Java program to delete certain text from a file

30 May, 2018
Prerequisite : PrintWriter , BufferedReader
Given two files input.txt and delete.txt. Our Task is to perform file extraction(Input-Delete) and save the output in file say output.txt
Example :
 
Naive Algorithm :
1. Create PrintWriter object for output.txt
2. Open BufferedReader for input.txt
3. Run a loop for each line of input.txt
   3.1 flag = false
   3.2 Open BufferedReader for delete.txt
   3.3 Run a loop for each line of delete.txt
      ->  If  line of delete.txt is equal to current line of input.txt 
            -> flag = true
            -> break loop

4. Check flag, if false
     -> write current line of input.txt to output.txt
5. Flush PrintWriter stream and close resources.

To successfully run the below program input.txt and delete.txt must exits in same folder OR provide full path for them.
// Java program to perform file// operation output = input-delete  import java.io.*;  public class FileOperation{    public static void main(String[] args) throws IOException     {        // PrintWriter object for output.txt        PrintWriter pw = new PrintWriter("output.txt");                  // BufferedReader object for input.txt        BufferedReader br1 = new BufferedReader(new FileReader("input.txt"));                  String line1 = br1.readLine();                  // loop for each line of input.txt        while(line1 != null)        {            boolean flag = false;                          // BufferedReader object for delete.txt            BufferedReader br2 = new BufferedReader(new FileReader("delete.txt"));                          String line2 = br2.readLine();                          // loop for each line of delete.txt            while(line2 != null)            {                if(line1.equals(line2))                {                    flag = true;                    break;                }                                  line2 = br2.readLine();            }                          // if flag = false            // write line of input.txt to output.txt            if(!flag)                pw.println(line1);                          line1 = br1.readLine();                      }                  pw.flush();                  // closing resources        br1.close();        pw.close();                  System.out.println("File operation performed successfully");    }}
Output:
File operation performed successfully

Note : If output.txt exist in cwd(current working directory) then it will be overwritten by above program otherwise new file will be created.
A better solution is to use HashSet to store each line of delete.txt and then while looping through lines of input.txt ,check if it is in hashset. If not present, write that line into output.txt.
To successfully run the below program input.txt and delete.txt must exits in same folder OR provide full path for them.
// Efficient Java program to perform file// operation output = input-delete  import java.io.*;import java.util.HashSet;  public class FileOperation{    public static void main(String[] args) throws IOException     {        // PrintWriter object for output.txt        PrintWriter pw = new PrintWriter("output.txt");                  // BufferedReader object for delete.txt        BufferedReader br2 = new BufferedReader(new FileReader("delete.txt"));                  String line2 = br2.readLine();                  // hashset for storing lines of delete.txt        HashSet<String> hs = new HashSet<String>();                  // loop for each line of delete.txt        while(line2 != null)        {            hs.add(line2);            line2 = br2.readLine();        }                              // BufferedReader object for input.txt        BufferedReader br1 = new BufferedReader(new FileReader("input.txt"));                  String line1 = br1.readLine();                  // loop for each line of input.txt        while(line1 != null)        {            // if line is not present in delete.txt            // write it to output.txt            if(!hs.contains(line1))                pw.println(line1);                          line1 = br1.readLine();        }                  pw.flush();                  // closing resources        br1.close();        br2.close();        pw.close();                  System.out.println("File operation performed successfully");    }}
Output:
File operation performed successfully

Note : If output.txt exist in cwd(current working directory) then it will be overwritten by above program otherwise new file will be created.
This article is contributed by Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
java-file-handling
Java-I/O
Java
Java
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
Interfaces in Java
Queue Interface In Java
Multidimensional Arrays in Java
HashMap in Java with Examples
Math pow() method in Java with Example
PriorityQueue in Java
Stack Class in Java
List Interface in Java with Examples
Initialize an ArrayList in Java
ArrayList in Java