Since the object content of the String class cannot be changed, a new String object will be constructed every time it is spliced, which is time-consuming and a waste of memory space
At this time, you need to use java The provided StringBuild class solves this problem
StringBuilder is also called a variable character sequence. It is a string buffer similar to String and can be regarded as a container. Many strings can be held in the container
Variable means that the content in the StringBuilder object is variable
Construction method
public StringBuilder(): Create an empty buffer
public StringBuilder(String srt): Create a buffer that stores str
public StringBuilding reverse(): Reverse the buffer data
String string = new StringBuilder(abc).reverse().toString();
System.out.println(string);
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Date class
java.util.Date Represents a specific moment, accurate to milliseconds
Construction method
public Date(): The current date object, the millisecond value elapsed from the time when the program is run to the time origin, is converted into a Date object, allocates a Date object and initializes this object to represent the time at which it is allocated (accurate to milliseconds).
public Date(long date): Convert the millisecond value date of the specified parameter into a Date object, allocate the Date object and initialize this object
Time origin : January 1, 1970 00:00:00 China is in the East 8th District. Strictly speaking, it is January 1, 1970 00:08:00 1s = 1000ms
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
}
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getTime
long getTime(): Get the millisecond value of the date object
// 获取从 时间原点 到 当前日期 的毫秒值
long time = nowTime.getTime();
System.out.println(time);
java.text.DateFormat is an abstract class of date/time formatting subclass, we can use this class to help us complete it Conversion between date and text, that is, you can convert back and forth between Date object and String object.
SimpleDateFormat
Since DateFormat is an abstract class and cannot be used directly, a commonly used subclass java.text.SimpleDateFormat is required. This class requires a pattern (format) to specify the standard for formatting or parsing.
Construction method
public SimpleDateFormat(): Constructs SimpleDateFormat using the default mode and locale date format symbols.
The default format is: (year)-(month)-(day) (am/pm)xx:xx
public SimpleDateFormat(String pattern) : Constructs a SimpleDateFormat with the given pattern and date format symbols of the default locale.
The parameter pattern is a string representing a custom format for date and time.
Commonly used format rules are:
Identifier letters (case sensitive)
Meaning
y
年
M
月
d
日
H
hours
m
minutes
s
Seconds
Note: For more detailed format rules, please refer to the API documentation of the SimpleDateFormat class.
Convert date object to string
public String format(Date date): Pass the date object and return the formatted string.
// 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));
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Convert string to date object
public Date parse(String source) Pass the string and return the date object
// 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);
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Calendar class
java.util.Calendar Calendar calendar class, replaces many Date methods
It is an abstract class, but provides static methods to create objects, and also provides Many static attributes
Month 0-11 represents January-December The first day of the week abroad is Sunday
getInstance
public static Calendar getInstance(): Get a calendar using the default time zone and locale.
Calendar c = Calendar.getInstance();
System.out.println(c);
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Static attributes and their corresponding fields
Use class name.property name to call, representing the given calendar field:
Field value
Meaning
YEAR
年
MONTH
Month (starts from 0, can be used as 1)
DAY_OF_MONTH
Day of the month (number)
HOUR
hour (12-hour format)
HOUR_OF_DAY
hour (24-hour format)
MINUTE
Minutes
SECOND
Seconds
##DAY_OF_WEEK
Week Day in (day of the week, Sunday is 1, you can use -1)
get
int get(int field): 返回给定日历字段的值
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");
void set(int year, int month, int date) 直接设置年月日为指定值
// 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));
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getTime
Date getTime(): 将日历对象转为日期对象
Date date = c.getTime();
System.out.println(date);
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练习
定义一个方法, 使用StringBuild将数组转换为 [元素1,元素2...] 的格式
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();
}
}
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计算一个人活了多少天
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;
}
}
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计算指定年份的2月有多少天
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);
}
}
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