Java чтение и запись в файл

При написание программы по распознаванию рукописного текста :

        ArrayList<TrainingSet> trainingSets = new ArrayList<>();
        for (int i = 0; i < 26; i++) {
            char letterValue = (char) (i + 65);
            String letter = String.valueOf(letterValue);
            for (ArrayList<Integer> list : readFromFile("/resources/" + letter + ".txt")) {
                trainingSets.add(new TrainingSet(list, GoodOutputs.getInstance().getGoodOutput(letter)));
            }
        }
        return trainingSets;
    }

Чтение в файле происходит успешно. Но когда я хочу записать в файл новую информацию:

        try {
            File file = new File("/resources/" + filename + ".txt");
            PrintWriter pw = new PrintWriter(new FileOutputStream(file, true));
            for (Integer i : input) {
                pw.write(Integer.toString(i));
            }
            pw.write("\n");
            pw.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

Вылезает куча ошибок:

java.io.FileNotFoundException: /resources/O.txt (No such file or directory)
    at java.base/java.io.FileOutputStream.open0(Native Method)
    at java.base/java.io.FileOutputStream.open(FileOutputStream.java:299)
    at java.base/java.io.FileOutputStream.<init>(FileOutputStream.java:238)
    at data.ReadWriteFile.saveToFile(ReadWriteFile.java:54)
    at gui.MainGui.lambda$setOnClicks$1(MainGui.java:155)
    at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967)
    at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308)
    at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
    at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
    at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:270)
    at java.desktop/java.awt.Component.processMouseEvent(Component.java:6589)
    at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342)
    at java.desktop/java.awt.Component.processEvent(Component.java:6354)
    at java.desktop/java.awt.Container.processEvent(Container.java:2261)
    at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:4966)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2319)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4798)
    at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4914)
    at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4543)
    at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4484)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2305)
    at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2772)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4798)
    at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772)
    at java.desktop/java.awt.EventQueue.access$600(EventQueue.java:97)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:97)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742)
    at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

Метод ReadFromFile:

private static ArrayList<ArrayList<Integer>> readFromFile(String filename) {
        ArrayList<ArrayList<Integer>> inputs = new ArrayList<>();
        try {
            InputStream in = ReadWriteFile.class.getResourceAsStream(filename);
            BufferedReader reader = new BufferedReader(new InputStreamReader(in));
            String line;
            while ((line = reader.readLine()) != null) {
                ArrayList<Integer> input = new ArrayList<>();
                for (int i = 0; i < line.length(); i++) {
                    int value = 0;
                    try {
                        value = Integer.parseInt(String.valueOf(line.charAt(i)));
                    } catch (Exception e) {
                    }
                    input.add(value);
                }
                inputs.add(input);
            }
            reader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return inputs;
    }