ListView add item android studio : XML

    <?xml version="1.0"?>
    <FrameLayout xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">
        
    <android.support.constraint.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorBackground">
        
    <android.support.design.widget.FloatingActionButton
    android:id="@+id/fab"
    android:layout_width="51dp"
    android:layout_height="51dp"
    android:layout_gravity="bottom"
    android:layout_margin="16dp"
    android:layout_marginBottom="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginRight="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    android:clickable="true"
    android:onClick="add_item"
    android:src="@mipmap/plus_two"
    app:backgroundTint="@color/colorPrimary"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintHorizontal_bias="0.984"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintVertical_bias="0.982" />
    </android.support.constraint.ConstraintLayout>
        
    <ListView
    android:id="@+id/lv"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:drawSelectorOnTop="false"
    tools:layout_editor_absoluteX="16dp"
    tools:layout_editor_absoluteY="16dp" />
        
    </FrameLayout>

http://prntscr.com/fuk5x8

I would like to add an item when I click the FAB, And even when you left the app, it kept your data saved. I've been trying all afternoon and I was not successful ;-;


thanks in advance


