Resolved merge conflict by incorporating both suggestions.

app/build.gradle

@@ -26,6 +26,7 @@ dependencies {
     implementation fileTree(dir: "libs", include: ["*.jar"])
     implementation 'androidx.appcompat:appcompat:1.1.0'
     implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
+    implementation 'androidx.legacy:legacy-support-v4:1.0.0'
     testImplementation 'junit:junit:4.13'
     androidTestImplementation 'androidx.test.ext:junit:1.1.1'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

app/src/main/java/com/example/instagramclone_android/HomeFragment.java

@@ -0,0 +1,54 @@
+package com.example.instagramclone_android;
+
+import android.os.Bundle;
+
+import androidx.fragment.app.Fragment;
+import androidx.recyclerview.widget.LinearLayoutManager;
+import androidx.recyclerview.widget.RecyclerView;
+
+import android.view.LayoutInflater;
+import android.view.View;
+import android.view.ViewGroup;
+
+
+/**
+ * A simple {@link Fragment} subclass.
+ * Use the {@link HomeFragment#newInstance} factory method to
+ * create an instance of this fragment.
+ */
+public class HomeFragment extends Fragment {
+    private RecyclerView recycler_view_story_section, recycler_view_post_section;
+
+
+    public HomeFragment() {
+    }
+
+    public static HomeFragment newInstance() {
+        HomeFragment fragment = new HomeFragment();
+        Bundle args = new Bundle();
+        fragment.setArguments(args);
+        return fragment;
+    }
+
+    @Override
+    public void onCreate(Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+    }
+
+    @Override
+    public View onCreateView(LayoutInflater inflater, ViewGroup container,
+                             Bundle savedInstanceState) {
+        View view = inflater.inflate(R.layout.fragment_home, container, false);
+        recycler_view_story_section = view.findViewById(R.id.recycler_view_story_section);
+        recycler_view_story_section.setHasFixedSize(true);
+        recycler_view_story_section.setLayoutManager(new LinearLayoutManager(view.getContext(), LinearLayoutManager.HORIZONTAL, false));
+        String[] usernames = {"gaurav","nihar","kunal","ajinkya","sakshi","nikita","aaditya","sandeep","mihir"};
+        recycler_view_story_section.setAdapter(new StoryAdapter(usernames));
+
+        recycler_view_post_section = view.findViewById(R.id.recycler_view_post_section);
+        recycler_view_post_section.setHasFixedSize(true);
+        recycler_view_post_section.setLayoutManager(new LinearLayoutManager(view.getContext(), LinearLayoutManager.VERTICAL, false));
+        recycler_view_post_section.setAdapter(new PostAdapter(usernames));
+        return view;
+    }
+}

app/src/main/java/com/example/instagramclone_android/LoginScreen.java

@@ -14,11 +14,8 @@ public class LoginScreen extends AppCompatActivity {
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
-        if(getSupportActionBar()!=null){
-            getSupportActionBar().hide();
-        }
         setContentView(R.layout.activity_login_screen);
-        Button button=(Button)findViewById(R.id.button);
+        Button button=(Button)findViewById(R.id.login_button);
         button.setOnClickListener(new View.OnClickListener()
         {
             @Override

app/src/main/java/com/example/instagramclone_android/MainScreen.java

@@ -1,20 +1,36 @@
 package com.example.instagramclone_android;
 
+import androidx.annotation.Nullable;
 import androidx.appcompat.app.AppCompatActivity;
+import androidx.appcompat.app.AppCompatDelegate;
+import androidx.fragment.app.Fragment;
+import androidx.fragment.app.FragmentManager;
+import androidx.fragment.app.FragmentTransaction;
 import androidx.recyclerview.widget.LinearLayoutManager;
 import androidx.recyclerview.widget.RecyclerView;
 
+<<<<<<< HEAD
 import android.content.DialogInterface;
 import android.content.Intent;
 import android.os.Bundle;
 import android.view.MotionEvent;
 import android.view.View;
 import android.widget.Button;
 import android.widget.ImageButton;
+=======
+import android.content.ContentResolver;
+import android.content.Intent;
+import android.content.pm.PackageManager;
+import android.os.Bundle;
+import android.os.PersistableBundle;
+import android.view.View;
+>>>>>>> 6d79e5d8589baed2e7762b3c0bd203346847ef50
 import android.widget.ImageView;
 import android.widget.LinearLayout;
 
