Wrapper class, Math, date format processing
- Packing class
- Math:random (), round ()
- Date format processing
Java advocates that thought is everything, but our basic data type is not a class, that is, there is no object concept, and Java advocated the idea of violating, in order to solve this contradiction, Java provides 8 kinds of basic data types of packaging classes.
Serial number |
Basic data types |
Packing class |
1 |
Int |
Integer |
2 |
Char |
Character |
3 |
Float |
Float |
4 |
Long |
Long |
5 |
Short |
Short |
6 |
Double |
Double |
7 |
Boolean |
Boolean |
8 |
Byte |
Byte |
Public Static voidMain (string[] args) {intI=Ten; Boolean flag=false; Doublej=10.00; Integer k=NewInteger (i);//basic data types are converted to wrapper class objects (boxing)Boolean f=NewBoolean (flag); Double D=NewDouble (j); intX=k.intvalue ();//wrapper classes are converted to basic data types called unboxingBoolean bo=F.booleanvalue (); DoubleDe=d.doublevalue ();//to disassemble the box}
In addition to solving the ideas that Java advocates, what can you do with the packaging class?
- • Boxing: Converting basic data types to wrapper class objects
- • Unboxing: Converting wrapper class objects to basic data types
- • Convert strings to basic data types
The above packing and unpacking operation is done manually, after JDK1.5, the packing and unpacking operation can be carried out automatically.
Public Static voidMain (string[] args) {intI=Ten; Boolean flag=false; Doublej=10.00; Integer k=i;//basic data types are converted to wrapper class objects (boxing)Boolean f=Flag; Double D=J; intX=k;//wrapper classes are converted to basic data types called unboxingBoolean bo=F; DoubleDe=d;//to disassemble the box}
// Convert a string to a base data type Private void Mian () { // TODO auto-generated method stub String s="1122" ; int x=integer.parseint (s); int y=integer.valueof (s); }
When developing, it is important to note that when converting, pay attention to the appearance of anomalies, for example, if we convert the string "aabb" to int type, then it will be an error (numberformatexception).
2Math
The Math class defines a series of mathematical operations, and is a method that uses the static modifier, which means that it can be accessed directly through the math class. We generally only need to focus on two of the development:
- Rounded
- • Get random numbers
Public Static void Main (string[] args) { double d=100.53; int i = (int) Math.Round (d); System. out . println (i); }
The other is the random number: Using random () to take an arbitrary number is 0-1 decimal, is greater than or equal to 0, less than 1.
Example: Take a random number between 1-10
Public Static void Main (string[] args) { for (int1; i++) { Double d=math.random (); // take a random number from 0 to 1 int x= (int) math.round (d*); System. out. println (x+" "); } }
Processing of 3rd-year formats
We are mainly talking about the date class, the date class in the Java.util package, the Calendar class, and then the date format conversion class (DateFormat).
Date:Most of the methods in the date class are obsolete, and generally only the construction method is used to get the current time of the system.
· Public Long GetTime (): Gets the number of milliseconds since the beginning of 1970, January, 1th
Public Static void Main (string[] args) { new Date (); SYSTEM.OUT.PRINTLN (date); System.out.println (Date.gettime ()); }
What if we're going to get the exact date object's date, seconds, minutes, milliseconds? We know that the methods in the date class are obsolete, so we are now using the Calendar Calendar class to complete the operation. This class has a number of ways to do this:
· Public abstract void Add (int field, int amount): Adds the specified value to the specified calendar field
· public int get (int field): Gets the value of the specified calendar field
· Public Final Date getTime (): Convert to date type
· Public void Set (int field, int value): Specifies the value of the Calendar field setting specified
· Public final void settime (date date): sets the date type to the calendar object
Example: Accurate month-on-day seconds
Public Static voidMain (string[] args) {Date date=NewDate (); SYSTEM.OUT.PRINTLN (date); System.out.println (Date.gettime ()); Calendar CA=NewGregorianCalendar ();//Get the Calendar object (you can also get the current time of the system)System.out.println ("Year" +Ca.get (calendar.year)); System.out.println ("Month" +ca.get (calendar.month) +1);//0-11 monthsSystem.out.println ("Day" +Ca.get (calendar.day_of_month)); System.out.println ("When" +Ca.get (calendar.hour_of_day)); System.out.println ("Min" +Ca.get (Calendar.minute)); System.out.println ("Seconds" +Ca.get (Calendar.second)); System.out.println ("Milliseconds" +Ca.get (Calendar.millisecond)); System.out.println ("The first few weeks of the year" +Ca.get (calendar.week_of_year)); System.out.println ("The first few weeks of the month:" +Ca.get (calendar.week_of_month)); }
3. Conversion of date formats
In Java using dateformat for date format conversion, this class is also an abstract class, when instantiated must rely on subclasses:SimpleDateFormat, commonly used construction methods:
· Public SimpleDateFormat (String pattern): Conversion rules for dates represented by parameters
Rules:y year,M-month,D-Day,H-hour,m-minute,S-second,S-millisecond
Common methods of DateFormat:
· Public final string format (date date): Converts a Date object to a string
· Public Date Parse (string source) throws ParseException: Converts a string to a date
Example: Converting a date type to a string
Public Static void Main (string[] args) { new Date (); New SimpleDateFormat ("Yyyy-mm-dd HH:mm:ss. SSS "); // Defining conversion Formats = Df.format (date); // to convert System.out.println (s); }}
Example: Converting a string into a date
String s = "1998/10-10"; New SimpleDateFormat ("Yyyy/mm-dd"); // Defining conversion Formats Try { = Df.parse (s); converts a string into a date System.out.println (date) ; Catch (Exception e) { e.printstacktrace (); }
In the development process, it is important to pay attention to the SimpleDateFormat definition of the conversion rules, if the conversion rules defined errors, then the date conversion exception can not be avoided
Summary
• Use wrapper classes to convert strings to basic data types
• Use the random () and round () methods in math
• Learn to use some of the methods in the Calendar class
• Learn to use DateFormat for conversion of date formats
Java Basics wrapper class, Math, date format processing