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How to save checkbox states using simple_list_item_multiple_choice?

I'm doing a grocery list app and i want all the list to be sort by department (Meat, fruit, bakery etc.). I try to do the first list which is the fruit list everything is working i can add item the checkbox is there i can use it no problem. the problem is that when i check items and i press the previous button on the phone and then i click back on the button to access the fruit list the items are not checked anymore.
I use the android.R.layout.simple_list_item_multiple_choice for the checkbox.
I try to use onSaveinstance but i cant get it to work ...i would like to save it without using SQl now my array are save in a file for my list.
I eard about shared preference but i'm not sure its what i'm looking for and i would need some help to understand how to use it.
This is the class that call the activity where the fruit list is displayed with the checkbox:
public class SelectDoActivity extends AppCompatActivity {
/*Set the instance variable*/
private ImageButton btn_FruitList;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate (savedInstanceState);
setContentView (R.layout.activity_select_do);
btn_FruitList = (ImageButton) findViewById (R.id.btn_FruitList);
/*Create the method that call the activity*/
btn_FruitList.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
openFruitList_Activity();
}
});
}
public void openFruitList_Activity() {
Intent intent = new Intent (this, FruitList_Activity.class);
startActivity(intent);
}
}
This is the class that display the fruit list with the check mark.
public class FruitList_Activity extends AppCompatActivity {
private ListView fruitsList;
private ArrayAdapter<String> adapterFruit;
private Button btn_Delete;
private Button btn_SelectAll;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate (savedInstanceState);
setContentView (R.layout.activity_fruit_list_);
fruitsList = findViewById(R.id.list_Fruits);
fruitsList.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
btn_Delete = findViewById (R.id.btn_delete);
CreateActivity.itemsFruit = FileHelper.readData(this);
adapterFruit = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_multiple_choice, CreateActivity.itemsFruit);
fruitsList.setAdapter(adapterFruit);
/*button to Remove items*/
btn_Delete.setOnClickListener(new View.OnClickListener()
{
public void onClick(View v)
{
SparseBooleanArray checkedItemPositions = fruitsList.getCheckedItemPositions();
int itemCount = fruitsList.getCount();
for(int i=itemCount-1; i >= 0; i--){
if(checkedItemPositions.get(i)){
fruitsList.setItemChecked(i,true);
adapterFruit.remove(CreateActivity.itemsFruit.get(i));
FileHelper.writeData(CreateActivity.itemsFruit, FruitList_Activity.this );
}
}
adapterFruit.notifyDataSetChanged();
}
});
}
}
I've been stuck with this since 2 weeks i would we really like some precious help i'm ne to java and android so thanks for taking the time to explain how to fix this.

ListView will refresh already existing item and won't add a new one

I am trying to make a To Do List app and I have an EditText field where I enter the task. When I press the "add" floating action button the first task goes in the listview perfectly but when I try to add a second task, it will change the first task to the second and so on. So the thing is that I can only have on task at a time but what I want to have is a list of tasks.
This is my onCreate method, where I declare the floating action button and set an OnClickListener for it:
DBHelper dbHelper;
ArrayAdapter<String> mAdapter;
ListView listView;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main_menu);
// Finds you button from the xml layout file
FloatingActionButton faButton = (FloatingActionButton) findViewById(R.id.fab);
// Waits for you to click the button
faButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
// Starts the function below
addNotification();
listView.setAdapter(mAdapter);
addToDoElement(null);
}
});
listView = (ListView)findViewById(R.id.lstTask);
}
And this is the method that adds the task from the EditText to the ListView
ArrayList<String> taskList;
public void addToDoElement(View view) {
listView = (ListView) findViewById(R.id.lstTask);
taskList = new ArrayList<>();
mAdapter = new ArrayAdapter<String>(this, R.layout.row, R.id.task_title, taskList);
listView.setAdapter(mAdapter);
//Gets TASK text from EditText
EditText mEdit = (EditText)findViewById(R.id.titlu);
Log.v("EditText", mEdit.getText().toString());
taskList.add(mEdit.getText().toString());
mEdit.setText("");
mAdapter.notifyDataSetChanged();
}

The part where you initialize your taskList and mAdapter needs to be done only once, in your onCreate method. So move this block
listView = (ListView) findViewById(R.id.lstTask);
taskList = new ArrayList<>();
mAdapter = new ArrayAdapter<String>(this, R.layout.row, R.id.task_title, taskList);
listView.setAdapter(mAdapter);
in your onCreate method. Your current code is resetting the adapter and task list on each click event.

