HiddenKeyListener for JavaFX

HiddenKeyListener.java

import javafx.event.Event;
import javafx.event.EventHandler;
import javafx.scene.input.KeyCode;
import javafx.scene.input.KeyEvent;

import java.util.Collections;
import java.util.Optional;
import java.util.Set;

class HiddenKeyListener<T extends Event> implements EventHandler<T> {

    public interface Callback {
        void action();
    }

    private final String pattern = "[A-Za-z_0-9]+";

    private final char[] code;
    private final Callback callback;

    private boolean repeatable;
    private boolean enter;
    private int timeout;
    private Set<KeyCode> skippedKeys;

    private int position;
    private long millis;

    public HiddenKeyListener(String code, Callback callback, boolean repeatable, boolean enter, int timeout, Set<KeyCode> skippedKeys) {
        Optional.ofNullable(code)
                .filter(s -> s.matches(pattern))
                .orElseThrow(() -> new IllegalArgumentException("Code must match " + pattern));

        Optional.ofNullable(callback)
                .orElseThrow(() -> new IllegalArgumentException("Callback cannot be null"));

        this.code = code.toCharArray();
        this.callback = callback;
        this.repeatable = repeatable;
        this.enter = enter;
        this.timeout = timeout <= 0 ? 30000 : timeout * 1000;
        this.skippedKeys = skippedKeys == null ? Collections.emptySet() : skippedKeys;

        this.position = 0;
        this.millis = 0;
    }

    public HiddenKeyListener(String code, Callback callback) {
        this(code, callback, true, false, 30, Collections.emptySet());
    }

    @Override
    public void handle(T t) {
        if (t instanceof KeyEvent) {
            KeyEvent keyEvent = (KeyEvent) t;

            if (skippedKeys.contains(keyEvent.getCode()))
                return;

            if (code.length == 1) {
                if (keyEvent.getText().charAt(0) == code[0]) {
                    callback.action();
                    return;
                }
            }

            if (millis + timeout <= System.currentTimeMillis())
                position = 0;

            if (keyEvent.getText().matches(pattern))
                check(keyEvent.getText().charAt(0));
            else if (keyEvent.getCode().equals(KeyCode.ENTER))
                check('\n');
            else if (!keyEvent.isShiftDown())
                position = 0;
        }
    }

    private void check(char input) {
        if (position == 0) {
            if (input == code[0]) {
                position = 1;
                millis = System.currentTimeMillis();
            }
        } else if (position > 0 && position < code.length) {
            if (input == code[position++]) {
                if (position == code.length && !enter) {
                    action();
                }
            } else {
                position = 0;
                check(input);
            }
        } else if (position == code.length) {
            if (input == '\n') {
                action();
            } else {
                position = 0;
                check(input);
            }
        }
    }

    private void action() {
        if (repeatable)
            position = 0;
        else
            position++;
        callback.action();
    }
}