an object-oriented approach. You pass an object of some class. The timer then calls one of the methods on that object. Passing an object is more flexible than passing a function because the object can carry additional information.

Of course, the timer needs to know what method to call. The timer requires that you specify an object of a class that implements the ActionListener interface of the java.awt.event package. Here is that interface:

public interface ActionListener
   void actionPerformed(ActionEvent event);

The timer calls the actionPerformed method when the time interval has expired.

Suppose you want to print a message “At the tone, the time is ...”, followed by a beep, once every 10 seconds. You would define a class that implements the ActionListener interface. You would then place whatever statements you want to have executed inside the actionPerformed method.

class TimePrinter implements ActionListener
   public void actionPerformed(ActionEvent event)
      System.out.println("At the tone, the time is " + new Date());

Note the ActionEvent parameter of the actionPerformed method. This parameter gives information about the event, such as the source object that generated it—see Chapter 11 for more information. However, detailed information about the event is not important in this program, and you can safely ignore the parameter.

Next, you construct an object of this class and pass it to the Timer constructor.

ActionListener listener = new TimePrinter();
Timer t = new Timer(10000, listener);

The first parameter of the Timer constructor is the time interval that must elapse between notifications, measured in milliseconds. We want to be notified every 10 seconds. The second parameter is the listener object.

Finally, you start the timer.


Every 10 seconds, a message like

At the tone, the time is Wed Apr 13 23:29:08 PDT 2016

is displayed, followed by a beep.

Listing 6.3 puts the timer and its action listener to work. After the timer is started, the program puts up a message dialog and waits for the user to click the OK button to stop. While the program waits for the user, the current time is displayed at 10-second intervals.

Be patient when running the program. The “Quit program?” dialog box appears right away, but the first timer message is displayed after 10 seconds.

Note that the program imports the javax.swing.Timer class by name, in addition to importing javax.swing.* and java.util.*. This breaks the ambiguity between javax.swing.Timer and java.util.Timer, an unrelated class for scheduling background tasks.

Listing 6.3 timer/TimerTest.java

 1   package timer;
 3   /**
 4      @version 1.01 2015-05-12
 5      @author Cay Horstmann
 6   */
 8   import java.awt.*;
 9   import java.awt.event.*;
10   import java.util.*;
11   import javax.swing.*;
12   import javax.swing.Timer;
13   // to resolve conflict with java.util.Timer
15   public class TimerTest
16   {
17      public static void main(String[] args)
18      {
19         ActionListener listener = new TimePrinter();
21         // construct a timer that calls the listener
22         // once every 10 seconds
23         Timer t = new Timer(10000, listener);
24         t.start();
26         JOptionPane.showMessageDialog(null, "Quit program?");
27         System.exit(0);
28      }
29   }
31   class TimePrinter implements ActionListener
32   {
33      public void actionPerformed(ActionEvent event)
34      {
35         System.out.println("At the tone, the time is " + new Date());
36         Toolkit.getDefaultToolkit().beep();
37      }
38   }

6.2.2 The Comparator Interface

In Section 6.1.1, “The Interface Concept,” on p. 288, you have seen how you can sort an array of objects, provided they are instances of classes that implement the Comparable interface. For example, you can sort an array of strings since the String class implements Comparable<String>, and the String.compareTo method compares strings in dictionary order.

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.

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 6.3, “Lambda Expressions,” on p. 314 how to use a Comparator much more easily with a lambda expression.

6.2.3 Object Cloning

In this section, we discuss the Cloneable interface that indicates that a class has provided a safe clone method. Since cloning is not all that common, and the details are quite technical, you may just want to glance at this material until you need it.

To understand what cloning means, recall what happens when you make a copy of a variable holding an object reference. The original and the copy are references to the same object (see Figure 6.1). This means a change to either variable also affects the other.

Employee original = new Employee("John Public", 50000);
Employee copy = original;
copy.raiseSalary(10); // oops--also changed original
Figure 6.1

Figure 6.1 Copying and cloning

If you would like copy to be a new object that begins its life being identical to original but whose state can diverge over time, use the clone method.

Employee copy = original.clone();
copy.raiseSalary(10); // OK--original unchanged

But it isn’t quite so simple. The clone method is a protected method of Object, which means that your code cannot simply call it. Only the Employee class can clone Employee objects. There is a reason for this restriction. Think about the way in which the Object class can implement clone. It knows nothing about the object at all, so it can make only a field-by-field copy. If all data fields in the object are numbers or other basic types, copying the fields is just fine. But if the object contains references to subobjects, then copying the field gives you another reference to the same subobject, so the original and the cloned objects still share some information.

To visualize that, consider the Employee class that was introduced in Chapter 4. Figure 6.2 shows what happens when you use the clone method of the Object class to clone such an Employee object. As you can see, the default cloning operation is “shallow”—it doesn’t clone objects that are referenced inside other objects. (The figure shows a shared Date object. For reasons that will become clear shortly, this example uses a version of the Employee class in which the hire day is represented as a Date.)

Figure 6.2

Figure 6.2 A shallow copy

Does it matter if the copy is shallow? It depends. If the subobject shared between the original and the shallow clone is immutable, then the sharing is safe. This certainly happens if the subobject belongs to an immutable class, such as String. Alternatively, the subobject may simply remain constant throughout the lifetime of the object, with no mutators touching it and no methods yielding a reference to it.

Quite frequently, however, subobjects are mutable, and you must redefine the clone method to make a deep copy that clones the subobjects as well. In our example, the hireDay field is a Date, which is mutable, so it too must be cloned. (For that reason, this example uses a field of type Date, not LocalDate, to demonstrate the cloning process. Had hireDay been an instance of the immutable LocalDate class, no further action would have been required.)