App crashes if I try going to a specific activity

So I am making a simple app. It's just basically making a list of win with a custom list view at the end.
It starts off on the main screen where there are two buttons, one is an "Add" button which takes you to the Add activity. If you push this it'll take you to a page where you type in the name,price, description of the wine and then hit a submit button to the list. The other button on the main screen is a "Go to List" button which just takes you directly to the list activity.
If I go through the Add button, add a wine, and then go to the list, it works fine. I can see the list. It even works if I don't add anything to the list. I can see the empty list activity.
When I push the "Go to List" button on the main screen though, it crashes and says "The application has stopped".
I don't get why I can go through the Add button to get to the list fine, but this button doesn't work at all.
Could I get some help?
Here are the three relevant activities, the AddActivity, the ListActivity, and the MainActivity.
AddActivity:
public class AddActivity extends AppCompatActivity {
EditText editWineName;
EditText editWinePrice;
EditText editWineDescription;
Button btnSubmit;
Button btnGoToList;
String stringWineName;
String stringWinePrice;
String stringWineDescription;
ArrayList<String> listWineName = new ArrayList<>();
ArrayList<String> listPrice = new ArrayList<>();
ArrayList<String> listWineDescription = new ArrayList<>();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_add);
setVariables();
btnSubmit.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
setVariables();
listWineName.add(stringWineName);
listPrice.add(stringWinePrice);
listWineDescription.add(stringWineDescription);
Toast.makeText(AddActivity.this, stringWineName + " was added to the list.", Toast.LENGTH_SHORT).show();
clearEditText();
}
});
btnGoToList.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
Intent intentGoToList = new Intent(AddActivity.this,ListActivity.class);
intentGoToList.putStringArrayListExtra("WINENAME", listWineName);
intentGoToList.putStringArrayListExtra("WINEPRICE", listPrice);
intentGoToList.putStringArrayListExtra("WINEDESCRIPTION", listWineDescription);
startActivity(intentGoToList);
}
});
}
private void setVariables(){
editWineName = (EditText) findViewById(R.id.editName);
editWinePrice = (EditText) findViewById(R.id.editPrice);
editWineDescription = (EditText) findViewById(R.id.editDescription);
btnSubmit = (Button) findViewById(R.id.btnSubmit);
btnGoToList = (Button) findViewById(R.id.btnGoToList);
stringWineName = editWineName.getText().toString();
stringWinePrice = "$" + editWinePrice.getText().toString();
stringWineDescription = editWineDescription.getText().toString();
}
private void clearEditText() {
editWineName.getText().clear();
editWinePrice.getText().clear();
editWineDescription.getText().clear();
}
}
ListActivity:
public class ListActivity extends AppCompatActivity {
ListView wineList;
ArrayAdapter adapter;
Button btnBacktoMain;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_list);
setVariables();
ArrayList<String> listWineName = getIntent().getStringArrayListExtra("WINENAME");
ArrayList<String > listWinePrice = getIntent().getStringArrayListExtra("WINEPRICE");
ArrayList<String> listWineDescription = getIntent().getStringArrayListExtra("WINEDESCRIPTION");
adapter = new CustomAdapter(this, listWineName, listWinePrice, listWineDescription);
wineList.setAdapter(adapter);
btnBacktoMain.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
Intent intentBackToMain = new Intent(ListActivity.this,MainActivity.class);
startActivity(intentBackToMain);
}
});
}
private void setVariables (){
btnBacktoMain = (Button) findViewById(R.id.btnBackToMain);
wineList = (ListView) findViewById(R.id.listWine);
}
}
MainActivity:
public class MainActivity extends AppCompatActivity {
Button btnAdd;
Button btnList;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
setVariables();
btnAdd.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) { //Goes to the add activity
Intent intentAdd = new Intent(MainActivity.this, AddActivity.class);
startActivity(intentAdd);
}
});
btnList.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) { //Goes to the list activity
Intent intentList = new Intent(MainActivity.this, ListActivity.class);
startActivity(intentList);
}
});
}
private void setVariables(){
btnAdd = (Button) findViewById(R.id.btnAddWine);
btnList = (Button) findViewById(R.id.btnViewList);
}
}

Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:344)
at android.widget.ListView.setAdapter(ListView.java:493)
at com.example.jeremy.mywine.ListActivity.onCreate(ListActivity.java:33)
Your crash indicates that the data in the adapter given to the ListView in ListActivity is null. Make it not null. Start at ListActivity.java at line 33 and go backwards to find where you are (or this case are not) initializing the data in the list adapter.
In you case, you are expecting data in your intent. OK, where is your intent set up? In your MainActivity click. Well, there you just launch the activity without passing any data in the intent extras, hence there is nothing to pull out from the intent in ListActivity, hence your crash.
So you need to initialize the data in MainActivity and set it as extras in the Intent you use to launch ListActivity.
Since the ListActivity is expecting this:
ArrayList<String> listWineName = getIntent().getStringArrayListExtra("WINENAME");
ArrayList<String > listWinePrice = getIntent().getStringArrayListExtra("WINEPRICE");
ArrayList<String> listWineDescription = getIntent().getStringArrayListExtra("WINEDESCRIPTION");
You would update your MainActivity to do something like this (where getDescriptions() is a fictitious method you would create to return a list of strings)
btnList.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
//Goes to the list activity
Intent intentList = new Intent(MainActivity.this, ListActivity.class);
intentList.putExtra("WINENAME", new ArrayList<String>()); // Empty list
intentList.putExtra("WINEPRICE", Arrays.asList("Foo", "Bar")); // Explicit list named items
intentList.putExtra("WINEDESCRIPTION", getDescriptions()); // Get list from private method
startActivity(intentList);
}
});
Also check this post my be useful for learning how to read and understand a crash log.
And check the documentation on how to start activities.
Hope that helps!

Trying to get string variables from a seperate class causing app to crash

I have simple page to start up the app that has a spinner,editText, and a button. When the user clicks the button I want the app take the selection of the spinner and text and set them to text views in the new class. I tried using getters but when I try this the app crashes.
First Class:
public class MainActivity extends AppCompatActivity {
String pullerName;
String storeName;
Spinner spinner;
EditText etPuller;
public String getStoreName(){
return storeName;
}
public String getPullerName(){
return pullerName;
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
spinner = (Spinner) findViewById(R.id.spinner);
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this,
R.array.store_list, android.R.layout.simple_spinner_item);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(adapter);
Button create = (Button) findViewById(R.id.btnCreate);
etPuller = (EditText) findViewById(R.id.editText);
create.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
pullerName = etPuller.getText().toString();
storeName = spinner.getSelectedItem().toString();
Intent i = new Intent(MainActivity.this,MainActivity2.class);
startActivity(i);
}
});
}
}
Second Class:
public class MainActivity2 extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.dialog_main);
MainActivity main = new MainActivity();
TextView user = (TextView) findViewById(R.id.tvUser);
TextView store = (TextView) findViewById(R.id.tvStore);
user.setText(main.getPullerName());
store.setText(main.getStoreName());
When this code runs it crashes as soon as i click the button to move to the next activity.

