【SC随笔】open and read txt file in java

• Java BufferedReader class
• • Example
• Java Scanner class
• • Example
• Java Desktop class
• • Example
• Java FileInputStream class
• • Example
• Java FileReader class
• • Example

—2022.4.30更新—
打开文件经常需要抛出异常，但try函数内部声明的成员又不能在函数外使用，try catch目前还没有学，干脆在类开头加上throws IOException，避免了try函数

read text from a character input stream.
belong to a java.io package.

public BufferedReader(Reader in)

import java.io.*;
import java.util.Scanner;

public class OpenFileExample1 throws IOException{
	BufferedReader reader = new BufferedReader(new FileReader("demo.txt"));
}

used for opening and reading a file
belongs to java.util package

public scanner (File source) throws FileNotFoundException  

import java.io.*;
import java.util.Scanner;

public class OpenFileExample2 throws IOException{
	Scanner scanner = new Scanner(new FileReader("demo.txt"));
}

provide an open() method to open a file.
belongs to a java.awt package
have to check the operating system supports Desktop or not

public void open (File file) throws IOException  

这个方案抛出好几种异常

• NullPointerException （文件为空）
• IllegalArgumentException（文件不存在）
• IOException（文件类型不支持）
• UnsupportedOperationExecution（当前平台不支持）

import java.awt.Desktop;  
import java.io.*;  
public class OpenFileExample3{  
	public static void main(String[] args){  
		try{  //constructor of file class having file as argument  
			File file = new File("demo.txt");   
			if(!Desktop.isDesktopSupported()){
				//check if Desktop is supported by Platform or not  
				System.out.println("not supported");  
				return;  
			}  
			Desktop desktop = Desktop.getDesktop();  
			if(file.exists())//checks file exists or not  
				desktop.open(file); //opens the specified file  
		}
		catch(Exception e){  
			e.printStackTrace();  
		}  
	}  
} 

public FileInputStream(File file) throws FileNotFoundException  

import java.io.*;
import java.util.Scanner;

public class OpenFileExample4 throws IOException{
	public static void main(String args[]){
		FileInputStream fis = new FileInputStream("demo.txt");
		char r = fis.read();
	}
}

used for opening and reading a file.
belongs to a java.io package.
used for reading raw bytes using the FileInputStream class

public FileReader(File file) throws FileNotFoundException  

import java.io.*;
import java.util.Scanner;

public class OpenFileExample5 throws IOException{
	FileReader fr=new FileReader("demo.txt");
	char r = fr.read();
}