Mutable string class
The basic concept
- Because the contents of the String described by the String class are constant and immutable, when a large number of similar strings need to be described in Java code, they can only be applied for and stored separately, resulting in a waste of memory space.
- To solve the above problem, the java.lang.StringBuilder class and the java.lang.StringBuffer class can be used to describe strings in which character sequences can be changed.
- The StringBuffer class has been around since jdk1.0 and is thread-safe and therefore inefficient.
- The StringBuilder class is a non-thread-safe class that has been around since JDK1.5.
A common constructor of the StringBuilder class
| Method statement | Function is introduced |
|---|---|
| StringBuilder() | Construct an object with no arguments and capacity of 16 |
| StringBuilder(int capacity) | The object is constructed based on the capacity specified by the parameter, which specifies the size |
| StringBuilder(String str) | Constructs an object from the string specified by the argument. The capacity is 16+ string length |
Member methods commonly used by the StringBuilder class
| Method statement | Function is introduced |
|---|---|
| int capacity() | Used to return the capacity of the calling object |
| int length() | Used to return the length of the string, that is, the number of characters |
| StringBuilder insert(int offset, String str) | Inserts a string and returns a reference to the calling object, itself |
| StringBuilder append(String str) | Append string |
| StringBuilder deleteCharAt(int index) | Removes a single character at index position from the current string |
| StringBuilder delete(int start, int end) | Delete string |
| StringBuilder replace(int start, int end, String str) | Substitution string |
| StringBuilder reverse() | String inversion |
-
Pay attention to
When passed as an argument, the internal String of the method does not change its value; StringBuffer and StringBuilder change their value.
Design of the return value
- Many methods of StringBuilder return values of type StringBuilder. Each of these methods returns the following statement: return this.
- As you can see, these methods return a reference to the object after making changes to the sequence of characters encapsulated by StringBuilder. The purpose of this design is to be able to call continuously.
Date related classes prior to Java8
An overview of the System class
The basic concept
- The java.lang. System class provides some useful class fields and methods.
Common methods
| Method statement | Function is introduced |
|---|---|
| static long currentTimeMillis() | Returns the time difference in milliseconds between the current time and 00:00:00:00 on January 1, 1970 |
An overview of the Date class
The basic concept
- The java.util.Date class is mainly used to describe specific moments, that is, the year, month, day, hour, minute, second, accurate to the millisecond.
Common methods
| Method statement | Function is introduced |
|---|---|
| Date() | The object is constructed with no arguments, which is the current system time |
| Date(long date) | Constructs an object based on the number of milliseconds specified by the argument, which is the number of milliseconds from 00:00 00:00 00:00 January 1, 1970 |
| long getTime() | Gets the number of milliseconds from the calling object to 00:00:00:00, January 1, 1970 |
| void setTime(long time) | Set the calling object to the point in time milliseconds from the base time |
An overview of the SimpleDateFormat class
The basic concept
- Java. Text. SimpleDateFormat classes is mainly used for implementation date and transformation between the text.
Common methods
| Method statement | Function is introduced |
|---|---|
| SimpleDateFormat() | Constructs objects with no arguments |
| SimpleDateFormat(String pattern) | The object is constructed according to the mode specified by the parameter. The mode is y- year M- month D – day H- hour M- minute S – second |
| final String format(Date date) | Used to convert a date type to a text type |
| Date parse(String source) | Used to convert text type to date type |
An overview of the Calendar class
The basic concept
- The java.util.Calender class is primarily used to describe specific moments, replacing outdated methods in the Date class for globalization.
- This class is abstract and therefore cannot be instantiated. Its concrete subclasses are for the calendar systems of different countries, among which the most widely used is GregorianCalendar, which corresponds to the standard calendar system used by most countries/regions in the world.
Common methods
| Method statement | Function is introduced |
|---|---|
| static Calendar getInstance() | To get a reference to the Calendar type |
| void set(int year, int month, int date, int hourOfDay, int minute, int second) | Set the year, month, day, hour, minute, second information |
| Date getTime() | Use to convert a Calendar type to a Date type |
| void set(int field, int value) | Sets the value of the specified field |
| void add(int field, int amount) | Adds a value to the specified field |
The use of polymorphism
- Polymorphism is formed through the parameter transfer of the method.
public static void draw(Shape s) {
s.show();
}
draw(new Rect(1.2.3.4));
Copy the code- Use polymorphic syntactic formats directly in the method body
Account acc = new FixedAccount();
Copy the code- Polymorphism is generated by the return value type of a method
Calender getInstance(a) {
return new GregorianCalendar(zone, aLocale);
}
Copy the codeDate-related classes in Java8
Java8 date class origin
-
A java.util.date class was included in JDK 1.0, but most of its methods were deprecated after the introduction of the Calendar class in JDK 1.1. And Calendar is not much better than Date. The problems they face are:
- The year in the Date class starts at 1900 and the month starts at 0.