+
 public class MainScreen extends AppCompatActivity {
+<<<<<<< HEAD
     LinearLayout main_screen_layout;
     ImageView ib1;
     @Override
@@ -28,11 +44,171 @@ protected void onCreate(Bundle savedInstanceState) {
         recycler_view_story_section.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false));
         String[] usernames = {"gaurav", "nihar", "kunal", "ajinkya", "sakshi", "nikita", "aaditya", "sandeep", "mihir"};
         recycler_view_story_section.setAdapter(new StoryAdapter(usernames));
+=======
+    public static final int DEFAULT_TAB_ID = -1;
+    public static final int HOME_TAB_ID = 0;
+    public static final int ADD_IMAGE_TAB_ID = 1;
+    public static final int PROFILE_TAB_ID = 2;
+    public static final int SEARCH_TAB_ID = 3;
+    public static final int ACTIVITIES_TAB_ID = 4;
+    private static final int UPLOAD_REQ = 1;
+    private static final String CURRENT_STATE_TAG = "currentTabState";
+    public static int currentTabState = DEFAULT_TAB_ID;
+    public static String currentUserId;
+    public static ContentResolver cr;
+    public static PackageManager pm;
+    public static MainScreen self;
+    public static FragmentManager fm;
+    private ImageView addButton, profileButton, homeButton, searchButton, activitiesButton;
+
+    @Override
+    protected void onCreate(Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+        setContentView(R.layout.activity_main_screen);
+        initials();
+        onClickListeners();
+        AppCompatDelegate.setCompatVectorFromResourcesEnabled(true);
+       /* Fragment home_fragment;
+        home_fragment = new HomeFragment();
+        loadFragment(home_fragment);*/
+    }
+    private void initials() {
+        self = this;
+        pm = getPackageManager();
+        cr = getContentResolver();
+        fm = getSupportFragmentManager();
+        addButton = findViewById(R.id.add_tab);
+        homeButton = findViewById(R.id.home_tab);
+        profileButton = findViewById(R.id.profile_tab);
+        searchButton = findViewById(R.id.search_tab);
+        activitiesButton = findViewById(R.id.activity_tab);
+
+        onHomeButtonClicked();
+
+    }
+    private void onClickListeners() {
+
+        homeButton.setOnClickListener(new View.OnClickListener() {
+            @Override
+            public void onClick(View v) {
+                onHomeButtonClicked();
+            }
+        });
+        addButton.setOnClickListener(new View.OnClickListener() {
+            @Override
+            public void onClick(View v) {
+                if (changeBackOtherImageResources(ADD_IMAGE_TAB_ID)) {
+                    /*addButton.setImageResource(R.drawable.plus_icon_fill);
+                    AddImageFragment addImageFragment = new AddImageFragment();
+                    getSupportFragmentManager().beginTransaction().addToBackStack(null);
+                    getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, addImageFragment).commit();
+                    Intent intent = new Intent(getApplicationContext(), UploadActivity.class);
+                    startActivityForResult(intent, UPLOAD_REQ);*/
+                }
+            }
+        });
+        profileButton.setOnClickListener(new View.OnClickListener() {
+            @Override
+            public void onClick(View v) {
+                if (changeBackOtherImageResources(PROFILE_TAB_ID)) {
+                    profileButton.setImageResource(R.drawable.profile_fill);
+                    /*ProfileFragment profileFragment = ProfileFragment.newInstance(MainActivity.currentUserId, ProfileType.LOGGED_IN_USER_PROFILE);
+                    getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, profileFragment).commit();*/
+                }
+            }
+        });
+
+        searchButton.setOnClickListener(new View.OnClickListener() {
+            @Override
+            public void onClick(View v) {
+                if (changeBackOtherImageResources(SEARCH_TAB_ID)) {
+                    searchButton.setImageResource(R.drawable.search_fill);
+                   /* PersonalsFragment personalsFragment = PersonalsFragment.newInstance(PersonalFragmentType.SEARCH_FRAGMENT, null);
+                    getSupportFragmentManager().beginTransaction().addToBackStack(null);
+                    getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, personalsFragment).commit();*/
+                }
+            }
+        });
+        activitiesButton.setOnClickListener(new View.OnClickListener() {
+            @Override
+            public void onClick(View v) {
+                if (changeBackOtherImageResources(ACTIVITIES_TAB_ID)) {
+                    activitiesButton.setImageResource(R.drawable.likes_fill);
+                    /*PersonalsFragment personalsFragment = PersonalsFragment.newInstance(PersonalFragmentType.ACTIVITY_FRAGMENT, null);
+                    getSupportFragmentManager().beginTransaction().addToBackStack(null);
+                    getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, personalsFragment).commit();*/
+                }
+            }
+        });
+
+    }
+    public void onHomeButtonClicked() {
+        if (changeBackOtherImageResources(HOME_TAB_ID)) {
+            homeButton.setImageResource(R.drawable.home_fill);
+            HomeFragment homeFragment = HomeFragment.newInstance();
+            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, homeFragment).commit();
+        }
+    }
+    @Override
+    public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
+        super.onSaveInstanceState(outState, outPersistentState);
+        outState.putInt(CURRENT_STATE_TAG, currentTabState);
+    }
+    private boolean changeBackOtherImageResources(int pressedIconState) {
+        int preTabState = currentTabState;
+        currentTabState = pressedIconState;
+        if (preTabState == currentTabState)
+            return false;
+        switch (preTabState) {
+            case HOME_TAB_ID:
+                homeButton.setImageResource(R.drawable.home_hollow);
+                break;
+            case ADD_IMAGE_TAB_ID:
+//                addButton.setImageResource(R.drawable.plus_icon_stroke);
+                break;
+            case PROFILE_TAB_ID:
+                profileButton.setImageResource(R.drawable.profile_hollow);
+                break;
+            case SEARCH_TAB_ID:
+                searchButton.setImageResource(R.drawable.search_hollow);
+                break;
+            case ACTIVITIES_TAB_ID:
+                activitiesButton.setImageResource(R.drawable.likes_hollow);
+                break;
+            default:
+                break;
+        }
+        return true;
+    }
+    @Override
+    protected void onDestroy() {
+        super.onDestroy();
+        currentTabState = -1;
+    }
+    @Override
+    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
+        super.onActivityResult(requestCode, resultCode, data);
+        switch (requestCode) {
+            case UPLOAD_REQ:
+                if (requestCode == RESULT_OK) {
+>>>>>>> 6d79e5d8589baed2e7762b3c0bd203346847ef50
 
