<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Open Checklist"
        tools:layout_editor_absoluteX="136dp"
        tools:layout_editor_absoluteY="407dp" />

</android.support.constraint.ConstraintLayout>


================================================================
package com.example.kandydatpl;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {


    private Button btn;

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

        btn = findViewById(R.id.button);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                openChecklist(v);
            }
        });
    }

    public void openChecklist(View view) {
        Intent intent = new Intent(this, TaskListActivity.class);
        startActivity(intent);
    }

}


====================================================================================

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".TaskListActivity">

    <LinearLayout
        android:id="@+id/linearLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <EditText
            android:id="@+id/item_edit_text"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="4"
            android:hint="Enter Item" />

        <Button
            android:id="@+id/add_btn"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Add" />

    </LinearLayout>

    <ListView
        android:id="@+id/items_list"
        android:layout_width="395dp"
        android:layout_height="667dp"
        android:layout_marginTop="8dp"
        android:layout_marginBottom="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/linearLayout"
        tools:layout_editor_absoluteX="8dp" />

</android.support.constraint.ConstraintLayout>

==============================================================================

package com.example.kandydatpl;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;

import java.io.File;
import java.util.ArrayList;

public class TaskListActivity extends AppCompatActivity implements View.OnClickListener, AdapterView.OnItemClickListener {

    private EditText itemET;
    private Button btn;
    private ListView itemsList;
    private ArrayList<String> items;
    private ArrayAdapter<String> adapter;

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

        itemET = findViewById(R.id.item_edit_text);

        btn = findViewById(R.id.add_btn);
        btn.setOnClickListener(this);

        itemsList = findViewById(R.id.items_list);

        items = FileHelper.readData(this);

        adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, items);
        itemsList.setAdapter(adapter);

        itemsList.setOnItemClickListener(this);
    }


    public void onClickAdd(View view)
    {
        switch (view.getId())
        {
            case R.id.add_btn:
                String itemEntered = itemET.getText().toString();
                adapter.add(itemEntered);
                itemET.setText("");
                FileHelper.writeData(items, this);
                Toast.makeText(this, "Item Added!", Toast.LENGTH_SHORT).show();
                break;
        }
    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        items.remove(position);
        adapter.notifyDataSetChanged();
        FileHelper.writeData(items, this);
        Toast.makeText(this, "Delete", Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onClick(View v) {
        onClickAdd(v);
    }
}