To pass data to a new Activity you need to use Intent.
This is what you need to do:
MainActivity.class:
create.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
pullerName = etPuller.getText().toString();
storeName = spinner.getSelectedItem().toString();
Intent i = new Intent(MainActivity.this,MainActivity2.class);
i.putExtra("puller-name", pullerName);
i.putExtra("store-name", storeName);
startActivity(i);
}
});
In the MainActivity2.class:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.dialog_main);
TextView user = (TextView) findViewById(R.id.tvUser);
TextView store = (TextView) findViewById(R.id.tvStore);
Intent intent = getIntent();
if(intent != null){ //I always check this to avoid Exceptions
String pullerName = intent.getStringExtra("puller-name");
String storeName = intent.getStringExtra("store-name");
if(pullerName != null) user.setText(pullerName);
if(storeName != null) store.setText(storeName);
}
}
You was creating a new instance of MainActivity in MainActivity2. So, when you called the getters of this mainActivity will return null, because these values were never setters. If you want to use your method, you need to pass the same instance of the activity to MainActivity2, but this is not recommended. The official method to pass data to another activity is via intent. To learn more about Intent learn the official Android documentation: https://developer.android.com/training/basics/firstapp/starting-activity.html

You have used wrong way to get values from MainActivity to MainActivity2. Simply you can pass values using the technique below.
Intent intent = new Intent(MainActivity.this,MainActivity2.class);
intent.putExtra("pullerName", pullerName);
To use this value pullerName in MainActivity2 class, just use the technique below.
String value = getIntent().getExtras().getString("pullerName");`

ListView doesn't show items when updated from a fucntion

I am using Android Studio, I have an activity with a list, when I add items to the list after filling a textbox and clicking a button the items show, but when I try to add items to the list from an SQL query the list does not show the items.
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Intent d = getIntent();
setContentView(R.layout.activity_add_group);
String choosengroup = d.getStringExtra("chosen");
groupn = (EditText) findViewById(R.id.gname);
soldiern = (EditText) findViewById(R.id.soldiername);
adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, list);
final Button savebutton = (Button) findViewById(R.id.savegroup);
final Button addbutton = (Button) findViewById(R.id.addsoldier);
final DatabaseHelper db;
db = new DatabaseHelper(getApplicationContext());
setListAdapter(adapter);
if(!choosengroup.equals("")){
groupn.setText(choosengroup);
list=getAllsoldiers(choosengroup,db);
// Log.d("list",list.toString());
onContentChanged ();
}
savebutton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
db.createGroup(new Groups(groupn.getText().toString())); //Creates a group in Groups table
// Creates a new table for the specific group
db.getWritableDatabase().execSQL(" CREATE TABLE `Group_"+groupn.getText().toString()+"` (`id` INTEGER PRIMARY KEY AUTOINCREMENT ,`name` TEXT,`ghours` INTEGER DEFAULT "+'"'+"0"+'"'+",`ahours`INTEGER DEFAULT "+'"'+"0"+'"'+", `khours` INTEGER DEFAULT "+'"'+"0"+'"'+");");
for (String name: list) {
// Log.d("names:",name.toString());
db.getWritableDatabase().execSQL("INSERT INTO `Group_"+groupn.getText().toString()+"` (name) VALUES('"+name+"');");
}
Intent act2 = new Intent(view.getContext(),ChooseGroup.class);
startActivity(act2);
}
});
addbutton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
// soldier.add(new Soldiers(soldiern.getText().toString()));
list.add(soldiern.getText().toString());
soldiern.setText("");
//setListAdapter(adapter);
Log.d("list",list.toString());
onContentChanged ();
}
});
The addbutton.setOnClickListener add items and they are shown on the list.
But this :
if(!choosengroup.equals("")){
groupn.setText(choosengroup);
list=getAllsoldiers(choosengroup,db);
// Log.d("list",list.toString());
onContentChanged ();
}
Does not show the items on the listview.
Any idea why?
Thanks in advance.

Try
list.addAll(getAllsoldiers(choosengroup,db));
instead of
list=getAllsoldiers(choosengroup,db);
My thinking is that the assignment:
list=getAllsoldiers(choosengroup,db);
changes the value of the reference held in list from the original list to a new reference, and the adapter is still using the reference to the original list.
ie when you created the adapter, you passed in the value of the reference to the original list
when you did the assignment, you just changed the value of reference stored in list, but the adapter is still holding the pointer to the original list
using addAll() will add the items to the original list which is still being referenced by your adapter.