날짜는 프로그램에서 자주 사용되는 데이터이다. 자바는 시스템의 날짜 및 시각을 읽을 수 있도록 Date 와 Calendar 클래스를 제공하고 있다. 이 두 클래스는 모두 java.util 패키지에 포함되어 있다.

 

1. Date 클래스

Date는 날짜를 표현하는 클래스이다. Date 클래스는 객체 간에 날짜 정보를 주고 받을 때 주로 사용된다. Date 클래스에는 여러 개의 생성자가 선언되어 있지만 대부분 Deprecated(비권장) 되어 현재는 Date() 생성자만 주로 사용한다. Date() 생성자는 컴퓨터의 현재 날짜를 읽어 Date 객체로 만든다.

Date now   =new Date();

현재 날짜를 문자열로 얻고 싶다면 toString() 메소드를 사용하면 된다. toString() 메소드는 영문으로 된 날짜를 리턴하는데 만약 특정 문자열 포맷으로 얻고 싶다면 java.text.simpleDateFormat 클래스를 이용하면 된다. SimpleDateFormat 클래스는 다음 절에서 설명한다.

 

public class DateExample {
    
    public static void main(String[] args) {
        
        Date now =new Date();
        
        String strNow1 =now.toString();
        System.out.println(strNow1);
        
        SimpleDateFormat sdf =
                new SimpleDateFormat("yyyy년 MM월 dd일 hh시 mm분 ss초");
        String strNow2 =sdf.format(now);
        System.out.println(strNow2);
        
    }    
}

 

Sat Jun 11 13:41:31 KST 2016
2016년 06월 11일 01시 41분 31초
 

2. Calendar 클래스

Calendar 클래스는 달력을 표현한 클래스이다. Calendar 클래스는 추상(abstract) 클래스이므로 new 연산자를 사용해서 인스턴스를 생성할 수 없다.

그 이유는 날짜와 시간을 계산하는 방법이 지역과 문화, 나라에 따라 다르기 때문이다. 우리나라만 해도 양렬과 음력이 동시에 사용되고 있다. 그래서 Calendar 클래스에는 날짜와 시간을 계산하는데 꼭 필요한 메소들만 선언되어 있고, 특정한 역법(날짜와 시간을 매기는 방법) 을 따르는 계산 로직은 하위 클래스에서 구현하도록 되어 있다. 특별한 역법을 사용하는 경우가 아니라면 직접 하위 클래스를 만들 필요는 없고 Calendar 클래스의 정적 메소드인 getInstance() 메소드를 이용하면 현재 운영체제에 설정되어 잇는 시간대 (TimeZone) 를 기준으로 한 Calendar 하위 객체를 얻을 수 있다.

Calendar now  =Calendar.getInstance();

 

calendar 객체를 얻었다면 get() 메소드를 이용해서 날짜와 시간에 대한 정보를 읽을 수 있다.

int year  =now.get(Calendar.year);

int month =now.get(Calendar.MONTH) +1;

int day =now.get(Calendar.DAY_OF_MONTH);

int week =now.get(Calendar.DAY_OF_WEEK);

int amPm =now.get(Calendar.AM_PM);

int hour =now.get(Calendar.HOUR);

int minute =now.get(Calendar.MINUTE);

int second =now.get(Calendar.SECOND);

 

get() 메소드를 호출할 때 사용한 매개값은 모두 Calendar 클래스에 선언되어 있는 상수들이다.

public class CaledarExample {
    
    public static void main(String[] args) {
        
        Calendar now =Calendar.getInstance();
        
        int year =now.get(Calendar.YEAR);
        int month=now.get(Calendar.MONTH)+1;
        int day =now.get(Calendar.DAY_OF_MONTH);
        
        int week =now.get(Calendar.DAY_OF_WEEK);
        String strWeek =null;
        
        switch(week){
            
            case Calendar.MONDAY :
                strWeek="월";
                break;
            
            case Calendar.TUESDAY :
                strWeek="화";
                break;
            
            case Calendar.WEDNESDAY :
                strWeek="수";
                break;
                
            case Calendar.THURSDAY :
                strWeek="목";
                break;
            case Calendar.FRIDAY:
                strWeek="금";
                break;
                
            case Calendar.SATURDAY :
                strWeek="토";
                break;
            
                default :
                    strWeek="일";
        }
        
        int amPm =now.get(Calendar.AM_PM);
        String strAmPm =null;
        if(amPm ==Calendar.AM){
            strAmPm ="오전";
        }else{
            strAmPm ="오후";
        }
        
        int hour =now.get(Calendar.HOUR);
        int minute =now.get(Calendar.MINUTE);
        int second =now.get(Calendar.SECOND);
        
        System.out.print(year + "년");
        System.out.print(month+ "월");
        System.out.print(day + "일");
        
        System.out.println(strWeek  + "요일");
        System.out.println(strAmPm  + "");
        System.out.println(hour  + "시");
        System.out.println(minute  + "분");
        System.out.println(second  + "초");
        
    }
    
}
2016년6월11일토요일
오후
2시
30분
50초
 

public class PrintTimeZoneID {

    public static void main(String[] args) {
        
        String[] availableIDs =TimeZone.getAvailableIDs();
        for(String id : availableIDs){
            System.out.println(id);
        }
        
    }
    
}
 

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