I've never really understood reading files from console

Question about the task

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I assume I am failing for writing my own shoddy method (fileAdd) and trying to be clever. But I'm not sure why it doesn't work. And my brain has been pummeled by these tasks already, so I'm gonna need some help on this one.

Make the joinData method transactional, i.e. if it fails, then the data must not be changed.
1. Read 2 file names from the console.
2. Read from the files line by line. Read from the first file into allLines, from the second file — into linesForRemoval.
In the joinData method:
3. If the allLines list contains all the lines from linesForRemoval, then remove from the allLines list all the lines that are in linesForRemoval.
4. If the condition in Item 3 is not satisfied, then:
4.1. clear the data in allLines
4.2. throw a CorruptedDataException
The joinData method should be called in main. Handle all exceptions in the main method.
Don't forget to close the streams.

Requirements:

The Solution class must contain a public static List<String> field called allLines.
The Solution class must contain a public static List<String> field called linesForRemoval.
The Solution class must have a public void joinData() method that can throw a CorruptedDataException.
The program should read the names of two files from the console.
The program must read line by line from the first file into the allLines list.
The program must read line by line from the second file into the linesForRemoval list.
The joinData method must remove from the allLines list all lines in the linesForRemoval list if allLines contains all the lines in the linesForRemoval list.
The joinData method must clear the allLines list and throw a CorruptedDataException, if allLines does not contain all the lines in the linesForRemoval list.
The joinData method should be called in main.

package com.codegym.task.task17.task1721; import java.io.FileReader; import java.io.IOException; import java.util.ArrayList; import java.util.List; import java.util.Scanner; /* Transactionality */ public class Solution { public static List<String> allLines = new ArrayList<>(); public static List<String> linesForRemoval = new ArrayList<>(); private static String fileName1, fileName2; public static void main(String[] args) throws IOException { Scanner sc = new Scanner(System.in); fileName1 = sc.nextLine(); fileName2 = sc.nextLine(); sc.close(); Solution solution = new Solution(); solution.joinData(); } public void joinData() throws CorruptedDataException { allLines = fileAdd(fileName1); linesForRemoval = fileAdd(fileName2); if (allLines.containsAll(linesForRemoval)) { allLines.removeAll(linesForRemoval); } else { allLines.clear(); throw new CorruptedDataException(); } } public static List<String> fileAdd (String file) { List<String> result = new ArrayList<>(); try { FileReader f = new FileReader(file); StringBuffer sb = new StringBuffer(); while (f.ready()) { char c = (char) f.read(); if (c == '\n') { result.add(sb.toString()); sb = new StringBuffer(); } } if (sb.length() > 0) result.add(sb.toString()); } catch (IOException e) {} return result; } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

1 March 2021, 16:17

#1 Remove line 17 and move the variables to the main method. Next remove lines 29 and 30 and move them, also, to the main method. This is because the joinData method must do as the task describes, and it does not say that it should read the data from the file. This would be a problem as the validator is going to test the method without your main method. It will fill AllLines, then fill linesForRemoval; both with its own implementation. Then it will call your joinData() to test the results. Because your joinData() doesn't do what the task says it should do, the validator will end up with incorrect results. #2 The fileAdd() method won't work either because lines don't just end with a new line character, they end with a carriage return and a new line character. I would suggest you either: a) do as above, but trim each string of carriage returns before adding them to the list b) use a buffered reader and read line by line. The internal mechanics will handle the proper trimming for you c) use a scanner and read line by line, this would be the same in essence as the buffered reader d) same as a but read all the bytes into a buffer first before converting to String a is going to be your slowest method. You will gain a lot of speed/performance by using b/c; while d will be the fastest.

Oliver Heintz

Level 18 , Mustang, United States

1 March 2021, 17:00

So, to trim off the carriage returns, would I insert a line: sb.trimToSize(); before my while loop? I mean, doing that allowed me to pass, but was that what you were referring to? And to do (d) on your list, would I make a byte array and read the data into that, and then convert it into a String? I'm guessing Level 18 will address a lot of this because these are the tasks that really throw me for a loop. I struggle immensely with input and output streams.

