3.Variables in Java Programming Language смотреть онлайн

Definition of Variable
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A variable is a container that holds values that are used in a Java program. Every variable must be declared to use a data type. For example, a variable could be declared to use one of the eight primitive data types: byte, short, int, long, float, double, char or boolean. And, every variable must be given an initial value before it can be used.

Examples:
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int myAge = 21;
The variable "myAge" is declared to be an int data type and initialized to a value of 21.

Variables in Java Programming Language
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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 cadence, speed, and gear, for example, are much more intuitive than abbreviated versions, such as s, c, and 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 gearRatio and currentGear are 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.

Rules for Defining a Variable in Java Programming Language
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1)Always begin your Java variable names with a letter, not "$" or "_"

2)The dollar sign character, by convention, is never used at all in Java variables. There may be situations where auto-generated names will contain the dollar sign, but your variable names should always avoid using it

3)It's technically legal to begin your variable's name with "_" but this practice is discouraged

4)Choosing full words instead of cryptic abbreviations for variable names are always considered good, it makes your code easier to read and understand

5)If chosen Java variable name consists of only one word, use all lower-case letters to spell it. If it consists of more than one word, capitalize the first letter of each subsequent word. The names firstChoice and reverseGear are good examples of this convention. For constant valued variables, such as static final int NUM_OF_CHOICES = 7, the convention changes slightly, capitalizing all letter and separating subsequent words with the underscore character. By convention, the underscore character is not used elsewhere.

As it has been said, above conventions are not rules therefore they can be compromised when needed.