Thinking in java

Integer i = null;

int j = i.intValue();

~!Null means no reference entity .It can be decided to some objects But in fact,i don't have any reference object.So intValue() method can't be operated;

Consequently,cause NullPointerException;~



Just neglect Autoboxing unboxing

Integer i1 = 100;

Integer i2 = 100;

Integer i3 = 200;

Integer i4 = 200;

sysout(i1 ==  i2);

sysout(i3 ==  I4);

How to  find surprises in the flesh .Just Coding will bring the answer to you

.There is no doubt that most of us will thinks the right answer is "True,True" but  the result must be unexpected.It is  'true, false." AutoBoxing takes special care for value begin from -128 to 127.These objects(In Java,Integer is a kind of Object )won't be reused.So i3 i4 reference is different/


JAVA如何过滤JPG文件

public class FileFilter { 
    public static void main(String[] args) { 
        String[] filenames = {"caterpillar.jpg", "cater.gif", 
                 "bush.jpg", "wuwu.jpg", "clockman.gif"};
        System.out.print("过滤出jpg文件:  "); 
        for(int i = 0; i < filenames.length; i++) {
            if(filenames.endsWith("jpg")) {
                System.out.print(filenames + " "); 
            }
        }
        System.out.println(""); 
    } 
} 
char charAt(int index);
int indexOf（int ch);
int IndexOf(String str)
int lastIndexOf(String str)
String substring(int beginIndex)
String substring(int 1,int 2);
char[] toCharArray(); 将字符串转为字符数组  

正则..正则设计是一门学问，如果要用到只能找到专门书籍学习如何设计

import java.io.*;
 
public class UseRegularExpression { 
    public static void main(String args[]) 
                              throws IOException { 
        BufferedReader reader = 
            new BufferedReader(
                new InputStreamReader(System.in));
 
        System.out.println("abcdefgabcabc".replaceAll(".bc", "###")); 
 
        String phoneEL = "[0-9]{4}-[0-9]{6}";
        String urlEL = "<a.+href*=*['\"]?.*?['\"]?.*?>";
        String emailEL = "^[_a-z0-9-]+(.[_a-z0-9-]+)*" + 
                         "@[a-z0-9-]+(.[a-z0-9-]+)*$";
        System.out.print("输入手机号码: "); 
        String input = reader.readLine();
 
        if(input.matches(phoneEL)) 
            System.out.println("格式正确"); 
        else 
            System.out.println("格式错误");
 
        System.out.print("输入href标签: "); 
        input = reader.readLine();
 
         // 验证href标签 
        if(input.matches(urlEL))
            System.out.println("格式正确"); 
        else
            System.out.println("格式错误");
 
        System.out.print("输入电子邮件: "); 
        input = reader.readLine();
 
        //验证电子邮件格式 
        if(input.matches(emailEL))
            System.out.println("格式正确"); 
        else
            System.out.println("格式错误"); 
    } 
}