Guadalupe Gagnon

Level 37 , Tampa, United States

1 March 2021, 17:12

with d, here is an example:

// create the stream
FileInputStream fis = new FileInputStream(file);

// create the buffer. The available() method gives the amount of unread bytes in the stream
// so using this will give the size the buffer needs to be
byte[] buffer = new byte[fis.available()];

// one line reads it all, just pass the buffer to the read() method
fis.read(buffer);

// always ensure streams are closed when you finish using them
fis.close();

// pass the buffer to String constructor, this will convert the bytes to a String
// replaceAll() is used here to remove all "\r", which is the sequence for carriage return
String data = new String(buffer).replaceAll("\r", "");

Guadalupe Gagnon

Level 37 , Tampa, United States

1 March 2021, 17:26

"sb.trimToSize();" This doesn't trim anything from the String that you are building. The StringBuilder class has an Array of characters internally that is used to efficiently build a String from pieces. The size is determined by an algorithm, which resizes as more data is added in. The trimToSize() method trims this internal Array down to the size of the data that you added in, which reduces the overall size that the object takes up in memory. https://www.geeksforgeeks.org/stringbuilder-trimtosize-method-in-java-with-examples/ If you didn't trim the carriage returns, then this task may not have them in the test data. Some of the tasks do, and some don't, but you should make it a default to handle them always.

Oliver Heintz

Level 18 , Mustang, United States

1 March 2021, 17:31

Every time I think I'm finally understanding how all this stuff works, something like this comes along and makes me want to cry. I think I follow what you're saying, but I know that in the future I'm still going to be unable to reproduce it.

Guadalupe Gagnon

Level 37 , Tampa, United States

1 March 2021, 17:37

Trust me... google Programming is a VERY large subject, and it grows larger every single day. Most people have this idea that learning to program means that you will know everything, but that couldn't be further from the truth. Effective programmers don't necessarily know everything, but know how to search for what they need. However, since you are still learning the basics, figuring out what you need is going to be a challenge. It does get easier over time, with practice (and failure), you start to retain a lot of information. Next task that requires you to read a lot of data you can remember that these techniques can be used to do that; and while you may not recall exactly how it was done, you will know what to look for through google to get you going.

Oliver Heintz

Level 18 , Mustang, United States

3 March 2021, 01:00

These are the sort of encouraging comments that they need to put in front of each new lesson still 😄 I'm concurrently taking courses in algorithms, and I'm able to get through the programming parts of that class so easy because of what I already know from CodeGym, so I know I'm learning a lot. I might not be advancing through this course as quickly as other students, but I am no less passionate about it.

Guadalupe Gagnon

Level 37 , Tampa, United States

3 March 2021, 03:13

1) https://www.youtube.com/watch?v=_KhZ7F-jOlI&t=20s 2) https://www.youtube.com/watch?v=ujb2CIWE8zY&t=470s 3) https://www.youtube.com/watch?v=oBt53YbR9Kk&t=3s Here are some videos I like. The first two are about 15 minutes a piece and explain how radix sort works by explaining how the sorting methods that build radix sort works. Neither of those have any code and just explain with examples. The third video is five hours long and explains dynamic programming. If you are interested in algorithms then this video has a lot to sink your teeth into. I watched it all and have already used a bunch of the techniques in my own code.

Oliver Heintz

Level 18 , Mustang, United States

5 March 2021, 11:07

I watched ALL of that 5 hour video. Even if he was using JavaScript, I think the logic there was great, and it makes me excited to tackle algorithm problems myself.

Guadalupe Gagnon

Level 37 , Tampa, United States

5 March 2021, 14:16

What I like about these is how simple the actual algorithms are that solve these complex looking problems. The 5 hour video, by itself, made me a better programmer.