-        RecyclerView recycler_view_post_section = (RecyclerView) findViewById(R.id.recycler_view_post_section);
-        recycler_view_post_section.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false));
-        recycler_view_post_section.setAdapter(new PostAdapter(usernames));
+                }
+                break;
+        }
 
+<<<<<<< HEAD
+=======
+    }
+   /* private void loadFragment(Fragment home_fragment) {
+        // load fragment
+        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
+        //frame_container is your layout name in xml file
+        transaction.replace(R.id.fragment_container, home_fragment);
+        transaction.addToBackStack(null);
+        transaction.commit();
+    }*/
+>>>>>>> 6d79e5d8589baed2e7762b3c0bd203346847ef50
 
         ib1 = (ImageView) findViewById(R.id.camera);
 

app/src/main/java/com/example/instagramclone_android/SplashScreen.java

@@ -14,9 +14,6 @@ public class SplashScreen extends AppCompatActivity {
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
-        if(getSupportActionBar()!=null){
-            getSupportActionBar().hide();
-        }
         setContentView(R.layout.activity_splash_screen);
         splash_screen_layout=(LinearLayout)findViewById(R.id.splash_screen_layout);
         splash_screen_layout.setOnClickListener(new View.OnClickListener() {

app/src/main/res/layout/activity_login_screen.xml

@@ -123,7 +123,7 @@
         android:layout_marginRight="30dp"
         android:textColor="#222121"/>
     <Button
-        android:id="@+id/button"
+        android:id="@+id/login_button"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:layout_marginRight="30dp"

app/src/main/res/layout/activity_main_screen.xml

@@ -8,7 +8,12 @@
     android:orientation="vertical"
     tools:context=".MainScreen"
     android:background="#fff">
-    <LinearLayout
+    <FrameLayout
+        android:id="@+id/fragment_container"
+        android:layout_width="match_parent"
+        android:layout_height="0dp"
+        android:layout_weight="1" />
+    <!--<LinearLayout
         android:layout_width="match_parent"
         android:layout_height="50dp"
         android:layout_gravity="top"
@@ -65,7 +70,7 @@
                 </androidx.recyclerview.widget.RecyclerView>
             </LinearLayout>
         </androidx.core.widget.NestedScrollView>
-    </FrameLayout>
+    </FrameLayout>-->
     <LinearLayout
         android:layout_width="match_parent"
         android:layout_height="50dp"