How to write a hashmap to a file and read it again from another intent

I have got a HashMap in Kotlin that I need to pass to another activity using intent, but its size exceeds the limit to using normal putExtra so I need to write it down in a local file then read it again when I open the second intent.

here is my hashMap private var optionsList: HashMap<String, List<String>> = hashMapOf() and am using HashMap.put method to add data to it .

I've tried to send it using intent.putExtra(HashMap) but it gives an error because it has hundreds of data.

Now all I need to know how to write it to a file and call it again from the second intent.

Solution

Generally you should review your app logic, because such a problem can be a sign of a poor design.

But anyway the next class can be used to store any data class on disk, it uses Gson library for json serialization:

import android.content.Context
import androidx.preference.PreferenceManager

import com.google.gson.Gson
import com.google.gson.JsonSyntaxException

class SharedSettingsProvider<T>(private val gson: Gson,
                                private val context: Context,
                                private val type: Class<T>,
                                private val key: String) {

    fun commit(model: T?) {
        PreferenceManager
                .getDefaultSharedPreferences(context)
                .edit()
                .putString(key, model?.let { gson.toJson(it) } ?: "")
                .apply()
    }

    fun retrieve(): T? {
        val mPrefs = PreferenceManager.getDefaultSharedPreferences(context)
        val gsonData = mPrefs.getString(key, "")
        return try {
            gson.fromJson(gsonData, type)
        } catch (ex: JsonSyntaxException) {
            null
        }
    }
}

The usage:

data class Data(val optionsList: Map<String, List<String>>)
val data = Data(mapOf(...))

const val DATA_KEY = "DATA_KEY"
val provider = SharedSettingsProvider(gson, context, Data::class.java, DATA_KEY)

// write data
provider.commit(data)

// read data
val data1 = provider.retrieve()