What I left out. Inappropriate char range?

In each while cycle you read 4 bytes. To stay with your example... In the first loop iteration you read in line 17 Q, this is >= 65, then you read W. It is <= 90 ---> counter++ Second iteration: E >= 65 (true), R <= 90 (true) --> counter++ Third iteration: T >= 65 (true), Y <= 90 (true) --> counter++ Fourth iterarion (the linebreak): 13 >= 65 (false) -> elseif 10 >= 97 (false) Fifth iteration: q >= 65 (true), w <= 90 false -> elseif e >= 97 (true), r <= 122 (true) --> couter++ Sixth iteration: t >= 65 (true), y <= 90 false -> elseif -1 >= 97 (false) You can go through all the chars and end up with 4 as result. Just keep in mind that the linebreak is two bytes on Win system (13 10). When the stream ends and you still try to read values you'll get a -1 Because char is (in the end) a numeric value you could use that for more understandable code. E.g. 'A' translates to 65 and 'Z' translates to '90' therefore you could use 'A' and 'Z' inside your if conditinon (instead of 65 and 90).

What I noticed is. When I assign fileInputStrem.read() to variable, then counting goes correct. Similar is when I put to a List and filter that list with if condition. Gives me good result. So, When I don't assign Stream to variable, then that data goes to default memory / container with default capacity, 4 bytes? And then there is a problem with display that data. Because memory contains too much data and it prints random data. Some time gives me empty space, but should print mark. Because is contained in if statement with range. And with every loop, my amount of available bytes is reduced. What if I create a buffer with 127 amount? (now it's popped into my head) I tried with 1000 capacity and 10 and 1. But it gives me fail result.

int counter = 0;
int loop = 0;       // test print

while (fileInputStream.available() > 0) {

    loop++;     // test print
    System.out.println("\nloop(" + loop + ")");     // test print

    if (fileInputStream.read() >= 'A' && fileInputStream.read() <= 'Z') {
        counter++;

        char c = (char) fileInputStream.read();     // test print
        System.out.println(c + "=" + fileInputStream.read() + " counter:" + counter);       // test print

    } else if (fileInputStream.read() >= 'a' && fileInputStream.read() <= 'z') {
        counter++;

        char c = (char) fileInputStream.read(); // test print
        System.out.println(c + "=" + fileInputStream.read() + " counter:" + counter);  // test print
    }

loop(1)
R=84 counter:1

loop(2)

loop(3)
y=13 counter:2

loop(4)

output solution: 2

Process finished with exit code 0

With buffer 127 still fail. Gives empty one loop. So the best way is to use variable or list? I will wait a while with verification.

The problem is that you think the InputStream.read() method always returns the same byte (in one loop cycle). But it doesn't. It returns the next byte each time you access it. So you need to save the result in a variable like

int b = fileInputStream.read();

Then you can access each time you need it that recently read byte. Example:

if (b >= 'a' && b <= 'z' || b >= 'A' && b <= 'Z')
    counter++;

Thanks, Thomas. Now is clear for me. 👍

One of the mistakes I made myself. And I wasn't able to imagine how to do it properly :)