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The conceptual framework for exceptions predates Java. So, too, does the incorporation of exceptions into other programming languages. For instance, exceptions were added to C++ several years before Java was created. What makes Java s approach to exceptions important is that they were part of the original design. They were not added after the fact. Exceptions are fully integrated into Java and form one of its foundational features. A key aspect of Java s exception mechanism is that its use is not optional. In Java, handling errors through the use of exceptions is the rule. This differs from C++, for example, in which exceptions are supported but are not fully integrated into the entire programming environment. Consider the common situations of opening or reading from a file. In Java, when an error occurs during one of these operations, an exception is thrown. In C++, the methods that open or read from a file report an error by returning a special error code. Because C++ did not originally support exceptions, its library still relies on error return codes rather than exceptions, and your program must constantly check for possible errors manually. In Java, you simply wrap the file-handling code within a try/catch block. Any errors will automatically be caught.
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Polymorphism is the attribute of object-oriented programming that allows one interface to be used by multiple methods. Java supports polymorphism with a variety of features, but two stand out. The first is the fact that every method (other than one marked final) can be overridden by a subclass. The second is the interface keyword. Let s examine each a bit closer. Because methods in a superclass can be overridden by those in a derived class, it s trivially easy to create class hierarchies in which subclasses are specializations of the superclass. Recall that a superclass reference can be used to refer to any subclass of that superclass. Furthermore, a call to a method on a subclass object, through a superclass reference, automatically executes the overridden version of that method. Thus, a superclass can define the form of an object and provide a default implementation. This default implementation can then be customized by a subclass to better meet the needs of a specific situation. Thus, the same interface, in this case the one defined by the superclass, can be the basis for multiple implementations. Of course, Java takes the concept of one interface, multiple methods a step further. It defines the interface keyword, which allows you to fully separate a class methods from their implementations. Although an interface is abstract, you can still declare a reference of an interface type. This reference can then be used to refer to any object that implements the interface. This is a very powerful concept because it streamlines and facilitates the use of polymorphism. As long as a class implements an interface, an object of that class can be
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used by any code that requires the functionality provided by the interface. For example, assuming an interface called MyIF, consider the following method:
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void myMeth(MyIF ob) { // ... }
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Any object that implements the MyIF interface can be passed to myMeth( ). It doesn t matter what other capabilities (if any) that object has. If it implements MyIF, then myMeth( ) can operate on it.
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Despite all of its powerful features, Java may not have been much more than a footnote in programming history if it were not for one important but nearly transparent part of the language: bytecode. As all Java programmers know, the output of the Java compiler is not machine code that can be directly executed by a CPU. Instead, it is a highly optimized set of portable instructions, called bytecode, which are executed by the Java Virtual Machine (JVM). The original JVM was simply an interpreter for bytecode. Today, the JVM also applies on-the-fly compilation of bytecode into executable code. Whatever process is used to execute bytecode, its advantages are enormously important to the success of Java. The first advantage is portability. By compiling a Java program into bytecode, it can be executed on any computer, with any type of CPU (and operating system) as long as a JVM is available for that environment. In other words, once a JVM has been implemented for a specific environment, any Java program can run in that environment. It is not necessary to create a separate executable for each different environment. The same bytecode can be run in all environments. Therefore, through the use of bytecode, Java offered programmers the ability to write once, run anywhere. The second advantage achieved by bytecode is security. Because execution of the bytecode is under the control of the JVM, the JVM can prevent a Java program from performing malicious acts that affect the host machine. The ability to ensure the security of the host computer was crucial to the success of Java because it enabled the creation of the applet. Because an applet is a small, dynamically downloaded program that comes across the Internet, some mechanism to prevent applets from doing harm was necessary. The combination of bytecode and the JVM provided the mechanism by which applets could be downloaded safely. Frankly, without bytecode, the Web would be a much different place today.
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