Java中Stringbuild,Date和Calendar类的用法详解

//public StringBuilder()：创建一个空白可变字符串对象，不含有任何内容
StringBuilder sb = new StringBuilder();
System.out.println("sb:" + sb);
System.out.println("sb.length():" + sb.length());

//public StringBuilder(String str)：根据字符串的内容，来创建可变字符串对象
StringBuilder sb2 = new StringBuilder("hello");
System.out.println("sb2:" + sb2);
System.out.println("sb2.length():" + sb2.length());

// 链式编程, 链式编程返回结果 看最后调用的方法
StringBuilder abc = new StringBuilder(stringBuilder.append(10).append("abc").append(10.1).append(new Object()).toString());
System.out.println("abc = " + abc);

String string = new StringBuilder(abc).reverse().toString();
System.out.println(string);

public static void main(String[] args) {
	// 创建日期对象，把当前的时间
	System.out.println(new Date()); // Tue Jan 16 14:37:35 CST 2020
	// 创建日期对象，把当前的毫秒值转成日期对象
	System.out.println(new Date(0)); // Thu Jan 01 08:00:00 CST 1970
}

// 获取从 时间原点 到 当前日期 的毫秒值
long time = nowTime.getTime();
System.out.println(time);

 // 设置偏移毫秒值为0, 即时间原点
nowTime.setTime(0);
System.out.println(nowTime);

// 将当前日期 转换为 x年x月x日 xx:xx:xx
Date nowTime = new Date();
DateFormat df = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss E");
System.out.println(df.format(nowTime));

// 获取sDate所代表的时间的毫秒值
String sDate = "1949-10-01";
DateFormat df2 = new SimpleDateFormat("yyyy-MM-dd");
// parse 若无法解析字符串会抛出异常 ParseException
Date date = df2.parse(sDate);
long time = date.getTime();
System.out.println(time);

Calendar c = Calendar.getInstance();
System.out.println(c);

int year = c.get(Calendar.YEAR);
// 0-11表示月份 需要+1
int month = c.get(Calendar.MONTH) + 1;
// DATE 和 DAY_OF_MONTH 的值是一样的
int day = c.get(Calendar.DAY_OF_MONTH);
System.out.println(year+"年"+month+"月"+day+"日");

TimeZone timeZone = c.getTimeZone();
System.out.println(timeZone);

// 将日历设置为2000.5.1, 当前时间为2023.4.5
// add方法设置偏移量
c.add(Calendar.YEAR, -23);
c.add(Calendar.MONTH, 1);
c.add(Calendar.DATE, -4);
System.out.println(c.get(Calendar.YEAR)+"."+(c.get(Calendar.MONTH) + 1)+"."+c.get(Calendar.DAY_OF_MONTH));

// set(int field, int value)方法 将日历设置为2001.4.2
c.set(Calendar.YEAR, 2001);
c.set(Calendar.MONTH, 3);
c.set(Calendar.DAY_OF_MONTH, 2);
System.out.println(c.get(Calendar.YEAR)+"."+(c.get(Calendar.MONTH) + 1)+"."+c.get(Calendar.DAY_OF_MONTH));

// set(int year, int month, int date)方法 将日历设置为2003.10.1
c.set(2003, 9, 1);
System.out.println(c.get(Calendar.YEAR)+"."+(c.get(Calendar.MONTH) + 1)+"."+c.get(Calendar.DAY_OF_MONTH));

Date date = c.getTime();
System.out.println(date);

public class Demo {
    public static void main(String[] args) {
        int[] arr = {3,765,8234,1,23};
        System.out.println(arrayConcatToSting(arr));
    }

    public static String arrayConcatToSting(int[] arr) {
        StringBuilder stringBuilder1 = new StringBuilder("[");
        for (int i = 0; i < arr.length; i++) {
            stringBuilder1.append(arr[i]);
            if (i < arr.length - 1) {
                stringBuilder1.append(", ");
            } else if (i == arr.length - 1){
                stringBuilder1.append("]");
            }
        }
        return stringBuilder1.toString();
    }

}

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Scanner;
public class Demo {
    public static void main(String[] args) throws ParseException {
        Scanner sc = new Scanner(System.in);
        System.out.print("请输入您的生日(年.月.日): ");
        System.out.println("您活了"+howLongHaveYouLived(sc.nextLine())+"天");
    }

    public static long howLongHaveYouLived (String str) throws ParseException {
        DateFormat df = new SimpleDateFormat("yyyy.MM.dd");
        Date birthDay = df.parse(str);
        long birthDayTime = birthDay.getTime();
        long nowTime = new Date().getTime();
        return (nowTime - birthDayTime) / 1000 / 60 / 60 /24;
    }
}

import java.util.Calendar;
import java.util.Date;
import java.util.Scanner;
public class Demo {
    public static void main(String[] args) {
        // 计算指定年份的2月有多少天
        Scanner sc = new Scanner(System.in);
        System.out.print("请输入您要指定的年份: ");
        int inputYear = sc.nextInt();
        System.out.println(inputYear+"年的2月有"+getDay(inputYear)+"天");
    }

    public static int getDay(int year) {
        Calendar calendar = Calendar.getInstance();
        calendar.set(year, 2, 1);
        calendar.add(Calendar.DATE, -1);
        return calendar.get(Calendar.DATE);
    }
}