[Java] Data type ①-Basic type

Java variables

• Variable declaration: Notifies the name of the variable to Java and allocates an area to store the value in memory.
• ʻInt data (= initial value) `
• Multiple variables can be declared together
• ʻInt data1, date2 (= initial value) `
• Try to set the initial value
  • int data1= 108;
  • String message = “Hello World”;
• Identifier
• UCS2 is used inside Java.
• All Unicode characters are available
• Case Sensitive
• Variable names are basically readable and ideally readable in English * if (obj.isCompleted()){
• By convention, variables are ** camelClass **, classes are ** Pascal **, constants are ** uppercase + underscore notation **

constant

• ** Variables that cannot be reassigned **

//Bad example
int price = 1000;
double sum = price * 1.08;

• By using constants, the meaning of the calculation formula is easy to understand and it is easy to change it.
• I want to avoid typing a character string directly in the code → Put it out as a constant

//Refactoring example
final double TAX = 1.08;
int price = 1000;
double sum = price * TAX;
System.out.println(sum);

Java data type

• Statically typed language: Determines the type when declaring a variable
• ** Basic type **: The value itself is stored
• ** Reference type **: Stores information that indicates the storage location of the value. Java is basically a reference type
• There are basic types and reference types that correspond to all
• boolean: Basic type Boolean: Reference type
• int: Basic type Integer: Reference type

Integer type: Use basic int

• byte, char: Store byte data
• short: Represents a small integer
• long: Only when dealing with a numerical range that cannot be supported by int

Floating point type: use basic double

• Occupies 64bit memory

• For floating-point types, the jump width of the value increases as the absolute position increases.

• IEEE 754 defines the value retention format

• Normalized, decomposed into mantissa and exponent format and managed 123.75_{(10)}=1111011.11_{(2)}=1.11101111*2^{6}_{(2)}

• $ 1.23 * 10 ^ 1 $ ~ $ 1.24 * 10 ^ 1 $ and $ 1.23 * 10 ^ {100} $ ~ $ 1.24 * 10 ^ {100} $ will make a difference! !!

Character type

• ** Char type **: 16bit --Unicode (UCS2) 1 character
• ** String type **: Variable length, multiple of 2

Boolean value (boolean, logical type)

• Cannot be converted to a numeric type (true = 1, false = 0, etc.)

Type inference

• var variable name = initial value
• var i = 48;
• Data type can be omitted
• The compiler automatically infers the shoulder from the assigned value and determines = type inference
• The above example infers an int
• Initial value cannot be omitted
  • Cannot use var on variable without initializer
• Cannot declare multiple
• Only local variables (variables declared in methods) that cannot be used in field declarations

What is a literal?

• ** The value itself that can be stored in the data type, the representation method **
• Integer literals, floating point literals, truth literals, etc., depending on the data type

Integer literal

• Decimal / 16-base / 8-base / 2-base literal

Floating point literal

• $ E $
• 1.14142e-5 → 0.0000114142

System.out.println("You are \"GREATE\" !");
System.out.println("Welcome to wherever you are \nThis is your life, you made it this far");
var str = "Welcome, you got to believe " + "That right here, right now"; //Separated for readability improvement
System.out.println(str);

int i = 10;
// byte b = i ; //NG
byte b  = (byte)i ; //OK