Links & DependenciesNote.javaactivity_main.xmlMainActivity.java

Firebase setup guide with dependencies and console link:

firebase.google.com/docs/android/setup

package com.codinginflow.firestoreexampleproject;

public class Note {
    private String title;
    private String description;

    public Note() {
        //public no-arg constructor needed
    }

    public Note(String title, String description) {
        this.title = title;
        this.description = description;
    }

    public String getTitle() {
        return title;
    }

    public String getDescription() {
        return description;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:padding="16dp"
    tools:context="com.codinginflow.firestoreexampleproject.MainActivity">

    <EditText
        android:id="@+id/edit_text_title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Title"
        android:inputType="text" />

    <EditText
        android:id="@+id/edit_text_description"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Description"
        android:inputType="text" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="saveNote"
        android:text="Save" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="loadNote"
        android:text="Load" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="updateDescription"
        android:text="Update description" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="deleteDescription"
        android:text="Delete description" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="deleteNote"
        android:text="Delete note" />

    <TextView
        android:id="@+id/text_view_data"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="20sp" />

</LinearLayout>

package com.codinginflow.firestoreexampleproject;

import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.firebase.firestore.DocumentReference;
import com.google.firebase.firestore.DocumentSnapshot;
import com.google.firebase.firestore.EventListener;
import com.google.firebase.firestore.FieldValue;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.FirebaseFirestoreException;
import com.google.firebase.firestore.SetOptions;

import java.util.HashMap;
import java.util.Map;

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";

    private static final String KEY_TITLE = "title";
    private static final String KEY_DESCRIPTION = "description";

    private EditText editTextTitle;
    private EditText editTextDescription;
    private TextView textViewData;

    private FirebaseFirestore db = FirebaseFirestore.getInstance();
    private DocumentReference noteRef = db.document("Notebook/My First Note");

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

        editTextTitle = findViewById(R.id.edit_text_title);
        editTextDescription = findViewById(R.id.edit_text_description);
        textViewData = findViewById(R.id.text_view_data);
    }

    @Override
    protected void onStart() {
        super.onStart();
        noteRef.addSnapshotListener(this, new EventListener<DocumentSnapshot>() {
            @Override
            public void onEvent(DocumentSnapshot documentSnapshot, FirebaseFirestoreException e) {
                if (e != null) {
                    Toast.makeText(MainActivity.this, "Error while loading!", Toast.LENGTH_SHORT).show();
                    Log.d(TAG, e.toString());
                    return;
                }

                if (documentSnapshot.exists()) {
                    Note note = documentSnapshot.toObject(Note.class);

                    String title = note.getTitle();
                    String description = note.getDescription();

                    textViewData.setText("Title: " + title + "\n" + "Description: " + description);
                } else {
                    textViewData.setText("");
                }
            }
        });
    }

    public void saveNote(View v) {
        String title = editTextTitle.getText().toString();
        String description = editTextDescription.getText().toString();

        Note note = new Note(title, description);

        noteRef.set(note)
                .addOnSuccessListener(new OnSuccessListener<Void>() {
                    @Override
                    public void onSuccess(Void aVoid) {
                        Toast.makeText(MainActivity.this, "Note saved", Toast.LENGTH_SHORT).show();
                    }
                })
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        Toast.makeText(MainActivity.this, "Error!", Toast.LENGTH_SHORT).show();
                        Log.d(TAG, e.toString());
                    }
                });
    }

    public void updateDescription(View v) {
        String description = editTextDescription.getText().toString();

        //Map<String, Object> note = new HashMap<>();
        //note.put(KEY_DESCRIPTION, description);

        //noteRef.set(note, SetOptions.merge());
        noteRef.update(KEY_DESCRIPTION, description);
    }

    public void deleteDescription(View v) {
        //Map<String, Object> note = new HashMap<>();
        //note.put(KEY_DESCRIPTION, FieldValue.delete());

        //noteRef.update(note);
        noteRef.update(KEY_DESCRIPTION, FieldValue.delete());
    }

    public void deleteNote(View v) {
        noteRef.delete();
    }

    public void loadNote(View v) {
        noteRef.get()
                .addOnSuccessListener(new OnSuccessListener<DocumentSnapshot>() {
                    @Override
                    public void onSuccess(DocumentSnapshot documentSnapshot) {
                        if (documentSnapshot.exists()) {
                            Note note = documentSnapshot.toObject(Note.class);

                            String title = note.getTitle();
                            String description = note.getDescription();

                            textViewData.setText("Title: " + title + "\n" + "Description: " + description);
                        } else {
                            Toast.makeText(MainActivity.this, "Document does not exist", Toast.LENGTH_SHORT).show();
                        }
                    }
                })
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        Toast.makeText(MainActivity.this, "Error!", Toast.LENGTH_SHORT).show();
                        Log.d(TAG, e.toString());
                    }
                });
    }
}