For every class, you need to decide whether

  1. The default clone method is good enough;
  2. The default clone method can be patched up by calling clone on the mutable subobjects; and
  3. clone should not be attempted.

The third option is actually the default. To choose either the first or the second option, a class must

  1. Implement the Cloneable interface; and
  2. Redefine the clone method with the public access modifier.

In this case, the appearance of the Cloneable interface has nothing to do with the normal use of interfaces. In particular, it does not specify the clone method—that method is inherited from the Object class. The interface merely serves as a tag, indicating that the class designer understands the cloning process. Objects are so paranoid about cloning that they generate a checked exception if an object requests cloning but does not implement that interface.

Even if the default (shallow copy) implementation of clone is adequate, you still need to implement the Cloneable interface, redefine clone to be public, and call super.clone(). Here is an example:

class Employee implements Cloneable
   // raise visibility level to public, change return type
   public Employee clone() throws CloneNotSupportedException
      return (Employee) super.clone();

The clone method that you just saw adds no functionality to the shallow copy provided by Object.clone. It merely makes the method public. To make a deep copy, you have to work harder and clone the mutable instance fields.

Here is an example of a clone method that creates a deep copy:

class Employee implements Cloneable
   public Employee clone() throws CloneNotSupportedException
      // call Object.clone()
      Employee cloned = (Employee) super.clone();

      // clone mutable fields
      cloned.hireDay = (Date) hireDay.clone();

      return cloned;

The clone method of the Object class threatens to throw a CloneNotSupportedException—it does that whenever clone is invoked on an object whose class does not implement the Cloneable interface. Of course, the Employee and Date classes implement the Cloneable interface, so the exception won’t be thrown. However, the compiler does not know that. Therefore, we declared the exception:

public Employee clone() throws CloneNotSupportedException

Would it be better to catch the exception instead?

public Employee clone()
      Employee cloned = (Employee) super.clone();
   catch (CloneNotSupportedException e) { return null; }
   // this won't happen, since we are Cloneable

This is appropriate for final classes. Otherwise, it is a good idea to leave the throws specifier in place. That gives subclasses the option of throwing a CloneNotSupportedException if they can’t support cloning.

You have to be careful about cloning of subclasses. For example, once you have defined the clone method for the Employee class, anyone can use it to clone Manager objects. Can the Employee clone method do the job? It depends on the fields of the Manager class. In our case, there is no problem because the bonus field has primitive type. But Manager might have acquired fields that require a deep copy or are not cloneable. There is no guarantee that the implementor of the subclass has fixed clone to do the right thing. For that reason, the clone method is declared as protected in the Object class. But you don’t have that luxury if you want users of your classes to invoke clone.

Should you implement clone in your own classes? If your clients need to make deep copies, then you probably should. Some authors feel that you should avoid clone altogether and instead implement another method for the same purpose. We agree that clone is rather awkward, but you’ll run into the same issues if you shift the responsibility to another method. At any rate, cloning is less common than you may think. Less than 5 percent of the classes in the standard library implement clone.

The program in Listing 6.4 clones an instance of the class Employee (Listing 6.5), then invokes two mutators. The raiseSalary method changes the value of the salary field, whereas the setHireDay method changes the state of the hireDay field. Neither mutation affects the original object because clone has been defined to make a deep copy.

Listing 6.4 clone/CloneTest.java

 1   package clone;
 3   /**
 4    * This program demonstrates cloning.
 5    * @version 1.10 2002-07-01
 6    * @author Cay Horstmann
 7    */
 8   public class CloneTest
 9   {
10      public static void main(String[] args)
11      {
12         try
13         {
14            Employee original = new Employee("John Q. Public", 50000);
15            original.setHireDay(2000, 1, 1);
16            Employee copy = original.clone();
17            copy.raiseSalary(10);
18            copy.setHireDay(2002, 12, 31);
19            System.out.println("original=" + original);
20            System.out.println("copy=" + copy);
21         }
22         catch (CloneNotSupportedException e)
23         {
24            e.printStackTrace();
25         }
26      }
27   }

Listing 6.5 clone/Employee.java

 1   package clone;
 3   import java.util.Date;
 4   import java.util.GregorianCalendar;
 6   public class Employee implements Cloneable
 7   {
 8      private String name;
 9      private double salary;
10      private Date hireDay;
12      public Employee(String name, double salary)
13      {
14         this.name = name;
15         this.salary = salary;
16         hireDay = new Date();
17      }
19      public Employee clone() throws CloneNotSupportedException
20      {
21         // call Object.clone()
22         Employee cloned = (Employee) super.clone();
24         // clone mutable fields
25         cloned.hireDay = (Date) hireDay.clone();
27         return cloned;
28      }
30      /**
31       * Set the hire day to a given date.
32       * @param year the year of the hire day
33       * @param month the month of the hire day
34       * @param day the day of the hire day
35       */
36      public void setHireDay(int year, int month, int day)
37      {
38         Date newHireDay = new GregorianCalendar(year, month - 1, day).getTime();
40         // Example of instance field mutation
41         hireDay.setTime(newHireDay.getTime());
42      }
44      public void raiseSalary(double byPercent)
45      {
46         double raise = salary * byPercent / 100;
47         salary += raise;
48      }
50      public String toString()
51      {
52         return "Employee[name=" + name + ",salary=" + salary + ",hireDay=" + hireDay + "]";
53      }
54   }
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