Android : Android: cannot resolve symbol "setImageResource"

I am trying to change the image for an ImageButton in my Java code, but I keep getting an error message "cannot resolve symbol "setImageResource".

The ImageButton is defined in my XML as follow:

<ImageButton
    android:id="@+id/button_ir2"
    android:layout_width="60dp"
    android:layout_height="50dp"
    android:layout_marginBottom="10dp"
    android:layout_marginRight="5dp"
    android:background="@android:color/white"
    android:src="@drawable/img1"
    android:scaleType="fitCenter"/>

And in my Java code:

            final View vz=findViewById(R.id.button_ir2);
            ImageButton aButton = (ImageButton)vz;
            aButton.setImageResource(R.drawable.img2);

I think I am typecasting this correctly, but maybe not. Spent almost a day on this, any help will be greatly appreciated.