java基础Demo

闲着没事，写写Demo!


1.字符串回文：

package com.org.momo.字符串;

import java.util.Scanner;

public class 判断字符串回文 {
   public static void main(String args[]){
	   Scanner scanner = new Scanner(System.in) ;
	   String str = scanner.nextLine() ;
	   boolean flag = new 判断字符串回文().isPrime(str) ;
	   if(flag){
		   System.out.println("您输入的是回文字符串！") ;
	   }else{
		   System.out.println("您输入的不是回文字符串！") ;
	   }
   }
	
	
   public boolean isPrime(String s){
   boolean flag = false ;
   int i=0 ;
   for(;i<=(s.length()/2)-1;i++ ){
	   if(s.charAt(i) == s.charAt(s.length()-1-i)){
		   continue ;
	   }else break ;
   }
	   if(i==(s.length()/2)){
		   flag = true ;
	   }
   return flag ;
 }
}

2.几个整型排序（冒泡排序）

package com.org.momo.数组;

public class 数组排序 {
    public static void main(String args[]){
    	String str = new 数组排序().Sort(4, 2, 1, 3) ;
    	System.out.println(str) ;
    }
	
	public String Sort(int a,int b,int c,int d){
		String content = "" ;
		int[] src= new int[4] ;
		src[0] = a ;
		src[1] = b ;
		src[2] = c ;
		src[3] = d ;
		
		for(int i=0;i<src.length;i++){
			for(int j=0;j<src.length-1-i;j++){
				if(src[j]>src[j+1]){
					int temp = src[j+1] ;
					src[j+1] = src[j] ;
					src[j] = temp ;
				}
			}
		}
		for(int i=0;i<src.length;i++){
			content +=src[i]+" " ;	
		}
		return content ;
	}
}

3.用单例写一个获得数据库连接对象的类（singleton）

package com.org.momo.单例;

import java.sql.Connection;
import java.sql.DriverManager;

public class ConnectionDB {
     private static Connection conn ;
     private static ConnectionDB db ;
     private static String DriverName="com.mysql.jdbc.Driver" ;   //DriverName="org.gjt.mm.mysql.Driver"
     private String URL = "jdbc:mysql://localhost:3306/college" ;
     
     private ConnectionDB(){
    	 try {
			Class.forName(DriverName) ;
			conn = DriverManager.getConnection(URL,"momo","123456") ;
		} catch (Exception e) {
			e.printStackTrace();
		}   	 
     }
     
     public static Connection getConnection(){
    	 if(db==null){
    		 db = new ConnectionDB() ;
    	 }
    	 return conn ;
     }
}

4.从inuput.txt全部读取，取其倒数20行输出到output.txt文件中

package com.org.momo.io;

import java.io.BufferedReader;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.InputStream;
import java.io.StringBufferInputStream;
import java.util.ArrayList;
import java.util.List;

public class ReadContent {
    static List<String> list = new ArrayList<String>() ;
    static int a[] ;

	public static void main(String args[]){
    	try {
    		
    		//1.读取文件input.txt 保存在数组里面
			BufferedReader content = new BufferedReader(new FileReader("D:/input.txt")) ;
			while(content.ready()){
			   list.add(content.readLine()) ;	
			}
			if(!list.isEmpty()){
			 a = new int[list.size()] ;
			 for(int i=0;i<list.size();i++){
				 a[i] = Integer.parseInt(list.get(i)) ;
			 }
			 
			 //2.对数组进行 “快速排序”
			 quickSort(a,0,a.length-1) ;
			 
            //3.保存文件 output.txt
			 StringBuffer str = new StringBuffer() ;
			 for(int i:a){
			  str.append(i+"\n") ;
			 }
			 FileOutputStream fos = new FileOutputStream("D:/output.txt",true);
			 InputStream fis = new StringBufferInputStream(str.toString()) ;
			 byte[] buffer = new byte[1024];     //1024 byte = 1KB 
			  int len = 0;
			  while ((len = fis.read(buffer)) > 0)
			  {
				fos.write(buffer , 0 , len) ;
			  }
			}
		} catch (Exception e) {
			System.out.println("没有你指定的文件！") ;
			e.printStackTrace() ;
		}
    }
	
	public static void quickSort(int[] src, int begin, int end) { 
		  if (begin < end) { 
		   int key = src[begin]; 
		   int i = begin; 
		   int j = end;  

		   while (i < j) { 
		    while (i < j && src[j] > key) { 
		     j--; 
		    } 
		    if (i < j) { 
		     src[i] = src[j]; 
		     i++; 
		    } 

		    while (i < j && src[i] < key) { 
		     i++; 
		    } 
		    if (i < j) { 
		     src[j] = src[i] ; 
		     j--; 
		    }
		   } 
		   src[i] = key;
		   quickSort(src, begin, i - 1); 
		   quickSort(src, i + 1, end); 
		  } 
		 } 
}