Author: Nusrat Faria

  <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="16dp"
        android:src="@android:drawable/ic_dialog_email"/>

    FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // We are showing only toast message. However, you can do anything you need.
                Toast.makeText(getApplicationContext(), "You clicked Floating Action Button", Toast.LENGTH_SHORT).show();
            }
        });

/ Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
            classpath 'com.android.tools.build:gradle:3.2.1'
          classpath 'com.google.gms:google-services:4.2.0'


        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        jcenter()
        maven {
            url "https://maven.google.com" // Google's Maven repository
        }
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

    implementation 'com.google.firebase:firebase-core:16.0.9'
    implementation 'com.google.firebase:firebase-firestore:19.0.0'
    implementation 'com.google.firebase:firebase-database:17.0.0'
    implementation  'com.google.firebase:firebase-auth:17.0.0'

apply plugin: 'com.google.gms.google-services'

<?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">
    <TabHost android:id="@+id/tabhost"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >
        <LinearLayout
            android:orientation="vertical"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent">
            <TabWidget android:id="@android:id/tabs"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content" />
            <FrameLayout android:id="@android:id/tabcontent"
                android:layout_width="fill_parent"
                android:layout_height="fill_parent">
                <LinearLayout
                    android:id="@+id/tab1"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent">
                    <Button
                        android:id="@+id/button"
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:text="tab1" />
                </LinearLayout>
                <LinearLayout
                    android:id="@+id/tab2"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent">
                    <Button
                        android:id="@+id/button2"
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:text="tab2" />
                </LinearLayout>
            </FrameLayout>
        </LinearLayout>
    </TabHost>
</android.support.constraint.ConstraintLayout>

package com.example.ppc.tabhost;

import android.app.TabActivity;
import android.support.v4.app.Fragment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TabHost;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        TabHost tabs = (TabHost) findViewById(R.id.tabhost);
        tabs.setup();
        TabHost.TabSpec spec = tabs.newTabSpec("tag1");
        spec.setContent(R.id.tab1);
        spec.setIndicator("First");
        tabs.addTab(spec);
        spec = tabs.newTabSpec("tag2");
        spec.setContent(R.id.tab2);
        spec.setIndicator("second");
        tabs.addTab(spec);

        tabs.setOnTabChangedListener(new TabHost.OnTabChangeListener() {
            @Override
            public void onTabChanged(String tabId) {
                if (tabId.equals("tag1")){
                   Toast.makeText(getApplicationContext(),"This IS Tab One",Toast.LENGTH_LONG).show();
                }

                else if (tabId.equals("tag2")){
                    Toast.makeText(getApplicationContext(),"This IS Tab Two",Toast.LENGTH_LONG).show();
                }
            }
        });

    }
}

Solution : go To Project level Garadle.build And Change Your Gradle Version To If Say Any Update Dont update.
classpath 'com.android.tools.build:gradle:3.3.2'

 Solution : Add Dependency In App level gradle.build And Enable Multidex True in Default Config In App.build.gradle file .
 defaultConfig { 
  //...
  multiDexEnabled true 
 }
compile 'com.android.support:multidex:1.0.1' 

Solution: Added two lines in gradle.properties file:
 These two lines automatically resolved my dependency conflicts between google's files and third party dependencies. Here's the link to follow: https://developer.android.com/topic/libraries/support-library/androidx-overview#new-project

android.useAndroidX=true
android.enableJetifier=true

android.useAndroidX=true
android.enableJetifier=true