程序的性能瓶頸通常在I/O，
I/O示例

package qingke;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

public class LearnFile {

    public static void main(String[] args) {
        String folderPath ="C:\\Users\\qingke04\\Desktop\\newFolder";
        creatFolder(folderPath);
        
        String fileName1 = "C:\\Users\\qingke04\\Desktop\\newFolder\\file1.txt";
        String fileName2 = "C:\\Users\\qingke04\\Desktop\\newFolder\\file2.txt";
        creatFile(fileName1);
        creatFile(fileName2);
        //seeFile(folderPath);
        //delFile(fileName1);
        //delFile(folderPath);
        writeToFile(fileName1);
        //readToFile(fileName1);
        File file =new File(fileName1);
        copyFile(folderPath,file);
        
        
    }
    public static void creatFolder(String folderPath){
        File file =new File(folderPath);
        file.mkdir();
    }
    public static boolean creatFile(String fileName){
        File file =new File(fileName);
        try {
            System.out.println(file.getCanonicalPath());
        } catch (IOException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }
        boolean isSuccess =false;
        
        try {
            isSuccess=file.createNewFile();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return isSuccess;
    }
    public static void seeFile(String folderPath){
        File file =new File(folderPath);
        for(String filename:file.list() ){
            System.out.println(filename);
        }
    }
    public static void delFile(String foldPath){
        File file = new File(foldPath);
        for(String fileName:file.list()){
            File newfile =new File(foldPath+"\\"+fileName);
            newfile.delete();
        }
    }
    public static void writeToFile(String fileName){
        File file =new File(fileName);
        BufferedWriter bwriter=null;
        try {
            FileWriter writer =new FileWriter(file);
            bwriter = new BufferedWriter(writer);
            bwriter.write("床前明月光");
            bwriter.newLine();
            bwriter.write("疑是地上霜");
            bwriter.newLine();
            bwriter.write("舉頭望明月");
            bwriter.newLine();
            bwriter.write("低頭思故鄉(xiāng)");
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }finally{
            if(bwriter!=null){
                try {
                    bwriter.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    }
    
    public static void readToFile(String fielName){
        File file =new File(fielName);
        BufferedReader bread =null;
        try {
            FileReader read =new FileReader(file);
            bread =new BufferedReader(read);
            String line = bread.readLine();
            while(line!=null){
                System.out.println(line);
                line =bread.readLine();
            }
            
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }catch(IOException e){
            e.printStackTrace();
        }
    }
    public static void copyFile(String folderPath,File fileName){
        //String str = fileName.getName();
        File file1 =new File(folderPath+"\\(COPY)"+fileName.getName());
        ArrayList<String> list =new ArrayList<>();
        try {
            file1.createNewFile();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        BufferedWriter bwriter ;
        BufferedReader bread;
        try {
            FileWriter writer =new FileWriter(file1);
            bwriter =new BufferedWriter(writer);
            FileReader read =new FileReader(fileName);
            bread =new BufferedReader(read);
            String line =bread.readLine();
            while(line!=null){
                list.add(line);
//              bwriter.write(line);
//              bwriter.newLine();
                line =bread.readLine();
            }
            Collections.shuffle(list);
            for(String l:list){
                bwriter.write(l);
                bwriter.newLine();
            }
            bwriter.close();
            bread.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}