Android listView doesn't get update when adding new items

Question

I'm trying to create a app where you can add strings to a list from another activity, but every time i add a new string to the list it replaces the old one.
How can I fix this?

Code where the list is.

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_show_list);


        Button button = (Button) findViewById(R.id.addBtn);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
               startActivity(new Intent(ShowList.this, AddToList.class));
            }
        });

        Intent i = getIntent();
        Bundle b = i.getExtras();
        if(b!=null) {
            String textToList = b.getString("listText");
            List<String> stuff = new ArrayList<String>();
            final ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, stuff);
            ListView list = (ListView) findViewById(R.id.listView);
            list.setAdapter(adapter);
            adapter.add(textToList);
            adapter.notifyDataSetChanged();
        }

    }

the xml files code:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".ShowList">

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Add"
    android:id="@+id/addBtn"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceMedium"
    android:text="List:"
    android:id="@+id/textView"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<ListView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/listView"
    android:layout_centerHorizontal="true"
    android:layout_above="@+id/addBtn"
    android:layout_below="@+id/textView" />

Code for the file where you input text:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_add_to_list);

    final Bundle b = new Bundle();
    final Intent i = new Intent(AddToList.this, ShowList.class);
    final EditText text = (EditText)this.findViewById(R.id.listText);
    Button btn = (Button) findViewById(R.id.btnAdd);
    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String s = text.getText().toString();
            b.putString("listText", s);
            i.putExtras(b);
            startActivity(i);
        }
    });
}

And finally the xml code for the previous peace of code:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="com.example.mathias.listapp.AddToList">

<EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/listText"
    android:layout_alignParentTop="true"
    android:layout_marginTop="100dp"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true"
    android:inputType="text"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="ADD"
    android:id="@+id/btnAdd"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true" />

Answer 1

This happens because you are creating a new List everytime you enter in your activity. So the field in which you save your list has to be a "permanent" one. I recommend you to declare a static list and check if it's not null:

Code where your list is:

public class Nameofmyclass{
    static List<String> stuff;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_show_list);

        if(stuff==null){
            stuff = new ArrayList<String>();
        }

        ListView list = (ListView) findViewById(R.id.listView);
        Button button = (Button) findViewById(R.id.addBtn);

        final ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, stuff);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
               startActivity(new Intent(ShowList.this, AddToList.class));
            }
        });

        Intent i = getIntent();
        Bundle b = i.getExtras();
        if(b!=null) {
            String textToList = b.getString("listText");
            stuff.add(textToList);              
        }

        list.setAdapter(adapter);

    }
}

By the way there are several "problems" in your code:

1. You added the item to the adapter, not to the list
2. You initialized your listview only if your bundle isn't null, why? Your listview will remain everytime, you don't need to initialize it with this check
3. You created a final intent in your other class outside the click. This isn't programmatically wrong, but there isn't any motivation in this case for which you need to do it. It's better do it locally inside the click as you did in the first class

Answer 2

try this ,

String textToList = b.getString("listText");
List<String> stuff = new ArrayList<String>();
stuff.add(textToList)

final ArrayAdapter<String> adapter = new ArrayAdapter<>(this,    android.R.layout.simple_list_item_1, stuff);
ListView list = (ListView) findViewById(R.id.listView);
list.setAdapter(adapter);
adapter.notifyDataSetChanged();