If you believe the marketing hype put out by Oracle and others, you think that Java is the best thing to happen to computers since the invention of memory. Java may not be that revolutionary, but it does have many built-in features that set it apart from other languages. The following sections describe just three of the many features that make Java so popular.
Type checking
All programming languages must deal in one way or the other with type checking - the way that a language handles variables that store different types of data. Numbers, strings, and dates, for example, are commonly used data types available in most programming languages. Most programming languages also have several types of numbers, such as integers and real numbers.
All languages must check data types, so make sure that you don't try to do things that don't make sense (such as multiplying the gross national product by your last name). The question is, does the language require you to declare every variable's type so you can do type checking when it compiles your programs, or does the language do type checking only after it runs your program?
Some languages, such as Perl, are not as rigid about type checking as Java. For example, Perl does not require that you indicate whether a variable will contain an integer, a floating point number, or a string. Thus, all the following Java statements are allowed for a single variable named
$a:
$a = 5
$a = "Strategery"
$a = 3.14159
Here three different types of data - integer, string, and double - have been assigned to the same variable.
Java, on the other hand, does complete type checking when the program is compiled. As a result, you must declare all variables as a particular type so that the compiler can make sure you use the variables correctly. The following bit of Java code, for example, won't compile:
int a = 5;
String b = "Strategery";
This code creates a variable (newHire) that can hold only Employee objects. Then it tries to assign the number 21 to it. The Java compiler won't let you run this program because 21 is a number, not an employee.
[TECHNICAL STUFF
An important object-oriented programming feature of Java called inheritance adds an interesting - and incredibly useful - twist to type checking. Inheritance in Java is way too complicated to dive into just yet, so I'll be brief here: In Java, you can create your own data types that are derived from other data types. Employees are people, for example, and customers are people too, so you might create a Person class and then create Employee and Customer classes that both inherit the Person class. Then you can write code like this:
]
Person p;
Employee e;
Customer c;
p = e; // This is allowed because an Employee is also a Person.
c = e; // This is not allowed because an Employee is not a Customer.
Exception handling
As Robert Burns said, "The best-laid schemes o' mice an' men gang oft agley, an' lea'e us nought but grief an' pain, for promis'd joy!" When you tinker with computer programming, you'll quickly discover what he meant. No matter how carefully you plan and test your programs, errors happen, and when they do, they threaten to bring your whole program to a crashing halt.
Java has a unique approach to error handling that's superior (in my opinion) to that of any other language (except C#, which just copies Java's approach). In Java, the JRE intercepts and folds errors of all types into a special type of object called an exception object. After all, Java is object-oriented through and through, so why shouldn't its exception-handling features be object-oriented?
Java requires any statements that can potentially cause an exception to be bracketed by code that can catch and handle the exception. In other words, you, as the programmer, must anticipate errors that can happen while your program is running and make sure that those errors are dealt with properly. Although this necessity can be annoying, it makes the resulting programs more reliable.
About the Book Author
Doug Lowe began writing computer books before Java was invented. He's covered dinosaurs such as COBOL, FORTRAN, and IBM mainframe computers, as well as web programming, Microsoft PowerPoint, and networking. Doug has written more than 30 For Dummies computer guides.
Java is a vital skill for any programmer. This convenient one-stop guide brings together the essentials to take you from wide-eyed Java novice to skilled user with a minimum of frustration. You'll learn the basics of writing Java code and then apply those skills to real projects. Along the way learn the details of object-oriented programming, JavaFX, and programming for the web. You can dive even deeper and grab a free bonus section on files and databases at dummies.com. Before you know it, you'll be building your own web apps and databases.
7 Books Inside ...
• Java Basics
• Programming Basics
• Object-Oriented Programming
• Strings, Arrays, and Collections
• Programming Techniques
• JavaFX
• Web Programming
• Web bonus: Files and Databases