I need to create my own method hasNextLine with block reading and system.lineseparator

Question

I've been struggling with it for almost a week. I need to make a function in my class that returns true/false if there is a next line in system.in, in a string or in a file.(no matter if it will detect last empty line 'cause java original scanner does the same) It should be written according to my skills (2 months of java in university) - we have only started to write our classes and it is a homework. I don't need you to do it for yourself but I need an advice. I can't use buffered reader's next line. I need to use my own blocking read (array of chars ~256-1024 chars) and when I use that function/method I need to continue reading from the char that was the last in previous time when function was called.

I tried this: It doesn't work on mac but on windows sometimes it does depending on buffer's size (char array) but I cant understand why. Also there is a possibility that first read can have buffer[-1] = '\r' and '\n' can be in the second buffer so I need to create a stringbuilder that will store chars (tbh 1 char before the second is added to the stringbuilder) before they will be recognized as a line separator.

public boolean hasNextLine() {
        refillIfNeeded();
        StringBuilder sep = new StringBuilder();
        while (true) {
            if (getCurrentID() == lastFilled(block)) {
                refillIfNeeded();
                newCurrentID(0);
            }
            if (countofread == -1) {
                break;
            }
            for (int i = getCurrentID(); i < lastFilled(block); i++) {
                if (block[i] == '\n' || block[i] == '\r') {
                    sep.append(block[i]);
                    System.out.println(sep.length());
                    if (sep.toString().equals(separator)) {
                        sep.setLength(0);
                        newCurrentID(i + 1);
                        return true;
                    }
                }
            }
            newCurrentID(lastFilled(block));
        }
        return false;
    }

Functions and their code can be found below:

import java.io.*;

public class MyScanner {

    File source;
    String charsetName;
    String string;
    InputStream in;
    BufferedReader reader;
    int CurrentID = 0;
    char[] block = new char[16];
    int countofread = 0;

    String separator = System.lineSeparator();

    public MyScanner(File source, String charsetName) {
        this.source = source;
        this.charsetName = charsetName;
        try {
            reader = new BufferedReader(
                    new InputStreamReader(new FileInputStream(source), charsetName)
            );
        } catch (IOException e) {
            System.err.println("Unsupported Encoding or File Not Found");
        }
    }

    public MyScanner(File source) {
        this.source = source;
        try {
            reader = new BufferedReader(
                    new InputStreamReader(new FileInputStream(source))
            );
        } catch (FileNotFoundException e) {
            System.err.println("File Not Found");
        }
    }

    public MyScanner(String string) {
        this.string = string;
        reader = new BufferedReader(new StringReader(string));
    }

    public MyScanner(InputStream in) {
        reader = new BufferedReader(new InputStreamReader(in));
    }

    public MyScanner(InputStream in, String charsetName) {
        try {
            reader = new BufferedReader(
                    new InputStreamReader(in, charsetName)
            );
        } catch (UnsupportedEncodingException e) {
            System.err.println("Unsupported Encoding");
        }
    }

    public void close() {
        try {
            reader.close();
        } catch (IOException e) {
            System.err.println("Can't close the scanner");
        }
    }

    public int fillBlock(char[] block) {
        try {
            countofread = reader.read(block);
        } catch (IOException e) {
            System.err.println("Can't fill the block");
        }
        return countofread;
    }

    public int getCurrentID() {
        return CurrentID;
    }

    public void newCurrentID(int newID) {
        CurrentID = newID;
    }

    public int lastFilled(char[] arr) {
        int last = arr.length - 1;
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == Character.MIN_VALUE) {
                last = i - 1;
                return last;
            }
        }
        return last;
    }

    public void refillIfNeeded() {
        if (getCurrentID() >= lastFilled(block)) {
            fillBlock(block);
        }
    }



    public boolean hasNextLine() {
        refillIfNeeded();
        StringBuilder sep = new StringBuilder();
        while (true) {
            if (getCurrentID() == lastFilled(block)) {
                refillIfNeeded();
                newCurrentID(0);
            }
            if (countofread == -1) {
                break;
            }
            for (int i = getCurrentID(); i < lastFilled(block); i++) {
                if (block[i] == '\n' || block[i] == '\r') {
                    sep.append(block[i]);
                    System.out.println(sep.length());
                    if (sep.toString().equals(separator)) {
                        sep.setLength(0);
                        newCurrentID(i + 1);
                        return true;
                    }
                }
            }
            newCurrentID(lastFilled(block));
        }
        return false;
    }





}