Programming Examples of Java Files

We will see here some programming examples related to Java Files.

Problem 1: Write a program to display the contents of a file to the output stream.

FileTest05.java

// Displaying contents of a file to the output stream
import java.io.*;

public class FileTest05 {
    
    public static void main(String[] args) {
        File f;
        FileReader in = null;
        int c;
        
        try {
            f = new File("a.txt");
            in = new FileReader(f);
                       
            while((c = in.read()) != -1) {
                System.out.print((char)c);
            }

        }
        catch(IOException e) {
            System.out.println(e);
        }
        finally {
            try {
                in.close();
            }
            catch(Exception e) {  
                System.out.println(e);
            }
        }
    }
}

Suppose the contents of file “a.txt” is:

It’s Merry Christmas..

Output:

C:\file>java FileTest05
It's Merry Christmas..

Problem 2: Write a program to write keyboard input to a file.

FileTest06.java

// Writing keyboard input to a file
import java.io.*;

public class FileTest06 {
    
    public static void main(String[] args) {
        File f;
        FileWriter out = null;
        char c;
        
        try {
            f = new File("output.txt");
            out = new FileWriter(f);
            System.out.println("Enter characters (type '@' to quit): ");
            while((c = (char)System.in.read()) != '@') {
                out.write(c);
            }            
        }
        catch(IOException e) {
            System.out.println(e);
        }
        finally {
            try {
                out.close();
            }
            catch(Exception e) {  
                System.out.println(e);
            }
        }
    }
}

Output:

C:\file>java FileTest06
Enter characters (type '@' to quit): 
Hello from Tech Guru. Website is techguruspeaks.com. Its Good Bye from Me.@ 

The characters before the ‘@‘ symbol will now be written to “output.txt” file. Now, see this file for checking.

Problem 3: Write a program to merge contents of two files into a third one.

FileTest07.java

// Merging contents of two files into a third one 
import java.io.*;

public class FileTest07 {

    public static void main(String[] args) {
        File f1;
        File f2;
        File f3;
        FileReader in1 = null;
        FileReader in2 = null;
        FileWriter out = null;
        int c;
        
        try {
            f1 = new File("a.txt");
            f2 = new File("b.txt");
            f3 = new File("c.txt");
            in1 = new FileReader(f1);
            in2 = new FileReader(f2);
            out = new FileWriter(f3);
            
            //copy from 1st file
            while((c = in1.read()) != -1) {
                out.write(c);
            }
            
            //copy from 2nd file
            while((c = in2.read()) != -1) {
                out.write(c);
            }
        }
        catch(IOException e) {
            System.out.println(e);
        }
        finally {
            try {
                in1.close();
                in2.close();
                out.close();
            }
            catch(Exception e) {  
                System.out.println(e);
            }
        }
    }
}

Suppose the contents of file “a.txt” is:

It’s Merry Christmas..

And of file “b.txt” is:

Enjoy this festive season..

The contents of output file “c.txt” will be:

It’s Merry Christmas.. Enjoy this festive season..

Problem 4: Write a program to display size and length of a file.

FileTest08.java

// Displaying size and length of a file
import java.io.*;

public class FileTest08 {

    public static void main(String[] args) throws IOException {

        System.out.print("Enter file name : ");
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        File f = new File(in.readLine());

        if (f.exists()) {
            // calculating number of lines in the file
            FileReader fr = new FileReader(f);
            LineNumberReader line = new LineNumberReader(fr);
            int count = 0;
            while (line.readLine() != null) {
                count++;
            }
            // calculating size of the file
            long file_size = f.length();

            System.out.println("Size of the file : " + file_size);
            System.out.println("Total line no: " + count);
        } 
        else {
            System.out.println("File does not exist.");
            System.exit(0);
        }
    }
}

Output:

C:\file>java FileTest08
Enter file name : mytext.txt
Size of the file : 103
Total line no: 4