class

all objects of a class have the same methods and variables
but different objects can hold different values in the same variable

more about methods

2 types of methods
return type (int, string, etc...)
void type

public class SampleClass{
	public int x; 
    public void sayHello(int y) {
        System.out.println("Hello" + x + " " + y);
    }

    public int sqareX() {
        return x*x ; 
    } 
}

SampleClass c1 = new SampleClass();
c1.x = 6 ;
c1.sayHello(8); // print Hello 6 8
int z = c1.squareX(); // z=36

multiple classes in multiple java files

multiple classes in a single java file

local variables

variable declared within a method definition
all method parameters are local variables
NO global variable in java

Static

static 에서는 static만 부를 수 있기때문에 myMethod 도 static 으로 해야함

parameter과 argument는 혼용해서 사용

this parameter

class MyDate { 
	int year; 
    String month; 
    int day; 
    void setMyDate ( int newYear, String month, int day) {
    	year = newYear; 
        this.month = month; 
        this.day = day ; 
    }
}

equals & toString

public boolean equals (ClassName objectName)
== 는 주소가 같은지 확인
equals 는 내용이 같은지 확인 => boolean

if ~~~
System.out.println(date1 + ~~ ) 에서
date1 => date1.toString()

information hiding

public & private

public :
can be used outside the class
no restriction
private :
cannot be accessed by name outside the class

good programming practice (concept of oop) -
variables: private
method: public

accessor getXXX()

data can be read but not changed

mutator setXXX()

change the value of object's instance values in a controlled manner

import java.util.Scanner;

public class DateFifthTry
{
    private String month;
    private int day;
    private int year; //a four digit number.

  public void writeOutput( )
    {
        System.out.println(month + " " + day + ", " + year);
    }

    public void readInput( )
    {
        boolean tryAgain = true;
        Scanner keyboard = new Scanner(System.in);
        while (tryAgain)
        {
            System.out.println("Enter month, day, and year");
            System.out.println("as three integers:");
            System.out.println("do not use commas or other punctuations.");
            int monthInput = keyboard.nextInt( );
            int dayInput = keyboard.nextInt( );
            int yearInput = keyboard.nextInt( );
            if (dateOK(monthInput, dayInput, yearInput) )
            {
                setDate(monthInput, dayInput, yearInput);
                tryAgain = false;
            }
            else
                System.out.println("Illegal date. Reenter input.");
         }
    }

    public void setDate(int month, int day, int year)
    {
        if (dateOK(month, day, year))
        {
            this.month = monthString(month);
            this.day = day;
            this.year = year;
        }
        else
        {
            System.out.println("Fatal Error");
            System.exit(0);
        }
    }

    public void setMonth(int monthNumber)
    {
        if ((monthNumber <= 0) || (monthNumber > 12))
        {
            System.out.println("Fatal Error");
            System.exit(0);
        }
        else
            month = monthString(monthNumber);
    }

    public void setDay(int day)
    {
        if ((day <= 0) || (day > 31))
        {
            System.out.println("Fatal Error");
            System.exit(0);
        }
        else
            this.day = day;
    }

    public void setYear(int year)
    {
        if ( (year < 1000) || (year > 9999) )
        {
            System.out.println("Fatal Error");
            System.exit(0);
        }
        else
            this.year = year;
    }

    public boolean equals(DateFifthTry otherDate)
    {
        return ( (month.equalsIgnoreCase(otherDate.month))
                  && (day == otherDate.day) && (year == otherDate.year) );
    }
    public boolean precedes(DateFifthTry otherDate)
    {
        return ( (year < otherDate.year) ||
           (year == otherDate.year && getMonth( ) < otherDate.getMonth( )) ||
           (year == otherDate.year && month.equals(otherDate.month)
                                         && day < otherDate.day) );
    }

    public String toString( )
    {
        return (month + " " + day + ", " + year);
    }

    public int getDay( )
    {
        return day;
    }

    public int getYear( )
    {
        return year;
    }

    public int getMonth( )
    {
        if (month.equalsIgnoreCase("January"))
            return 1;
        else if (month.equalsIgnoreCase("February"))
            return 2;
        else if (month.equalsIgnoreCase("March"))
            return 3;
        else if (month.equalsIgnoreCase("April"))
            return 4;
        else if (month.equalsIgnoreCase("May"))
            return 5;
        else if (month.equals("June"))
            return 6;
        else if (month.equalsIgnoreCase("July"))
            return 7;
        else if (month.equalsIgnoreCase("August"))
            return 8;
        else if (month.equalsIgnoreCase("September"))
            return 9;
        else if (month.equalsIgnoreCase("October"))
            return 10;
        else if (month.equalsIgnoreCase("November"))
            return 11;
        else if (month.equalsIgnoreCase("December"))
            return 12;
        else
        {
            System.out.println("Fatal Error");
            System.exit(0);
            return 0; //Needed to keep the compiler happy
        }
    }


    private boolean dateOK(int monthInt, int dayInt, int yearInt)
    {
        return ( (monthInt >= 1) && (monthInt <= 12) &&
                 (dayInt >= 1) && (dayInt <= 31) &&
                 (yearInt >= 1000) && (yearInt <= 9999) );
    }

    private String monthString(int monthNumber)
    {
        switch (monthNumber)
        {
        case 1:
            return "January";
        case 2:
            return "February";
        case 3:
            return "March";
        case 4:
            return "April";
        case 5:
            return "May";
        case 6:
            return "June";
        case 7:
            return "July";
        case 8:
            return "August";
         case 9:
            return "September";
        case 10:
            return "October";
        case 11:
            return "November";

        case 12:
            return "December";
        default:
            System.out.println("Fatal Error");
            System.exit(0);
            return "Error"; //to keep the compiler happy
        }
    }
}

precondition postcondition in comments

/** 
precondition: ~~
postcondition: ~~
 **/

overloading

two or more methods in the same class have the same method name
parameter의 개수가 다름 / parameter의 type이 다름

you cannot overload based on the type returned

you cannot overload operators in java (C++과는 구분되는 특징)

constructors

special kind of method

public ClassName (anyParameters) {code}

ClassName objectName = new ClassName(anyArgs);

if a constructor is invoked again using new, the first obj is discarded and entirely new obj is created

import java.util.Scanner; 

public class Date {
	private String month;
    private int day;
    private int year;
    
    public Date() { //default constructor 
    	month = "January";
        day = 1; 
        year = 1000; 
    }
    
    public Date(int monthInt, int day, int year) { //constructor
    	setDate(monthInt, day, year); 
    }
    
    public Date(String monthString, int day, int year) {
    	setDate(monthString, day, year);
    }
    
    public Date(int year) {
    	setDate(1, 1, year); 
    }
    
    public Date(Date aDate) {  // copy constructor => ch5
    	if (aDate == null) {
        	System.out.println("FatalError");
            System.exit(0); 
        }
        month = aDate.monthl 
        day = aDate.day;
        year = aDate.year; 
    }
    ...
    public setDate ... 
    ...
    
}

public class ConstructorsDemo
{
	public static void main(String[] args)
    {
    	Date date1 = new Date("December", 16, 1770);
        Date date2 = new Date (1, 27, 1756);
        Date date3 = new Date(1882);
        Date date4 = new Date(); 
        
        System.out.println(date1);
        System.out.println(date2); 
        System.out.println(date3);
        System.out.println(date4); 
    }
}

default variable initializations

boolean => false
other primitives => zero
types => null
local variables are not automatically initialized

StringTokenizer

import java.util.Scanner;
import java.util.StringTokenizer

public class StringTokenizerDemo
{
	public static void main(String[] args)
    {
    	Scanner keyboard = new Scanner(System.in);
        System.out.println("Enter your last name, first name, middle name)
        System.out.println("if u have no middle name, enter \"none\".");
        String inputLine = keyboard.nextLine(); // 정, 문경 none
        String delimiters = ", "; //comma and black space
        StringTokenizer nameFactoru = new StringTokenizer(inputLine, delimiters);
        String lastName = nameFactory.nextToken();
        String firstName = nameFactory.nextToken(); 
        String middleName = nameFactory.nextToken();
        if(middleName.equalsIgnoreCase("None"))
        	middleName = ""; 
        System.out.println("Hello " + firstName + " " + middleName + " " + lastName); //hello 문경 정 //comma는 delimiter여서 안읽힘 
    }
}

StringTokenizer.nextToken();