The use of some common Java classes

 Public classDemo { Public Static voidMain (string[] args) {//DecimalFormat class under Java.text package, date formatting class SimpleDateFormat//#表示这一位存在就显示, does not exist on the display, #后的0表示存在就显示, does not exist display 0,. The following means to retain 2 decimal placesJava.text.DecimalFormat dfdecimalformat=NewJava.text.DecimalFormat ("#0.00"); System.out.println (Dfdecimalformat.format (12.43453));//12.43Java.text.DecimalFormat dfdecimalformat1=NewJava.text.DecimalFormat ("#.00"); System.out.println (Dfdecimalformat1.format (0.43453));//output. 43, front 0. A few words, 0 will be omitted.System.out.println (Dfdecimalformat1.format (4.43453));//output 4.43, so format ("#0.00") GoodSimpleDateFormat SF=NewSimpleDateFormat ("yyyy mm month DD Day"); Date D=NewDate (); System.out.println (Sf.format (d));//Formatting Date Objects January 10, 2015sf=NewSimpleDateFormat ("yyyy mm month DD Day HH:MM:SS"); System.out.println (Sf.format (d));//January 10, 2015 04:46:06//Java.lang Package: Math Mathematical Formula ClassSystem.out.println (Math.PI);//PiSystem.out.println (Math.Abs (-100));//Absolute ValueSystem.out.println (Math.ceil (4.1232));//Rounding up 5.0System.out.println (Math.ceil (-4.1232));//Rounding up -4.0System.out.println (Math.floor (4.1232));//Rounding down 4.0System.out.println (Math.floor (-4.1232));//Rounding down -5.0System.out.println (Math.Round (4.432));//Rounding 4System.out.println (Math.Round (4.654));//5System.out.println (MATH.SQRT (100));//Square root 10.0System.out.println (Math.pow (2, 3));//2 of the three-time Square 8.0System.out.println (Math.random ());//generate random numbers between 0 and 1//The random class under the Java.util package, date class, Calendar classRandom random=NewRandom (); System.out.println (Random.nextint ());//print random integers plus or minus, with no definite rangeSystem.out.println (Random.nextint (100));//print random integers [0 to]java.util.Date Date=NewJava.util.Date ();//get the current system date ObjectSYSTEM.OUT.PRINTLN (date);//Standard Time Format//Sat Jan 13:57:21 CST//constructs the Java.sql.Date object first through the Java.util.Data objectJava.sql.Date date2=Newjava.sql.Date (Date.gettime ());  System.out.println (DATE2); //2015-01-10Calendar Calendar=calendar.getinstance (); //Calendar provides a class method getinstance to get a generic object of this typeSystem.out.println (Calendar); intYear=Calendar.get (calendar.year); intMonth=Calendar.get (Calendar.month); intday=Calendar.get (Calendar.day_of_month); intHour=Calendar.get (Calendar.hour_of_day); intMinute=Calendar.get (Calendar.minute); intSecond=Calendar.get (Calendar.second); System.out.println ( Year+ "-" +month+ "-" +day+ "" +hour+ ":" +minute+ ":" +second); //the String class and the StringBuffer class//String: Content is immutable, changing content is creating new objects//stringbuffer: Content variable, change content does not change objectString s=NewString ("I Love You");//S is a reference to the stack memory, the right is the value of the heap memory, s points to the heap memory "I Love You"System.out.println (s);        System.out.println (S.hashcode ()); S+ = "China";        System.out.println (s); System.out.println (S.hashcode ());//Hashcode, the equivalent of creating a new objectStringBuffer SB=NewStringBuffer ("I Love You");        System.out.println (SB); System.out.println (Sb.hashcode ());//508839021Sb.append ("China"); System.out.println (SB);//I Love your ChinaSystem.out.println (Sb.reverse ());//inverse of string anihc uoy Evol ISystem.out.println (Sb.hashcode ());//508839021,hashcode, equivalent to appending on a far object, without creating a new object//StringBuilder: Thread non-secure, StringBuffer: Thread-Safe//StringBuffer and StringBuilder, they are string variables, are objects that can be changed, and whenever we use them to manipulate strings,//actually operates on an object and does not create as many objects as a string, so the speed is fast.//1. If you want to manipulate a small amount of data with = String//2. Manipulating large amounts of data under a single-threaded operation string buffer = StringBuilder//3. Multi-threaded operation string buffer operation large amount of data = StringBuffer    }

The use of some common Java classes