week4. Fundamental Data Types

◆ Number Types

- int : integer, no fractional parts

- double  : floating-point numbers (double precision)

※ Primitive Types

Type	Description	Size
int	The integer type, with range -2,147,483,648 . . . 2,147,483,647	4 bytes
byte	The type describing a single byte, with range -128 . . . 127	1 byte
short	The short integer type, with range -32768 . . . 32767	2 bytes
long	The long integer type, with range  -9,223,372,036,854,775,808 . . . 9,223,372,036,854,775,807	8 bytes
double	The double-precision floating-point type, with a range of about ±10308 and about 15 significant decimal digits	8 bytes
float	The single-precision floating-point type, with a range of about ±1038 and about 7 significant decimal digits	4 bytes
char	The character type, representing code units in the Unicode encoding scheme	2 bytes
boolean	The type with the two truth values false and true	1 bit

◆ Number Types : Floating-point Types

- Rounding errors occur when an exact conversion between numbers is not possible

- Java : Illegal to assign a floating-point expression to an integer variable

*** Java는 자동형변환을 지원하지 않는다. 캐스팅을 반드시 해주어여 한다.

◆ Constants : final

- A final variable is a constant (c에서 const가 있다면, java에선 final이 있다!)

- Once its value has been set, it cannot be changed

- Convention : Use all-uppercase names for constants

◆ Constants : static final

- Only 1 copy ~ sharing wit/ objects

- If constant values are needed in several methods, declare them together with the instance fields of a class and tag them as static and final

◆ Arithmetic Operator

- basic : +, -, *, /

- Priority of *, / is higher than +,-

- Parenthses control the order of subexpression computation :

ex. ( a + b ) / 2

◆ Increment and Decrement 

- item++   ->>> item = item + 1;

- item--     ->>> item = item - 1;

◆ Integer Division

- / is the division operator

- If both arguments are integer, the result is an integer. The remainder is discarded.

(정수끼리 연산일 때 몫만 계산)

◆ Powers and Roots

- Math class : contains methods sqrt and pow to compute square roots and powers

- To compute x^n, you write Math.pow(x,n)

- To take the square root of a number, use Math.sqrt ~ square root of x ~ Math.sqrt(x)

- Mathematical Methods

Function	Returns
Math.sqrt(x)	square root
Math.pow(x, y)	power xy
Math.exp(x)	ex
Math.log(x)	natural log
Math.sin(x), Math.cos(x), Math.tan(x)	sine, cosine, tangent  (x in radians)
Math.round(x)	closest integer to x
Math.min(x, y), Math.max(x, y)	minimum, maximum

◆ Cast and Round

- Cast converts a value to a different type

- Math.round converts a floating-point number to nearest integer

◆ Calling Static Methods

- A static method does not operate on an object (ex. Math Class)

- Static methods are declared inside classes

- Naming convention : Classes start with an uppercase letter; objects start with a lowercase

◆ The String Class

- A string  is a sequence  of characters

- Strings are objects of the String class

- A String literal is a sequence of characters enclosed in double quotation marks : "Hello, Emzei!"

- String length is the number of characters in the String

- Empty string : ""

◆ Concatenation

- use the + oeprator :

ex.

String name = "Emzei";

String message = "Hello, " + name;

// message is "Hello, Emzei"

- If one of the arguments of the + operator is a string, the other is converted to a string

ex.

String a = "Agent";

int n =7;

String bond = a + n ; // bond is "Agent7"

- in Print Statements

* Useful to reduce the number of System.out.print instructions

ex.

System.out.print("The total is ");

System.out.println(total);

versus

System.out.println("The total is "+total);

◆ Converting between Strings and Numbers

- Convert to number :

int n = Integer.parseInt(str);

double x = Double.parseDouble(x);

- Convert to string :

String str = "" + n;

str = Integer.toString(n);

- convert 할 때...

※ String과 숫자는 위의 함수들을 이용

※ Casting은 숫자와 숫자 사이에서 이용

◆ Substrings (부분문자열)

- String greeting = "Hello, World!";

  String sub = greeting.substring(0, 5); // sub is "Hello"

- Supply start and "past the end" position

- (string object's name).substring(starting index, ending index)

※ charAt(index) -> 해당 index의 char를 리턴

◆ Reading Input

- System.in has minimal set of features ㅡ it can only read on bye at a time

- In Java 5.0, Scanner class was added to read keyboard input in a convenient manner

ex. 

Scanner in = new Scanner(System.in);

System.out.println("Enter quantity:");

int quantity = in.nextInt();

* nextDouble reads a double

* nextLine read a line (until user hits Enter)

* next reads a word (until any white space)

◆ Reading Input From a Dialog Box

- String input = JOptionPane.showInputDialog(prompt)

- Convert strings to numbers if necessary:

int count = Integer.parseInt(input);

- Conversion throws an exception if user doesn't supply a number - see u ch.11

- Add System.exit(0) to the main method of any program that uses JOptionPane

※ 

JOptionPane ~ OptionPanel 만들어주는 API

System.exit(0) -> close program after saving correctly