Check out example codes for "set icon to fab programmatically in android studio". It will help you in understanding the concepts better.

Code Example 1

floatingActionButton.setImageDrawable(ContextCompat.getDrawable(getContext(), R.drawable.ic_full_sad));

Code Example 2

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            floatingActionButton.setImageDrawable(getResources().getDrawable(R.drawable.ic_full_sad, context.getTheme()));
        } else {
            floatingActionButton.setImageDrawable(getResources().getDrawable(R.drawable.ic_full_sad));
    }

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