Java Development Common Classes

Object class: Superclass, parent class for all classes
Equals (String str): Used to compare whether two objects (strings) are equal
ToString (): Returning a string
System classes: Systems classes
Out: Output
In: Receive keyboard input
Exit (): Stop program
Currenttimemillis (): Returns the current time in milliseconds
Math class: Algorithm class
Ceil (double D): Zero integer After the decimal point, not zero-rounding
Double d = 212.9;
Math.ceil (d);---->213;
Double D2 = 212.1;
Math.ceil (D2);--->213
Floor (double D): Rounding down
Double d = 212.9;
Math.floor (d);---->212;
Double D2 = 212.1;
Math.floor (D2);--->212;
Round (Double R): rounding
Double d = 212.9;
Math.Round (d);---->213;
Double D2 = 212.1;
Math.Round (D2);--->212;
Max (double d,double D2): Returns the maximum value of two values
Min (Double d,double D2): Returns the minimum value of two values
Random (): Generating random numbers between 0.0--1.0
Date Class: Time action Class (Java.util.Date)
Date date = new Date ();//Get current time
Date date = new Date (int year, int month, int day);//Create a specified date
Note: The year Y is represented by an integer y-1900.
The month is represented by an integer from 0 to 11, 0 is a month, 1 is February, etc., so 11 is December.

System.out.println ("Year:" + (Date.getyear () +1900));
System.out.println ("Month:" + (Date.getmonth () +1));
System.out.println ("Date:" +date.getdate ());
System.out.println ("When:" +date.gethours ());
System.out.println ("min:" +date.getminutes ());
System.out.println ("SEC:" +date.getseconds ());
System.out.println ("Week:" +date.getday ());

  Time Format: DateFormat
  dateformat df = dateformat.getdateinstance (constant value);
  Time format: SimpleDateFormat
  simpledateformat sdf = new SimpleDateFormat ();
   non-parametric formatting: generated format:   14-9-30 morning 10:18
  simpledateformat sdf = new SimpleDateFormat ("Yyyy-mm-dd");
   Parameter formatting: The generated format is the form specified by the parameter

   generally use SimpleDateFormat to create an object using its parent class
  dateformat df = new SimpleDateFormat ();
  Calendar abstract class (Calendar)
   The first instantiation method:
  calendar cal = Calendar.getinstance ();//Get instance method of calender
  
   Second instantiation mode:
  calendar cal2=new GregorianCalendar ();
   System.out.println ("Year:" +cal.get (Calendar.year));//Get Year
  system.out.println ("Month:" + (Cal.get ( Calendar.month) +1));
  system.out.println ("Day:" +cal.get (calendar.date));
  system.out.println ("When:" +cal.get (Calendar.hour_of_day));
  system.out.println ("Cent:" +cal.get (Calendar.minute));
  system.out.println ("Seconds:" +cal.get (Calendar.second));
  system.out.println ("Week:" + (Cal.get (Calendar.day_of_week)-1));

Note: Get month: plus 1
Week: minus 1;
Random: Classes that generate random numbers
Instantiated:
Random rd = new Random ();
Generate integer random number: 1.rd.nextint ();
2.rd.nextint (int d);
The second, which is used to generate a random number of the specified size, between the specified number of 0--, excluding the specified number
Generate double random number (between 0.0--1.0): Rd.nextdouble ();
Equivalent to: Math.random ();

Object
System
Math
Date
SimpleDateFormat
Calendar
Random

String: String manipulation class
 1.length (): To find the length of the string: Here is the method, and the array length is the property
 2.startswith (String val To determine whether the string starts with Val, returns True No returns False
 3.endswith (string val); Determines whether the string ends in Val, and returns True No returns False
 4.charat (int index): Searches for subscript content in a string based on index value, cannot be negative, and cannot exceed string length
 5.indexof (character): Returns the subscript position for the first occurrence of this character in a string, and does not return-1
 6. IndexOf (String): Returns the subscript position for the first occurrence of this substring in the string, and does not return-1
 7.lastindexof (): Returns the subscript for the first occurrence of this character in the string from the back, and does not return-1
 8. subString (int index): intercepts a string (starting from index subscript to end)
 9.substring (int beginindex,int endindex): Intercepts a string (from beginindex subscript to Endindex end (excluding endindex subscript))
 10.concat (String str): to concatenate str into the specified string to generate a new