example_widget.xmlExampleAppWidgetProvider.javaexample_appwidget_info.xmlAndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/example_widget_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="Press me" />

</RelativeLayout>

package com.codinginflow.widgetexample;

import android.app.PendingIntent;
import android.appwidget.AppWidgetManager;
import android.appwidget.AppWidgetProvider;
import android.content.Context;
import android.content.Intent;
import android.widget.RemoteViews;


public class ExampleAppWidgetProvider extends AppWidgetProvider {

    @Override
    public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds) {
        for (int appWidgetId : appWidgetIds) {
            Intent intent = new Intent(context, MainActivity.class);
            PendingIntent pendingIntent = PendingIntent.getActivity(context, 0, intent, 0);

            RemoteViews views = new RemoteViews(context.getPackageName(), R.layout.example_widget);
            views.setOnClickPendingIntent(R.id.example_widget_button, pendingIntent);

            appWidgetManager.updateAppWidget(appWidgetId, views);
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
    android:initialLayout="@layout/example_widget"
    android:minHeight="40dp"
    android:minResizeWidth="40dp"
    android:minWidth="110dp"
    android:previewImage="@drawable/widget_preview"
    android:resizeMode="vertical|horizontal"
    android:updatePeriodMillis="3600000"
    android:widgetCategory="home_screen">

</appwidget-provider>

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.codinginflow.widgetexample">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".MainActivity"
            android:launchMode="singleTop">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <receiver android:name=".ExampleAppWidgetProvider">
            <intent-filter>
                <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
            </intent-filter>
            <meta-data
                android:name="android.appwidget.provider"
                android:resource="@xml/example_appwidget_info" />
        </receiver>
    </application>

</manifest>