Why doesn't this work?

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I found a method I vaguely understand on Baeldung that I repurposed for this task, but I get the gist of it. We check the string at the end of the last "." and if it equals txt then we create a new FileInputStream, otherwise we close and throw the exception. It seems straightforward. Did I miscalculate the index or something?

Change the TxtInputStream class so that it only works with txt files (* .txt).
For example, first.txt or name.1.part3.txt.
If a non-txt file is passed (e.g. file.txt.exe), then the constructor should throw
an UnsupportedFileNameException.
Think about what else you need to do if an exception is thrown.

Requirements:

The TxtInputStream class must inherit the FileInputStream class.
If a txt file is passed to the constructor, TxtInputStream should behave like a regular FileInputStream.
If a non-txt file is passed to the constructor, an UnsupportedFileNameException should be thrown.
If an exception is thrown, then you must also call super.close().

package com.codegym.task.task18.task1814; import java.io.FileInputStream; import java.io.IOException; import java.util.Optional; /* UnsupportedFileName */ public class TxtInputStream extends FileInputStream { public TxtInputStream(String fileName) throws IOException, UnsupportedFileNameException { super(fileName); if (GetExtension(fileName).equals("txt")) new FileInputStream(fileName); else { super.close(); throw new UnsupportedFileNameException(); } } public Optional<String> GetExtension (String fileName) { return Optional.ofNullable(fileName).filter(f -> f.contains(".")).map(f -> f.substring(fileName.lastIndexOf("."))); } public static void main(String[] args) { } }

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Nouser

Level 36 , Germany

12 March 2021, 20:24

solution

try endsWith instead of your Optional. Or, if that is to simple for you, then use substring like in the lambda you used String ext = fileName.substring(fileName.lastIndexOf(".")); but I'd try endsWith first If you still want to go the Optional route, then you get the value of that Optional with get()

Oliver Heintz

Level 18 , Mustang, United States

12 March 2021, 20:34

Yeah, I knew using endsWith would work, and it was a simple change that took all of 5 s to implement, but I still want to know WHY what I tried first didn't work. For future applications I might need to handle all sorts of different file extensions differently, and this seems to limit me to only a few very specific extensions.

Nouser

Level 36 , Germany

12 March 2021, 20:36

as said, get() and I changed one fileName to f but it works your way, too, cause you just have that one name.

String fileName = "myName.txt";
Optional<String> op = Optional.ofNullable(fileName).filter(f -> f.contains(".")).map(f -> f.substring(f.lastIndexOf(".")));
System.out.println(op.get());

You compare that Optional object with a String and that doesn't work.

Oliver Heintz

Level 18 , Mustang, United States

12 March 2021, 21:28

You're right, that is far too confusing for me to fully understand completely. I understand what you're saying, but am not sure it's going to stick for me to remember the next time I encounter this issue.