All computer programs, regardless of the programming language they are written in, read data from somewhere (a file, the keyboard, the mouse, the Internet, etc.), process the data, and then write the data somewhere else such as the computer monitor or a database file on the hard drive.
In Java, like most other programming languages, data is kept in variables. Yes variables – like X and Y and all those other letters you thought you would never use again in high school algebra. As you create Java programs, you have to declare variables, assign values to them, and execute operations using these variables.
Every variable in a Java program has a certain data type. The data type determines what type of data can be contained within that variable and what operations can be executed on the variable. A variable could be a number. Numbers are further broken down into categories based on their size. A variable can also be a string of text. Although you cannot perform mathematical equations on strings, you can use the “+” operator to concatenate two or more strings together.
int cadence = 0; int speed = 0; int gear = 1;
What are the rules and conventions for naming a field? Besides
int, what other data types are there? Do fields have to be initialized when they are declared? Are fields assigned a default value if they are not explicitly initialized? We’ll explore the answers to such questions in this lesson, but before we do, there are a few technical distinctions you must first become aware of. In the Java programming language, the terms “field” and “variable” are both used; this is a common source of confusion among new developers, since both often seem to refer to the same thing.
The Java programming language defines the following kinds of variables:
- Instance Variables (Non-Static Fields) A simple definition is that instance variables are things an object knows about itself, but the class does not know about. All instances of an object have their own copies of instance variables, even if the value is the same from one object to another. One object instance can change values of its instance variables without affecting all other instances. Changing the value of a class variable changes that value for all objects. Instance variables can be used by all methods of a class unless the method is declared as static. You access instance variables directly from their containing object instances.
- Class Variables (Static Fields) A class variable is any field declared with the
staticmodifier; this tells the compiler that there is exactly one copy of this variable in existence, regardless of how many times the class has been instantiated. A field defining the number of gears for a particular kind of bicycle could be marked as
staticsince conceptually the same number of gears will apply to all instances. The code
static int numGears = 6;would create such a static field. Additionally, the keyword
finalcould be added to indicate that the number of gears will never change.
- Local Variables Similar to how an object stores its state in fields, a method will often store its temporary state in local variables. The syntax for declaring a local variable is similar to declaring a field (for example,
int count = 0;). There is no special keyword designating a variable as local; that determination comes entirely from the location in which the variable is declared — which is between the opening and closing braces of a method. As such, local variables are only visible to the methods in which they are declared; they are not accessible from the rest of the class.
- Parameters You’ve already seen examples of parameters, both in the
Bicycleclass and in the
mainmethod of the “Hello World!” application. Recall that the signature for the
public static void main(String args). Here, the
argsvariable is the parameter to this method. The important thing to remember is that parameters are always classified as “variables” not “fields”. This applies to other parameter-accepting constructs as well (such as constructors and exception handlers) .
Every programming language has its own set of rules and conventions for the kinds of names that you’re allowed to use, and the Java programming language is no different. The rules and conventions for naming your variables can be summarized as follows:
- Variable names are case-sensitive. A variable’s name can be any legal identifier — an unlimited-length sequence of Unicode letters and digits, beginning with a letter, the dollar sign “
$“, or the underscore character “
_“. The convention, however, is to always begin your variable names with a letter, not “
$” or “
_“. Additionally, the dollar sign character, by convention, is never used at all. You may find some situations where auto-generated names will contain the dollar sign, but your variable names should always avoid using it. A similar convention exists for the underscore character; while it’s technically legal to begin your variable’s name with “
_“, this practice is discouraged. White space is not permitted.
- Subsequent characters may be letters, digits, dollar signs, or underscore characters. Conventions (and common sense) apply to this rule as well. When choosing a name for your variables, use full words instead of cryptic abbreviations. Doing so will make your code easier to read and understand. In many cases it will also make your code self-documenting; fields named
gear, for example, are much more intuitive than abbreviated versions, such as
g. Also keep in mind that the name you choose must not be a keyword or reserved word.
- If the name you choose consists of only one word, spell that word in all lowercase letters. If it consists of more than one word, capitalize the first letter of each subsequent word. The names
currentGearare prime examples of this convention. If your variable stores a constant value, such as
static final int NUM_GEARS = 6, the convention changes slightly, capitalizing every letter and separating subsequent words with the underscore character. By convention, the underscore character is never used elsewhere.