Common library of Basic Java Knowledge (1)

StringBuffer

1, StringBuffer and String are not directly related
2, StringBuffer to avoid the sring content changes in the phenomenon of garbage generation.
3. A StringBuffer object can call the ToString () method to convert to a string object.
The following is a small example of how the StringBuffer class is related:

Package stringbuffer; Public classStringbufferdemo { Public Static void Main(string[] args) {//TODO auto-generated method stub        the structure of//* StringBufferStringBuffer BUF1 =NewStringBuffer ();//equivalent to an empty stringStringBuffer buf2 =NewStringBuffer ("123");//Construct StringBuffer object by stringSystem. out. println ("Below you can see a blank line plus the string" 123 ""+"\ n"+buf1+"\ n"+BUF2);/ * * Common methods * * */        //append () method to add charactersBuf1.append ("123"). Append (' C '). Append (23.2). Append (false); System. out. println ("Below you can see the string that is bridged by the Append () method:"+"\ n"+BUF1);//insert () method inserts a stringBuf1.insert (0,true). Insert (0,"First word Fu Chan insert"); System. out. println ("Below you can see the string after inserting a character before the specified position"+"\ n"+BUF1);The //delete () method deletes a substring of the specified range (a character starting at subscript 0)Buf1.delete (0,1); System. out. println ("Below you can see the string after deleting the specified position character"+"\ n"+BUF1);//reverse () method reverses string contentsBuf1.reverse (); System. out. println ("Below you can see the inverted string"+"\ n"+BUF1);//replace () method substitution characterBuf1.replace (0,1,"7788"); System. out. println ("You can see the replaced string below."+"\ n"+BUF1); }}

Runtime

Directly on the example, look at the effect!

 Public  class runtimedemo {    /** * @param args * @throws ioexception * @throws interruptedexception */     Public Static void Main(string[] args)throwsIOException, Interruptedexception {//TODO auto-generated method stubRuntime r = Runtime.getruntime ();//Get Runtime instanceProcess Pro = R.exec ("Notepad");//Perform a native program "Notepad"        //thread.sleep (5000);//sleep 5 secondsPro.destroy ();//Destroy the processSystem.out.println ("Native Memory:"+r.totalmemory ()); }}

System

 Public  class systemdemo {    /** * Calculates the run time of a program * @param args */     Public Static void Main(string[] args) {//TODO auto-generated method stub        LongStart = System.currenttimemillis (); System.out.println ("Current time:"+start); for(inti =0; I < -; i++) { for(intj =0; J < -;            J + +) {j + +; }        }LongEnd = System.currenttimemillis (); System.out.println ("Program Execution time:"+ (End-start)); }}

Date, Calendar

 Public  class datedemo {    /** * @param args * *     Public Static void Main(string[] args) {//TODO auto-generated method stub        //Take a look at today's date, the format is a bit weirdSystem.out.println (NewDate ());the//calendar class, which is an abstract class that needs to be instantiated with its subclassesCalendar C =NewGregorianCalendar (); StringBuffer s =NewStringBuffer (); S.append (C.get (calendar.year) +"Year"+ (C.get (calendar.month) +1)+"Month"+ C.get (calendar.day_of_month) +"Day"+"  "+c.get (Calendar.hour) +":"+c.get (Calendar.minute));    System.out.println (s); }}

The date of the above System.out.println (new date ()) output is not a habit, and the following will make you more vulnerable:

DateFormat

import Java.text.dateformat;import java.util.Date; public  class  Formatdemo {public  static  void  main  (string[] args) {//TODO auto-generated method Stub  Date d = new  date ();  DateFormat f = dateformat.getdateinstance (); //get the default date formatting instance  DateFormat ff = dateformat.getdatetimeinstance (); //get the default datetime formatted instance  System. out . println (F.format (d)); //formatted after output  System. out . println (Ff.format (d)); //formatted after output }} 

The output is not accustomed to, but its format can not change it? The answer is yes, keep looking:

Import Java.text.dateformat;import java.util.Date; Public classFormatdemo { Public Static void Main(string[] args) {//TODO auto-generated method stubDate d =NewDate ();//instancing Date        //The following method gives the parameters of theDateFormat f = dateformat.getdateinstance (Dateformat.full);//Get the default date format instanceDateFormat ff = dateformat.getdatetimeinstance (dateformat.full,dateformat.full);//Get the default datetime format instanceSystem. out. println (F.format (d));//Formatted outputSystem. out. println (Ff.format (d));//Formatted output}}

In fact, we can also have more formats, such as: 2015/5/26, next time to continue (-_-)

Common library of Basic Java Knowledge (1)