《java基础知识》Java IO流详解

Posted 2022-10-13 jssj

Java IO概念

1. 用于设备之间的数据传输。

2. Java 将操作数据流的功能封装到了IO包中。

3. 数据流流向分：输入流和输出流，操作对象为文件。

4. 流按照操作数据分：字节流（通用）和字符流。

5. 将计算机语言：二进制数据转换成文件显示到电脑上。

 

IO包：继承关系图：

 

字符流：

Reader ：读取字符流，方法见API。

Writer ：写入字符流，方法见API。

案例（Writer ）：

import java.io.*;

public class var 
    public static void main(String[] agrs)
        Writer writer = null;
        try
            writer = new FileWriter("Demo.txt");
            // writer = new FileWriter("Demo.txt",true); 文件续写功能，否则会覆盖。
            writer.write("今天天气真好！");
            writer.flush();
         catch (IOException e) 
            e.printStackTrace();
        finally 
            if(writer != null)
                try 
                    writer.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
        
    

运行结果：

 

案例（Reader）：

文件数据：

import java.io.*;

public class var 
    public static void main(String[] agrs)
        Reader reader = null;
        try
            reader = new FileReader("Demo.txt"); 
            char[] arr = new char[5];
            int red = reader.read(arr); //red是装到数组的长度。
            while(red != -1 )
                System.out.println(new String(arr,0,red));
                red = reader.read(arr);
            
         catch (IOException e) 
            e.printStackTrace();
        finally 
            if(reader != null)
                try 
                    reader.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
        
    

运行结果：

 

实现一个完整的文件复制。

import java.io.*;

public class var 
    public static void main(String[] agrs)
        Reader reader = null;
        Writer writer = null;
        try
            reader = new FileReader("Demo.txt");
            writer = new FileWriter("Dome1.txt"); //
            char[] arr = new char[5];
            int red = 0; //red是装到数组的长度。
            while((red = reader.read(arr)) != -1 )
                writer.write(new String(arr,0,red));
                writer.flush();
            
         catch (IOException e) 
            e.printStackTrace();
        finally 
            if(reader != null)
                try 
                    reader.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
            if(writer != null)
                try 
                    writer.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
        
    

运行结果：

 

优化文件复制案例（缓冲流）：

import java.io.*;

public class var 
    public static void main(String[] agrs)
        BufferedReader bufferedReader = null; //缓冲读取流
        BufferedWriter bufferedWriter = null; //缓冲写入流
        try
            bufferedReader = new BufferedReader(new FileReader("Demo.txt"));
            bufferedWriter = new BufferedWriter(new FileWriter("Dome2.txt"));
            String str ;
            while((str = bufferedReader.readLine()) != null )
                bufferedWriter.write(str);
                bufferedWriter.newLine();   //换行。
                bufferedWriter.flush();
            
         catch (IOException e) 
            e.printStackTrace();
        finally 
            if(bufferedReader != null)
                try 
                    bufferedReader.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
            if(bufferedWriter != null)
                try 
                    bufferedWriter.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
        
    

运行结果：

 

上述IO无法处理视频，图片等一些文件，拷贝出来的文件也无法打开，由此我们引出字节流。

字节流：

OutputSteam：写入字节流，方法见API。

InputSteam: 读取字节流，方法见API。

案例（OutputSteam）：

public class var 
    public static void main(String[] agrs)
        FileOutputStream fileOutputStream = null;
        try 
            fileOutputStream = new FileOutputStream("demo.txt");
            fileOutputStream.write("今天天气真好，我们去玩吧！".getBytes());
            //不需要刷新。
         catch (FileNotFoundException e) 
            e.printStackTrace();
         catch (IOException e) 
            e.printStackTrace();
        finally 
            if(fileOutputStream != null)
                try 
                    fileOutputStream.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
        
    

运行结果：

 

 案例（InputSteam）：

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

public class PublicTest 
    public static void main(String[] args) 
        FileInputStream fileInputStream = null;
        try 
            fileInputStream = new FileInputStream("demo.txt");
            int ch =0;
            byte[] arr = new byte[fileInputStream.available()];  //正式写代码不要使用fileInputStream.available()，因为文件比较大的时候内存会不够用。
            while((ch = fileInputStream.read(arr)) != -1)
                System.out.println(new String(arr,0,ch));
            
         catch (FileNotFoundException e) 
            e.printStackTrace();
         catch (IOException e) 
            e.printStackTrace();
         finally 
            if(fileInputStream != null)
                try 
                    fileInputStream.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
        
    

运行结果：

案例：复制一个视频文件

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

public class PublicTest 
    public static void main(String[] args) 
        FileOutputStream fileOutputStream = null;
        FileInputStream fileInputStream = null;
        try 
            fileInputStream = new FileInputStream("video.avi");
            fileOutputStream = new FileOutputStream("video1.avi");
            int ch =0;
            byte[] arr = new byte[fileInputStream.available()];  //正式写代码不要使用fileInputStream.available()，因为文件比较大的时候内存会不够用。
            while((ch = fileInputStream.read(arr)) != -1)
                fileOutputStream.write(arr);
            
         catch (FileNotFoundException e) 
            e.printStackTrace();
         catch (IOException e) 
            e.printStackTrace();
         finally 
            if(fileInputStream != null)
                try 
                    fileInputStream.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
            if(fileOutputStream != null)
                try 
                    fileOutputStream.close();
                 catch (IOException e) 
                    e.printStackTrace();
                
            
        
    

运行结果：

 

转换流（键盘输入和控制台输出作为案例）：

import java.io.*;

public class var 
    public static void main(String[] agrs)
        OutputStream out = System.out;  //控制台打印
        OutputStreamWriter outputStreamWriter = null;
        outputStreamWriter = new OutputStreamWriter(out);  // outputStreamWriter = new OutputStreamWriter(out,"GBK");  可以指定写的编码
        BufferedWriter bufferedWriter = new BufferedWriter(outputStreamWriter);

        InputStream in = System.in;   //键盘输入流。
        InputStreamReader inputStreamReader = new InputStreamReader(in); // InputStreamReader inputStreamReader = new InputStreamReader(in,"BGK"); 同样可以指定编码
        BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
        try 
            String line = null;
            while((line = bufferedReader.readLine()) != null)
                if("exit".equals(line))
                    break;
                
                bufferedWriter.write(line);
                bufferedWriter.flush();
            
         catch (IOException e) 
            e.printStackTrace();
        finally 
            try 
                if(bufferedWriter != null)
                    bufferedWriter.close();
                
                if(bufferedReader != null)
                    bufferedReader.close();
                
             catch (IOException e) 
                e.printStackTrace();
            
        
    

运行结果：

IO流先讲到这里，Java IO流还有很多有意思的类和方法见API。

参考：https://www.cnblogs.com/runningTurtle/p/7088125.html