How to save Objects to a file in Android Kotlin

I have A RecordDAO.kt which was given to me by my professor and we should edit it so the program works as it should. The program itself is kind of a student performance logger where I can add new student performances to show them in a ListView.

The RecordDAO looks like this:

class RecordDAO(private val ctx: Context) {
    private var nextId = 0
    private val records = initRecords()

    fun findAll(): List<Record> = records.toList()

    fun findById(id: Int): Record? {
        var record: Record? = Record()
        records.forEach { if(it.id == id) record = it }
        return record
    }
    /**
     * Ersetzt das übergebene [Record] Objekt mit einem bereits gespeicherten [Record] Objekt mit gleicher id.
     *
     * @param record
     * @return true = update ok, false = kein [Record] Objekt mit gleicher id im Speicher gefunden
     */
    fun update(record: Record): Boolean {
        var existing = false
        records.forEach {
            if(it.id == record.id){
                val index= records.indexOf(it)
                records[index] = record
                existing = true
            }
        }
        return existing
    }

    /**
     * Persistiert das übergebene [Record] Objekt und liefert die neue id zurück.
     * (Seiteneffekte: record.id = nextId; nextId += 1 )
     *
     * @param record
     * @return neue record id
     */
    fun persist(record: Record): Int {
        saveRecords()
        record.id = nextId
        records.add(record)
        return record.id!!
    }

    /**
     * Löscht das übergebene [Record] Objekt anhand der id aus dem Speicher.
     *
     * @param record
     * @return true = ok, false = kein [Record] Objekt mit gleicher id im Speicher gefunden
     */
    fun delete(record: Record): Boolean {
        var existing = false
        records.forEach {
            if(it.id == record.id){
                val index:Int = records.indexOf(it)
                records.removeAt(index)
                existing = true
            }
        }
        return existing
    }

    private fun initRecords(): MutableList<Record> {
        val records = mutableListOf<Record>()
        val f = ctx.getFileStreamPath(FILE_NAME)
        if (f.exists()) {
            ctx.openFileInput(FILE_NAME).use { fin ->
                ObjectInputStream(fin).readObject().let { obj ->
                    records.addAll(obj as MutableList<Record>)
                    // init next id
                    if (records.size > 0) {
                        nextId += 1
                    }
                }

            }
        }
        return records;
    }

    private fun saveRecords() {
        ctx.openFileOutput(FILE_NAME, Context.MODE_PRIVATE)
                .use { ObjectOutputStream(it).writeObject(records) }
    }

    companion object {
        private const val FILE_NAME = "records.obj"
    }
}

I got some questions for this code, what exactly does the saveRecords function do, does it create the file and saves the list into it or do I have to create that file by my hand.

My Form Kotlin Form file looks like this:

class RecordFormActivity : AppCompatActivity() {

    private var record = Record()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_record_form)

        val names = resources.getStringArray(R.array.module_names)

        val adapter = ArrayAdapter(this, simple_dropdown_item_1line, names)
        moduleName.setAdapter(adapter)
    }

    fun onSave(view: View) {
        var isValid = true

        record.moduleNum = moduleNum.text.toString().trim().ifEmpty {
            moduleNum.error = getString(R.string.module_num_not_empty)
            isValid = false
            ""
        }

        record.moduleName = moduleName.text.toString().ifEmpty {
            moduleName.error = getString(R.string.module_name_not_empty)
            isValid = false
            ""
        }

        if(isValid){
            if(record.id == null) RecordDAO(this).persist(record)
            else(RecordDAO(this).update(record))
        }

        finish()

    }
}

Where do I have to call the saveRecords method so the data gets saved to the file and gets retrieved after I close and open the app again ?

Should I post my layout XML files aswell?