CodeGym/Help with Java Tasks/Help me please with n3 & n4

Level 32

Budapest

03.09.2024
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Help me please with n3 & n4

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I've written this code, but I can't pass the 3d and 4th requirements, i.e. I need to make sure the product list remains unchanged when you run the program without arguments, and when the program is run with the arguments "-c productName price quantity", a new line with the corresponding product should be added to the end of the file. I thought the line if (!args[0].equals("-c")) return; was enough to make sure nothing happens when there are no arguments or the arguments are different from what is expected. I probably misunderstand the task. For instance, I'm not really getting how I am supposed to follow this instruction: "Before adding a new line, re-write all of its contents to the file". To me, adding a line (to the file) equals to rewriting the contents of this line to the file. How can one be done before the other? Is that where my mistake is?

CrUD for a table inside a file.
Read a file name for CrUD operations from the console.

The program is started with the following arguments:
-c productName price quantity

Argument values:
where id is 8 characters.
productName is 30 characters.
price is 8 characters.
quantity is 4 characters.
-c adds the product with the specified parameters to the end of the file, and generates an id by incrementing the maximum id found in the file.

The file data is stored in the following order (without separating spaces):
id productName price quantity

Each data field is padded with spaces up to its length.

Before adding a new line, re-write all of its contents to the file.

Example:
19847 Swim trunks, blue 159.00 12 198479 Swim trunks, black with printe173.00 17 19847983Snowboard jacket with reflecti10173.991234

Requirements:

The program should read a file name for CrUD operations from the console.
The Solution class should not use static variables.
When you run the program without arguments, the product list must remain unchanged.
When the program is run with the arguments "-c productName price quantity", a new line with the corresponding product should be added to the end of the file.
The product must have the next id after the maximum id found in the file.
The format of the new product line must precisely match that format specified in the task.
The file streams must be closed.

package com.codegym.task.task18.task1827; /* Prices */ import java.io.*; import java.nio.Buffer; public class Solution { public static void main(String[] args) throws Exception { //• The program should read a file name for CrUD operations from the console. BufferedReader breader = new BufferedReader(new InputStreamReader(System.in)); String fName = breader.readLine(); FileReader freader = new FileReader(fName); BufferedReader breader2 = new BufferedReader(freader); int idMax = 0; while (breader2.ready()) { String line = breader2.readLine(); String regex1 = line.substring(0,8); String digits = regex1.trim(); int id = Integer.parseInt(digits); idMax = idMax > id? idMax : id; } //• The Solution class should not use static variables. - OK //• When you run the program without arguments, the product list must remain unchanged. //• When the program is run with the arguments "-c productName price quantity", //a new line with the corresponding product should be added to the end of the file. BufferedWriter bwriter = new BufferedWriter(new FileWriter(fName)); if (!args[0].equals("-c")) return; //The width is the minimum number of characters to be written to the output. // If the length of the converted value is less than the width // then the output will be padded by ' ' ('\u0020') // until the total number of characters equals the width. // The padding is on the left by default. // If the '-' flag is given, then the padding will be on the right. If the width is not specified then there is no minimum. String s1 = String.valueOf(idMax + 1); //String.format("%1$" + length + "s", inputString); String ID = String.format("%1$-" + 8 + "s", s1); /* if (s1.length() < 8) { for (int i = 0; i < (8 - s1.length()); i++) { s1 = s1 + " "; } } else s1 = s1.substring(0, 8);*/ String s2 = args[1]; /*if (s2.length() < 30) { for (int i = 0; i < (30 - s2.length()); i++) { s2 = s2 + " "; } } else s2 = s2.substring(0, 30);*/ String productName = String.format("%1$-" + 30 + "s", s2); String s3 = args[2]; /*if (s3.length() < 8) { for (int i = 0; i < (8 - s3.length()); i++) { s3 = s3 + " "; } } else s3 = s3.substring(0, 8);*/ String price = String.format("%1$-" + 8 + "s", s3); String s4 = args[3]; /*if (s4.length() < 4) { for (int i = 0; i < (4 - s4.length()); i++) { s4 = s4 + " "; } } else s4 = s4.substring(0, 4);*/ String quantity = String.format("%1$-" + 4 + "s", s4); String result = ID + productName + price + quantity; bwriter.write(result); bwriter.write(System.lineSeparator()); //• The product must have the next id after the maximum id found in the file. //• The format of the new product line must precisely match that format specified in the task. //• The file streams must be closed. breader.close(); freader.close(); breader2.close(); bwriter.close(); } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

3 September, 13:35

for your first issue try to make sure that the args length is 4 and that the first value is "-c":

if(args.length does not equal 4 OR args[0] does not equal "-c") return;

The reason your code does not work is because if there are no arguments then the array would have no indices (would have a length of 0). Therefore checking if the first index has the value "-c" would result in an ArrayIndexOutOfBoundsException being thrown. Your second question is that the program just wants you to overwrite the file each time. So if the file has: line 1 info line 2 info line 3 info Then your program should read all of that, determine what to do with the passed in arguments, then write: line 1 info line 2 info line 3 info line 4 info <-- the new line info

Guadalupe Gagnon

Level 37 , Tampa, United States

3 September, 13:45

Your code properly adds spaces when an item is shorter than what is specified; however, a problem I see is that if any of the items have larger lengths than specified your code does not fix that. For example, if the new product has a product description that is 40 characters long then your code would write all 40 characters instead of only writing 30 as the conditions specify.