- Formatting is only useful for the Date class, not the Calendar class.
- Non-thread-safe, etc.
An overview of Java8 date classes
- Java 8 further enhances Date and Time handling with the release of a new date-time API.
- Java.time package: The base package for this package’s date/time API.
- Java.time. chrono package: This package provides access to different calendar systems.
- Java.time. format package: This package formats and parses date and time objects.
- Java.time. temporal package: This package contains the underlying framework and extended features.
- Java.time. zone package: This package supports classes for different time zones and related rules.
Overview of the LocalDate class
The basic concept
- The java.time.LocalDate class is mainly used to describe date information in year-month-day format. This class does not represent time and time zone information.
Common methods
| Method statement | Function is introduced |
|---|---|
| static LocalDate now() | Gets the current date from the system clock in the default time zone |
Overview of the LocalTime class
The basic concept
- The java.time.LocalTime class is used to describe time information, including hours, minutes, seconds, and nanoseconds.
Common methods
| Method statement | Function is introduced |
|---|---|
| static LocalTime now() | Gets the current time from the system time in the default time zone |
| static LocalTime now(ZoneId zone) | Gets the current time of the specified time zone |
An overview of the LocalDateTime class
The basic concept
- The java.time.LocalDateTime class is used to describe dates and times without time zones in the ISO-8601 calendar system, for example, 2007-12-03T10:15:30.
Common methods
| Method statement | Function is introduced |
|---|---|
| static LocalDateTime now() | Gets the current date and time from the system time of the default time zone |
| static LocalDateTime of(int year, int month, int dayOfMonth, int hour, int minute, int second) | Set the date and time according to the specified date, month, day, hour, minute, second |
| int getYear() | Gets the numeric value of the year field |
| int getMonthValue() | Gets the month field between 1 and 12 |
| int getDayOfMonth() | Get the date field |
| int getHour() | Hours of acquisition |
| int getMinute() | Get minutes |
| int getSecond() | Get number of seconds |
| LocalDateTime withYear(int year) | Sets to the year specified by the parameter |
| LocalDateTime withMonth(int month) | Sets to the month specified by the parameter |
| LocalDateTime withDayOfMonth(int dayOfMonth) | Sets to the day specified by the parameter |
| LocalDateTime withHour(int hour) | When set to the parameter specified |
| LocalDateTime withMinute(int minute) | Sets the score specified for the parameter |
| LocalDateTime withSecond(int second) | Sets to the second specified by the parameter |
| LocalDateTime plusYears(long years) | Plus the year specified by the argument |
| LocalDateTime plusMonths(long months) | Plus the month specified by the argument |
| LocalDateTime plusDays(long days) | Plus the day specified by the argument |
| LocalDateTime plusHours(long hours) | Plus the parameter specified |
| LocalDateTime plusMinutes(long minutes) | Plus the points specified by the argument |
| LocalDateTime plusSeconds(long seconds) | Plus the seconds specified by the argument |
| LocalDateTime minusYears(long years) | Subtract the year specified by the argument |
| LocalDateTime minusMonths(long months) | Subtract the month specified by the argument |
| LocalDateTime minusDays(long days) | Subtract the day specified by the argument |
| LocalDateTime minusHours(long hours) | When subtracting the parameter specified |
| LocalDateTime minusMinutes(long minutes) | Subtract the score specified by the argument |
| LocalDateTime minusSeconds(long seconds) | Subtract the seconds specified by the argument |
An overview of the Instant class
The basic concept
- The java.time.Instant class is used to describe Instant point in time information.
Common methods
| Method statement | Function is introduced |
|---|---|
| static Instant now() | Retrieves the current time from the system clock |
| OffsetDateTime atOffset(ZoneOffset offset) | Combine this moment with the offset to create the offset date time |
| static Instant ofEpochMilli(long epochMilli) | Constructs an object based on the number of milliseconds specified by the argument, which is the number of milliseconds from 00:00 00:00 00:00 on January 1, 1970 |
| long toEpochMilli() | Gets the number of milliseconds from 00:00:00, January 1, 1970 |
Overview of the DateTimeFormatter class
The basic concept
- Java. Time. The format. DateTimeFormatter class is mainly used for formatting and parsing the date.
Common methods
| Method statement | Function is introduced |
|---|---|
| static DateTimeFormatter ofPattern(String pattern) | Gets the object according to the mode specified by the argument |
| String format(TemporalAccessor temporal) | Converts the specified date and time to a string |
| TemporalAccessor parse(CharSequence text) | Converts the parameter specified string to date time |