android app keeps stopping when i try to open on emulator

Question

I am beginner in android development. I created an activity and added a button. Then I added an event listener to open another activity when the button is clicked. So far the two classes are

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button floating_action_button = findViewById(R.id.fab);
        floating_action_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, navActivity.class);
                startActivity(intent);
            }
        });
    }

}

and

public class navActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_nav);
    }
}

layout file for MainActivity is as follows

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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">

    <TextView
        android:layout_width="244dp"
        android:layout_height="78dp"
        android:text="Click on the red button to add the image you want to edit"
        android:textFontWeight="600"
        android:textSize="20dp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.497"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.484" />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="24dp"
        android:layout_marginRight="24dp"
        android:clickable="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.954"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.915"
        app:srcCompat="@android:drawable/ic_input_add"
        tools:layout_margin="16dp" />

</androidx.constraintlayout.widget.ConstraintLayout>

can anyone tell me what is wrong with this? And why does app keep stopping?

Answer 1

I found the error I had to use

FloatingActionButton floating_action_button = findViewById(R.id.fab); 

in place of Button floating_action_button = findViewById(R.id.fab) ; in even handler.