How to Open a File in Java

There are following ways to open a file in Java:

Java Desktop class
Java FileInputStream class
Java BufferedReader class
Java FileReader class
Java Scanner class
Java nio package

Java Desktop class

Java Desktop class provide an open() method to open a file. It belongs to a java.awt package. Desktop implementation is platform-dependent, so it is necessary to check the operating system supports Desktop or not. The Desktop class looks for an associated application registered on the native desktop to handle a file. If there is no associated application or application is fails to be launched, it throws the FileNotFoundException. Launches the user default browser to show a specified URI.

Launches the user default mail client with an optional mail-to URI.
Launches the registered application to open, edit, or print a specified file.

The open() method of Desktop class launches the associated application to open a file. It takes a file as an argument. The signature of the method is:

The method throws the following exceptions:

NullPointerException: If the file is null.
IllegalArgumentException: It is thrown when the file does not exist.
IOException: It is thrown when there is no application associated with the given file type.
UnsupportedOperationExecution: If the current platform does not support the Desktop.Action.Open action.

Example

import java.awt.Desktop;
import java.io.*;
public class OpenFileExample1 
{
public static void main(String[] args) 
{
try
{
//constructor of file class having file as argument
File file = new File("C:\\demo\\demofile.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();
}
}
}

When we run the above program, it opens the specified text file in the default text editor. We can also open the .docx, .pdf, and .jpg file.

Output:

Java FileInputStream class

Java FileInputStream class is used to open and read a file. We can open and read a file by using the constructor of the FileInputStream class. The signature of the constructor is:

It accepts a file as an argument. It throws FileNotFoundException if the file does not exist or file name is a directory.

Example

import java.io.*;
import java.util.Scanner;
public class OpenFileExample2
{	
public static void main(String args[])
{
try
{
//constructor of file class having file as argument
File file=new File("C:\\demo\\demofile.txt"); 
FileInputStream fis=new FileInputStream(file);     //opens a connection to an actual file
System.out.println("file content: ");
int r=0;
while((r=fis.read())!=-1)
{
System.out.print((char)r);		//prints the content of the file
}
}
catch(Exception e)
{
e.printStackTrace();
}
}
}

Output:

Java BufferedReader class

Java BufferedReader class reads text from a character input stream. It belongs to a java.io package. We use the constructor of the BufferedReader class to open or read a file. The signature of the constructor is:

It creates a buffering character-input stream that uses a default?sized input buffer. It uses a default size input buffer.

Example

import java.io.*;
import java.util.Scanner;
public class OpenFileExample3
{	
public static void main(String args[])
{
try
{
//constructor of File class having file as argument
File file=new File("C:\\demo\\demofile.txt"); 
//creates a buffer reader input stream
BufferedReader br=new BufferedReader(new FileReader(file));
System.out.println("file content: ");
int r=0;
while((r=br.read())!=-1)
{
System.out.print((char)r);
}
}
catch(Exception e)
{
e.printStackTrace();
}
}
} 

Output:

Java FileReader class

Java FileReader class is also used for opening and reading a file. It belongs to a java.io package. It is a convenience for reading characters of the files. It is used for reading raw bytes using the FileInputStream class. We use the constructor of the FileInputStream class to open and read a file. The signature of the constructor is:

It accepts a file as an argument. It throws the FileNotFoundException if the specified file does not exist or the file name is a directory.

Example

import java.io.*;
public class OpenFileExample4
{	
public static void main(String args[])
{
try
{
//constructor of the File class having file as an argument
FileReader fr=new FileReader("C:\\demo\\demofile.txt"); 
System.out.println("file content: ");
int r=0;
while((r=fr.read())!=-1)
{
System.out.print((char)r);	//prints the content of the file 
}
}
catch(Exception e)
{
e.printStackTrace();
}
}
}

Output:

Java Scanner class

Java Scanner class is also used for opening and reading a file. The Scanner class belongs to java.util package. The constructor of Scanner class is used for opening and reading a file. The signature of the constructor is:

It accepts a file (to be scanned) as an argument. It also throws FileNotFoundException, if the source of the file is not found.

Example

import java.io.File; 
import java.util.Scanner; 
public class OpenFileExample5
{ 
public static void main(String[] args) 
{ 
try
{
File file=new File("C:\\demo\\demofile.txt"); 
Scanner sc = new Scanner(file); 	//file to be scanned
while (sc.hasNextLine()) 		//returns true if and only if scanner has another token
System.out.println(sc.nextLine()); 	
}
catch(Exception e)
{
e.printStackTrace();
}
} 
}

Output:

Java nio package

readAllLines() method: The readAllLines() method is the method of File class. It reads all lines from a file and bytes from the file are decoded into characters using UTF-8 charset. It returns the lines from the file as a list. The signature of the method is:

Where path is the file path.

Above method is equivalent to invoking the following:

Collections.emptyList(): The emptyList() method is the method of Collection class which belong to java.util package. It is used to obtain an empty list. The signature of the method is:

Example

import java.util.*; 
import java.nio.charset.StandardCharsets; 
import java.nio.file.*; 
import java.io.*; 
public class OpenFileExample6
{ 
public static List<String> readFileInList(String fileName) 
{ 
List<String> lines = Collections.emptyList(); 
try
{ 
lines=Files.readAllLines(Paths.get(fileName), StandardCharsets.UTF_8); 
} 
catch (IOException e) 
{ 
e.printStackTrace(); 
} 
return lines; 
} 
public static void main(String[] args) 
{ 
List l = readFileInList("C:\\demo\\demofile.txt"); 
Iterator<String> itr = l.iterator(); 	  //access the elements
while (itr.hasNext()) 		//returns true if and only if scanner has another token
System.out.println(itr.next());      //prints the content of the file
} 
}

Output: