Java Reference
In-Depth Information
Good Programming Practice 2.1
Some organizations require that every program begin with a comment that states the pur-
pose of the program and the author, date and time when the program was last modified.
Error-Prevention Tip 2.1
As you write new programs or modify existing ones, keep your comments up-to-date with
the code. Programmers will often need to make changes to existing code to fix errors or to
enhance capabilities. Updating your comments helps ensure that they accurately reflect
what the code does. This will make your programs easier to understand and modify in the
future. Programmers using or updating code with out-of-date comments might make in-
correct assumptions about the code that could lead to errors or even security breaches.
Using Blank Lines
Line 3 is a blank line. Blank lines, space characters and tabs make programs easier to read.
Together, they're known as white space (or whitespace). The compiler ignores white space.
Good Programming Practice 2.2
Use blank lines and spaces to enhance program readability.
Declaring a Class
Line 4
public class Welcome1
begins a class declaration for class Welcome1 . Every Java program consists of at least one
class that you (the programmer) define. The class keyword introduces a class declaration
and is immediately followed by the class name ( Welcome1 ). Keywords (sometimes called
reserved words ) are reserved for use by Java and are always spelled with all lowercase let-
ters. The complete list of keywords is shown in Appendix C.
In Chapters 2-7, every class we define begins with the public keyword. For now, we
simply require public . You'll learn more about public and non- public classes in Chapter 8.
Filename for a public Class
A public class must be placed in a file that has a filename of the form ClassName .java , so
class Welcome1 is stored in the file Welcome1.java .
Common Programming Error 2.2
A compilation error occurs if a public class's filename is not exactly same name as the class
(in terms of both spelling and capitalization) followed by the .java extension.
Class Names and Identifiers
By convention, class names begin with a capital letter and capitalize the first letter of each
word they include (e.g., SampleClassName ). A class name is an identifier —a series of char-
acters consisting of letters, digits, underscores ( _ ) and dollar signs ( $ ) that does not begin
with a digit and does not contain spaces. Some valid identifiers are Welcome1 , $value ,
_value , m_inputField1 and button7 . The name 7button is not a valid identifier because
it begins with a digit, and the name input field is not a valid identifier because it contains
a space. Normally, an identifier that does not begin with a capital letter is not a class name.
 
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