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class, the methods in the subclass which override the corresponding methods in the superior class will insure that methods appropriate to the particular instance will be called. This is another way in which OO programming reduces the amount of code that must be written and tested, and also promotes reuse of existing code in new applications. INTERFACES In Java, we can also specify an interface to enforce a common set of behaviors among different classes. For example, think about the problem of sorting a group of Automobile objects. How should Automobiles be ordered By year Alphabetically by make By horsepower This problem arises frequently when we re working with objects, so the Java language specifies an interface that any new class can implement in order to facilitate sorting instances of the new class. An interface consists of one or more method signatures, but it does not contain any code implementing any of the methods. (An interface can include constants as well as method signatures, but we will focus on the methods.) A method signature is like the first line of a method; the signature specifies what the method will return, the name of the method, and what arguments the method expects when it is called. For example, Java provides the interface Comparable, and the Comparable interface specifies a single method called compareTo(). The compareTo() method expects an object as an argument (the object with which to compare the first instance) and it returns an int. The value of the returned int will be 0 if the two objects being compared are equal, -1 if the first object is smaller (should be ordered first), and +1 if the first object is larger (should be ordered second). We can implement the interface Comparable in our Automobile class by adding this code to our class Automobile: class Automobile implements Comparable<Automobile> { . . public int compareTo( Automobile car ) { return this.toString().compareTo( car.toString() ); } The new first line of our Automobile class now declares that the class Automobile will implement the interface Comparable. The <Automobile> syntax says that we will only be comparing an Automobile object with another Automobile. If, by some error of programming, our program tries to compare an Automobile object with a Thermostat object, the JVM will generate an error. The new compareTo() method implements the Comparable interface for the class Automobile. We ve taken a shortcut here which takes advantage of the toString() method we already have for Automobile. The phrase this.toString() will return a String object which represents this instance of an Automobile. The key word this always references the particular instance itself. The String returned will be of this form: 2002 VW Passat The year will be followed by the make and the model of the Automobile. Likewise, the phrase car.toString() will return a String representing the other Automobile, such as this: 2006 Kia Rio The String class has a compareTo() method that orders Strings in ASCIIbetical order, which is like alphabetical order, except that it follows the ASCII encoding values of characters. In ASCII the letters are coded in alphabetical order, but all uppercase letters come before any of the lower-case letters, and digits and symbols are included as well as letters. For our purposes of ordering Automobiles, we thought that the
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toString() representation of Automobiles sorted in ASCIIbetical order would be fine. Older cars will be ordered first, and among cars of the same year, cars will be sorted ASCIIbetically by make and model. The interface idea is similar to the idea of a class, in that an interface creates a new data type. When the class Automobile implements the interface Comparable, instances of Automobile can also be treated as instances of Comparable. Just as good design of a class hierarchy can reduce programming and improve reliability, good interface design can also. For instance, the sort() method of the Java Collections class will sort lists of Comparable objects. Our Automobile class implements Comparable, Java s String class implements Comparable, Java s Date class implements Comparable, etc. One sort() method will work for any group of objects whose class implements Comparable. That alone is a great example of code reuse. It s easy to design and use your own interfaces, too. However, in this brief chapter we will not be discussing that topic. ERROR HANDLING Java uses Exceptions to represent error conditions. Exception is actually a class in Java, and when a program creates an Exception, it creates a new object, which is an instance of the Exception class. An Exception object can have information about what went wrong, usually including an error message, and a stack trace showing which method created the error. Having created an Exception object when something goes wrong, the program throws the Exception using the key word throw. The JVM will print an error message and stop execution when a program throws an Exception, unless the programmer has provided code to catch the exception and handle the error condition in the program. This approach to handling program errors is called exception handling for obvious reasons, and it s a relatively modern idea. One advantage of handling errors this way is that code to handle error conditions will be segregated into code blocks separate from the main logic of the program. This makes it much easier to follow the intent of the programmer, both in the main logic and in the error handling code. Java provides many subclasses of Exception so that different problems result in different classes of Exception objects being thrown. Some examples include FileNotFoundException, NullPointerException, and NumberFormatException. Java recognizes two types of Exceptions, checked and unchecked. The names come from what the Java compiler does with them. The compiler checks for appropriate handling of checked Exceptions, but does not check for handling of unchecked Exceptions. An unchecked Exception represents an error that is probably too serious for an application to correct. An example is the NullPointerException, which occurs when the program tries to access a variable which should contain a reference to an object, but which contains null instead. The compiler assumes that such an exception should cause the program to terminate with an error condition, so it does not check to see that the program has code to handle that error condition itself. A checked exception such as FileNotFoundException represents an error condition from which the program potentially could recover. In the case of FileNotFoundException, for example, the program could prompt the operator for a new file name and try again. If the compiler recognizes that a method could encounter a checked exception, the compiler will require that the method either provide a handler for that exception or declare with its own throws declaration that it can itself generate the checked Exception. The way to add exception handling to a Java program is to enclose program statements which might generate an Exception within a try block. A try block begins with the key word try and an open curly brace. At the end of the code being included in the try block is a close curly brace. Immediately following the try block will be one or more catch blocks. A catch block contains the code to handle the error. Let s go back to our Automobile class and its accelerate() method. Instead of simply setting the speed of the Automobile to its maximum value when the argument to the accelerate() method is too large, we can have the Automobile class generate a special subclass of Exception appropriate to this application. Here is the code for our new ExcessiveSpeedException class:
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