public class MainActivity extends AppCompatActivity {

    private RichTextEditor editor;

    private GroupedCommandsEditorToolbar bottomGroupedCommandsToolbar;

    private IPermissionsService permissionsService = new PermissionsService(this);


    @Override
    public void onCreate(@Nullable Bundle savedInstanceState, @Nullable PersistableBundle persistentState) {
        super.onCreate(savedInstanceState, persistentState);
        setContentView(R.layout.activity_main);

        editor = (RichTextEditor) findViewById(R.id.editor);

        // this is needed if you like to insert images so that the user gets asked for permission to access external storage if needed
        // see also onRequestPermissionsResult() below
        editor.setPermissionsService(permissionsService);

        bottomGroupedCommandsToolbar = (GroupedCommandsEditorToolbar) findViewById(R.id.bottomGroupedCommandsToolbar);
        bottomGroupedCommandsToolbar.setEditor(editor);

        // you can adjust predefined toolbars by removing single commands
//        bottomGroupedCommandsToolbar.removeCommandFromGroupedCommandsView(CommandName.TOGGLE_GROUPED_TEXT_STYLES_COMMANDS_VIEW, CommandName.BOLD);
//        bottomGroupedCommandsToolbar.removeSearchView();


        editor.setEditorFontSize(20);
        editor.setPadding((4 * (int) getResources().getDisplayMetrics().density));

        // some properties you also can set on editor
//        editor.setEditorBackgroundColor(Color.YELLOW)
//        editor.setEditorFontColor(Color.MAGENTA)
//        editor.setEditorFontFamily("cursive")

        // show keyboard right at start up
//        editor.focusEditorAndShowKeyboardDelayed()

        // only needed if you allow to automatically download remote images
        editor.setDownloadImageConfig(new DownloadImageConfig(DownloadImageUiSetting.AllowSelectDownloadFolderInCode,
            new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES), "downloaded_images")));
        
        
        /*      Set listeners on RichTextEditor         */
        
        // get informed when edited HTML changed
        editor.addDidHtmlChangeListener(new DidHtmlChangeListener() {
            @Override
            public void didHtmlChange(boolean didHtmlChange) {
                // e.g. set save button to enabled / disabled
                // btnSave.setEnabled(didHtmlChange);
            }
        });
        
        // use this listener with care, it may decreases performance tremendously
        editor.addHtmlChangedListener(new HtmlChangedListener() {
            @Override
            public void htmlChangedAsync(@NotNull String html) {
                // htmlChangedAsync() gets called on a background thread, so if you want to use it on UI thread you have to call runOnUiThread()
            }
        });
    }


    // Important: Overwrite onBackPressed and pass it to toolbar.There's no other way that it can get informed of back button presses.
    @Override
    public void onBackPressed() {
        if(bottomGroupedCommandsToolbar.handlesBackButtonPress() == false) {
            super.onBackPressed();
        }
    }

    // only needed if you like to insert images from local device so the user gets asked for permission to access external storage if needed

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        permissionsService.onRequestPermissionsResult(requestCode, permissions, grantResults);

        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    }


    // then when you want to do something with edited html
    private void save() {
        editor.getCurrentHtmlAsync(new GetCurrentHtmlCallback() {

            @Override
            public void htmlRetrieved(@NotNull String html) {
                saveHtml(html);
            }
        });
    }

    private void saveHtml(String html) {
        // ...
    }

}

import android.content.Context;
import android.util.AttributeSet;

import net.dankito.richtexteditor.android.command.ToggleGroupedTextStylesCommand;
import net.dankito.richtexteditor.android.command.UndoCommand;

public class ExampleToolbar extends EditorToolbar {

    public ExampleToolbar(Context context) {
        super(context);

        initToolbar();
    }

    public ExampleToolbar(Context context, AttributeSet attrs) {
        super(context, attrs);

        initToolbar();
    }

    public ExampleToolbar(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);

        initToolbar();
    }

    public ExampleToolbar(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);

        initToolbar();
    }


    private void initToolbar() {
        addCommand(new ToggleGroupedTextStylesCommand());

        addCommand(new UndoCommand());

        addSearchView();
    }

}

RichTextEditor editor = ...;
editor.setTheme("green")