content_main.xml

<com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        android:layout_marginEnd="32dp"
        android:layout_marginBottom="36dp"
        app:backgroundTint="@color/colorPrimaryDark"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:maxImageSize="35dp"
        app:srcCompat="@drawable/ic_add_black_24dp" /> <!-- create this icon in drable folder  -->

MainActivity.java

FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //todo
            }
        });