Evgeniia Shabaeva

Level 32 , Budapest, Hungary

16 September, 10:15

Thank you very much, Thomas! I rewrote the condition the way you advised. Still didn't pass the 2-nd requirement, so tried wrapping the next code into an else-clause, and that worked. Also had to had to add "true" to the line with the BufferedWriter to pass the 4th requirement. I think think there have been tasks in which I didn't need to add this boolean value to prevent overwriting the information... But I might've misunderstood something. Well, this is the result:

if (args.length != 4 || !args[0].equals("-c")) return;
else {
            String ID = shapeLength(String.valueOf(idMax + 1), 8);
            String productName = shapeLength(args[1], 30);
            String price = shapeLength(args[2], 8);
            String quantity = shapeLength(args[3], 4);
            String result = ID + productName + price + quantity;
            BufferedWriter bwriter = new BufferedWriter(new FileWriter(fName, true));
            bwriter.write(System.lineSeparator() + result);
            productList.add(result);
            bwriter.close();
        }

Evgeniia Shabaeva

Level 32 , Budapest, Hungary

16 September, 10:18

Thank you very much, Guadalupe, for your comment. I decided to write a separate method for shaping the product information to avoid too many conditional statements in my code. This is what I came up with:

public static String shapeLength (String givenString, int givenLength) {
        String result = null;
        if (givenString.length() > givenLength) result = givenString.substring(0, givenLength);
        else if (givenString.length() < givenLength) result = String.format("%1$-" + givenLength + "s", givenString);
        else result = givenString;
        return result;
    }

There could have been a better way to do it, but mine worked as well. I'm so happy and relieved:) Thank you for your help!

Thomas

Level 41 , Bayreuth, Germany

16 September, 11:08

Guadalupe already gave you a lot of excellent advice. I'd just wanted to mention, that using FileReader in append mode is not what CG required (and I'm astonished that it validated ;) ) The requirement clearly is to rewrite the entire file and not to append just that single product passed inside the program arguments. Another suggestion is, to make use of some structure. That'll make your coder life a lot easier. Like using a switch-case in main. If the first argument matches -c, then you invoke a create method that does the create crud operation for you. As nothing else follows, the main method would just silently terminate if you did not pass a valid operation identifier. However you can add new operations with ease. Here's very basic pseudo code that will give you some ideas how to get some clear structure into your code. I also describe how to rewrite that file without that true (FileWriter append mode)

main:
    case(-c):
        switch create(fileName, args)

// crud method
create(fileName, args):void
    List<Product> products = readProducts(fileName)
    Product product = createProduct(args)
    products.add(product)
    writeProducts(fileName, products)

// main helper methods
createProduct(args):Product
    .. create the product from the args

readProducts(fileName):List<Product>
   ... read file contents to a list

writeProducts(fileName, products):void
    eg. PrintWriter out = new ...
    for (Product p : products)
        pw.println(p)

Evgeniia Shabaeva

Level 32 , Budapest, Hungary

16 September, 12:02

Thank you very much for your reply, Thomas. Probably, using FileReader in the append mode was accepted because the 4th requirement says that "when the program is run with the arguments '-c productName price quantity', a new line with the corresponding product should be added to the end of the file". Re-writing is only mentioned in the instruction which says, "before adding a new line [to the product list, as I understood], re-write all of its contents to the file". That just means to me that every new line with product info must be first added to the file, and then to the product list. By the way, I didn't create a separate Product class (as it wasn't in the task description, and I didn't think of it myself either). I just made a <String>arrayList and called it productList. Was it all the same, or was it cheating, I'm wondering now? Why is better to create a class? To be able to use its getters and setters, or are there other advantages? Also thank you for the recommendation to use switch. I still have a question about it. Is it really helpful when we only have two variants to switch between? I usually use it when there are too many options and I don't want to overload my code with if-clauses. Why is it better here?

Thomas

Level 41 , Bayreuth, Germany

16 September, 12:13

Product class: That's no cheating at all as it was not required. If I remember correctly this (or the next) task even fails if you use a dedicated class with a double price field as this probably may render differently when writing it back to the file compared to the orignal String. Like missing leading zeros or such. Yes, rewriting is mentioned just in the instruction and for me that is something like enhanced requirements. And if the clearly state: Before adding a new line, re-write all of its contents to the file, then I assume CG really means you should re-write all of the content before adding a new line. Whatever, it validated and you learned a lot of new stuff. And that counts :)

Thomas

Level 41 , Bayreuth, Germany

16 September, 12:20

Is it really helpful when we only have two variants to switch between? There is no fixed rule when to use switch-case of if-else if. Sometimes it is just better to read if you use swith-case. It also may be easier to modify or you directly can return values and save some lines (the new switch case from Jave 14, it also doesn't require break anymore). Sometimes you just can not use switch-case it if the type does not allow it (if it's not enum, string, primitive types and some wrappers) or if you need ranges.