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Interview Questions
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J2EE, J2ME, JAVA 4
What Is an Interface?

As you've already learned, objects define their interaction with the outside world through the methods that they expose. Methods form the object's interface with the outside world; the buttons on the front of your television set, for example, are the interface between you and the electrical wiring on the other side of its plastic casing. You press the "power" button to turn the television on and off.

Floating Point Variables

You may have noticed something a little funny about the above output. The numbers aren't exactly correct. Zero degrees Fahrenheit is actually -17.778 degrees Celsius, not -18 degrees Celsius as this program reports. The problem is that we used only integers here, not decimal numbers. In computer-speak decimal numbers are called "floating point numbers."

Floating point numbers can represent a broader range of values than integers. For example you can write very large numbers like the speed of light (2.998E8 meters per second) and very small numbers like Plank's constant (6.63E-27 ) using the same number of digits. On the other hand you lose some precision that you probably didn't need for such large and small numbers anyway.

What Is an Exception?

When an error occurs within a method, the method creates an object and hands it off to the runtime system. The object, called an exception object, contains information about the error, including its type and the state of the program when the error occurred. Creating an exception object and handing it to the runtime system is called throwing an exception.

Recursive Methods

Java supports recursive methods, i.e. even if you're already inside methodA() you can call methodA(). The easiest way I can think of to explain recursion is to look at a simple acronym, GNU. The GNU project, among other things, is trying to produce free versions of the Unix operating system and many Unix tools, such as lex, yacc, and cc. One minor problem with this effort is that the name Unix is trademarked so the GNU project can't use it. Hence, instead of Unix, we have GNU, where GNU stands for "Gnu's Not Unix." The definition of GNU refers to itself; that is, it's recursive. So what is GNU? One level deeper it's "(Gnu's Not Unix)'s Not Unix." One level deeper still, it becomes "((Gnu's Not Unix)'s Not Unix)'s Not Unix." And so on, ad infinitum. It's like standing between a pair of mirrors. The images just fade off into the distance with no clear end in sight. In computer programming recursion is achieved by allowing a method to call itself.

Advantages of Exceptions

Exceptions provide the means to separate the details of what to do when something out of the ordinary happens from the main logic of a program. In traditional programming, error detection, reporting, and handling often lead to confusing spaghetti code. For example, consider the pseudocode method here that reads an entire file into memory.

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