Basic Java Tutorials-Common classes (iv)

Iv. Common Categories

4.1 String-related classes (string stringbuffer)

4.1.1String class

　　　　

4.1.2StringBuffer class (representing variable)

Explain the mutable and immutable

String S1 = "Hello";

String s2 = "World";

S1+=S2;

It actually opens up a piece of memory, and the Hello and world copy go in, S1 point to the new memory

And StringBuffer just add a piece of memory behind the S1, add the world, do not need copy

　　String and StringBuffer only this certain difference.

4.2 Basic Data Type wrapper class (Integer,double,character, etc.) (Java.lang)

Encapsulate a base data type value as an object

The 4.3 file class (Java.io.File) represents the System file name (path and file name)

 String separator = file.separator;//file delimiter, Windows is \, and Linux is/, but both systems apply to/.    String filename = "MyFile.txt"; String directory = "Mydir1" + separator + "MYDIR2"; program full version package Bjsxt;import java.io.*;p ublic class Testfile {public STA    tic void Main (string[] args) {String separator = File.separator;    String filename = "MyFile.txt";    String directory = "Mydir1" + separator + "MYDIR2";    String directory = "Mydir1/mydir2";    String directory = "Mydir1\\mydir2";    File F = new file (directory, filename);      if (f.exists ()) {System.out.println ("file name:" + F.getabsolutepath ());    SYSTEM.OUT.PRINTLN ("File size:" + f.length ());      } else {f.getparentfile (). Mkdirs ();      try {f.createnewfile ();      } catch (IOException e) {e.printstacktrace (); }    }  }}

Print all things for a folder and its sub-files import java.io.*;p Ublic class FileList {public static void main (string[] args) {file F = new file ("d:/a") ; System.out.println (F.getname ()); Tree (f, 1);} private static void Tree (File f, int level) {String prestr = ' "; for (int i=0; i<level; i++) {prestr + = "    ";} file[] Childs = F.listfiles (); for (int i=0; i<childs.length; i++) {System.out.println (prestr + childs[i].getname ()); if (Childs[i].isdirectory ()) {tree (Childs[i], level + 1);}}}

4.4Enum Class (Java.lang.Enum)

Used to fetch only one of the specified values.

public enum MyColor {red, green, blue};//defines a type

MyColor m = mycolor.red;//value

Program full version public class Testenum {public enum MyColor {red, green, blue};p ublic enum Mydooropener {me, mywife}; public static void Main (string[] args) {MyColor m = Mycolor.red;switch (m) {case red:System.out.println ("Red"); Break;case Green: System.out.println ("green"); Break;default:system.out.println ("Default");} System.out.println (M);}}

Basic Java Tutorials-Common classes (iv)