Variables

자바의 변수에 대한 기본 개념 및 문법을 정리하였습니다.

 

 

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< Introduction >

To store information, use variables, named location in memory.

Storing a information allows us to use that information later, accessing the information we stored.

 

• When declare variable, specify the type of information we're storing. There's two main data-types.
  1. Primitive Type
  2. Non-Primitive Data Types(called Reference Type)

 

< Primitive Data Type >

Data-Type	Size	Description
byte	1 byte	Stores whole numbers from -128 to 127
short	2 bytes	Stores whole numbers from -32,768 to 32,767
int	4 bytes	Stores whole numbers from -2,147,483,648 to 2,147,483,647
long	8 bytes	Stores whole numbers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
float	4 bytes	Stores fractional numbers. Sufficient for storing 6 to 7 decimal digits
double	8 bytes	Stores fractional numbers. Sufficient for storing 15 decimal digits
boolean	1 bit	Stores true or false values
char	2 bytes	Stores a single character/letter or ASCII values

 

Example)

int year = 2000;
char myName = 'Lee';
boolean olderThanTwenty = true;

 

Reference : Java Data Types

Java Data Types

 

 

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< int >

To store Whole Number.

 

Example)

// int variable declaration
int yearJavaWasCreated;
// assignment
yearJavaWasCreated = 1996;
// declaration and assignment
int numberOfPrimitiveTypes = 8;

 

 

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< doubles >

To store Decimal.

 

Example)

// doubles can have decimal places:
double price = 8.99;
// doubles can have values bigger than what an int could hold:
double gdp = 12237700000;

 

 

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< booleans >

Two values : true or false (like yes or no)

 

Example)

boolean javaIsACompiledLanguage = true;
boolean javaIsACupOfCoffee = false;

 

 

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< char >

Hold any character, like a letter, space, or punctuation mark.

It must be surrounded by single quotes, ' ' not " "

 

Example)

char grade = 'A';
char firstLetter = 'p';
char punctuation = '!';

 

 

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< String >

String is an object, have built-in behavior (Reference Type)

String holds sequences of characters.

It must be surrounded by double quotes, " " not ' '

 

Example)

String greeting = "Hello World";

String salutations = new String("Hello World");

: Also create a new String object by calling the String class when declaring a String.

 

 

+. Escape Character : \

- To remove the meaning of the character that follows.

 

Example)

System.out.println("\"Hello World\"");
// "Hello World"

System.out.println("This is the backslash symbol: \\");
// This is the backslash symbol: \

System.out.println("Hello\nGoodbye");
// Hello
// Goodbye

: \n means Spacing

 

 

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< Naming >

Naming variables according to convention leads to clear, readable, and maintainable code.

 

• In Java, variable names are case-sensitive.
  myvariable is a different variable from myVariable
• A variable starts with a valid letter, or a $, or a _ , No other symbols or numbers can begin a variable name. 
  1stPlace and *Gazer are not valid variable names.
  
• Variable naming in Java follows camelCase style
  : Variable names of more than one word have the first letter lowercase while the beginning letter of each subsequent word is capitalized.

 

Example)

// bad naming
String data = "Delilah";

// good naming
String nameOfUser = "Delilah";

// good style
boolean isHuman;
 
// bad styles
// no capitalization for new word
boolean ishuman;
// first word should be lowercase
boolean IsHuman;
// underscores don't separate words
boolean is_human;