Invoke Virtual Method on Null Object

Question

I'm making a simple point of sales application for my employer. A simple login screen followed by a simple tab layout displaying all the items we sell. The MainActivity is the login page and upon successful login you are brought to the point of sales activity which had a tab layout with its corresponding fragments for each tab. I have no errors and the login page work fine, until I login successfully and the second activity (which contains the tablayout) is called. I then get a crash with this error,

Attempt to invoke virtual method 'void android.widget.TextView.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

Here is the line its pointing to,

tabMeals.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            viewPager.setCurrentItem(0);
        }
    });

Here is the entirety of my posScreen activity

public class posScreen extends AppCompatActivity {
ViewPager viewPager;
ViewPagerAdapter viewPagerAdapter;
private TextView tabMeals, tabSides, tabCheckout;

public posScreen() {
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_pos_screen);
    this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);

    //Tab Resources
    tabMeals = findViewById(R.id.meals_tab);
    tabSides = findViewById(R.id.sides_tab);
    tabCheckout = findViewById(R.id.checkout_tab);
    viewPager = findViewById(R.id.FragContainer);

    tabMeals.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            viewPager.setCurrentItem(0);
        }
    });
    tabSides.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            viewPager.setCurrentItem(1);
        }
    });
    tabCheckout.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            viewPager.setCurrentItem(2);
        }
    });

    viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
    viewPager.setAdapter(viewPagerAdapter);

    viewPager.addOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

        }

        @Override
        public void onPageSelected(int position) {
            onChangeTab(position);
        }

        @Override
        public void onPageScrollStateChanged(int state) {

        }
    });

}

private void onChangeTab(int position) {
    if(position == 0) {
        tabMeals.setTextSize(30);
        tabSides.setTextSize(20);
        tabCheckout.setTextSize(20);
    } else if(position == 1) {
        tabMeals.setTextSize(20);
        tabSides.setTextSize(30);
        tabCheckout.setTextSize(20);
    } else if(position == 2) {
        tabMeals.setTextSize(20);
        tabSides.setTextSize(20);
        tabCheckout.setTextSize(30);
    }
}

}

And here is my posScreen xml file

 <?xml version="1.0" encoding="utf-8"?>
<androidx.appcompat.widget.LinearLayoutCompat 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="wrap_content"
    tools:context=".posScreen">
    <androidx.viewpager.widget.ViewPager
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/FragContainer"
        android:layout_marginTop="50dp"/>

    <com.google.android.material.tabs.TabLayout
        android:id="@+id/tabLayout"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <com.google.android.material.tabs.TabItem
            android:id="@+id/meals_tab"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Meals" />

        <com.google.android.material.tabs.TabItem
            android:id="@+id/sides_tab"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Sides and Drinks" />

        <com.google.android.material.tabs.TabItem
            android:id="@+id/checkout_tab"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Checkout" />

    </com.google.android.material.tabs.TabLayout>

</androidx.appcompat.widget.LinearLayoutCompat>

Any help would be appreciated, I'm not super familiar with Java or Android Studio.

Answer 1

The problem is that you declare this:

private TextView tabMeals, tabSides, tabCheckout;

But actually, in the xml, i can see, those id do not correspond to TextViews but to TabItem. So on, try declaring them as:

private TabItem tabMeals, tabSides, tabCheckout;