Data in hashmap does not save after closing app

Question

I'm currently creating an app in android studio where I store user input into a hashmap and show it into a listview, but when I close the app and open it back up, the data is gone and it shows an empty listview.

Is there a way to solve this?

Answer 1

Suppose you have a hashMap val mp = HashMap<Int, String>() . Then you need to save the contents of this hashmap when your app closes, and restore data on app open. You can store the map inside sharedPreference or preferenceDataStore . I'll show you the sharedPreference way:

private val mp: HashMap<Int, String> = HashMap()
    private val preferenceName = "HASHMAP_PREFS"
    private val HASHMAP_KEY = "HASHMAP_KEY"
    private lateinit var sharedPreferences: SharedPreferences

    private fun initPref() {
        if(!this::sharedPreferences.isInitialized) {
            this.sharedPreferences = getSharedPreferences(preferenceName, Context.MODE_PRIVATE)
        }
    }

    private fun saveHashMap() {
        initPref()
        val editor = this.sharedPreferences.edit()
        val hashMapValue = Gson().toJson(mp)
        editor.putString(HASHMAP_KEY, hashMapValue)
        editor.apply()
    }

    private fun restoreHashMap() {
        initPref()
        val hashMapAsStringValue = this.sharedPreferences.getString(HASHMAP_KEY, null)
        if(hashMapAsStringValue!=null) {
            val hashMapType: Type = object : TypeToken<HashMap<Int, String>>() {}.type
            val tempHashMap: HashMap<Int, String> = Gson().fromJson(hashMapAsStringValue, hashMapType)
            this.mp.putAll(tempHashMap) // the hashMap is restored
        }
        Log.e("TAG", "hashMap = ${mp.toString()}")
    }

Here, the function names are self explainatory. Simply save data when app closes:

    override fun onDestroy() {
        saveHashMap()
        super.onDestroy()
    }

And restore data on app open:

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.main_activity)
     
        restoreHashMap()
        val key = System.currentTimeMillis().toInt() % 100
        val value = "${key*2}"
        mp.put(key, value)
        // todo: logic, other codes...
    }

Notice that in this example, I used HashMap<Int, String>. If you use map of other type, you should change the codes accordingly.

Edit

This is the java code can look like:

import android.content.Context;
import android.content.SharedPreferences;
import android.os.Bundle;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.util.HashMap;

public class DemoActivity extends AppCompatActivity {

    private HashMap<Integer, String> mp = new HashMap<>();
    private String preferenceName = "HASHMAP_PREFS";
    private String HASHMAP_KEY = "HASHMAP_KEY";

    private SharedPreferences sharedPreferences = null;

    private void initPreference() {
        if(sharedPreferences == null) {
            sharedPreferences = getSharedPreferences(preferenceName, Context.MODE_PRIVATE);
        }
    }

    private void saveHashMap() {
        initPreference();
        SharedPreferences.Editor editor = sharedPreferences.edit();
        Gson gson = new Gson();
        String hashMapValue = gson.toJson(mp);
        editor.putString(HASHMAP_KEY, hashMapValue);
        editor.apply();
    }

    private void restoreHashMap() {
        initPreference();

        String hashMapValue = sharedPreferences.getString(HASHMAP_KEY, null);
        if(hashMapValue!=null) {
            Type hashMapType = new TypeToken<HashMap<Integer, String>>(){}.getType();
            Gson gson = new Gson();
            HashMap<Integer, String> restoredHashMap = gson.fromJson(hashMapValue, hashMapType);
            mp.putAll(restoredHashMap);
        }
    }

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_activity);
        restoreHashMap();
    }

    @Override
    protected void onDestroy() {
        saveHashMap();
        super.onDestroy();
    }
}