How to set margins for TextView programmatically?

set to LayoutParams.

LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
params.setMargins(10,10,10,10);
tv1.setLayoutParams(params);

It depends according to your parent view.

If you using LinearLayout on your textview as a parent view give params like below

LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
params.setMargins(10,10,10,10);
tv1.setLayoutParams(params);

If you using RelativeLayout on your textview as a parent view give params like below

RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(RelativeLayout.LayoutParams.WRAP_CONTENT, RelativeLayout.LayoutParams.WRAP_CONTENT);
params.setMargins(10,10,10,10);
tv1.setLayoutParams(params);

All these answers are great, but I was using ConstraintLayout, so here is code for that:

ConstraintLayout.LayoutParams params = new ConstraintLayout.LayoutParams(ConstraintLayout.LayoutParams.WRAP_CONTENT, ConstraintLayout.LayoutParams.WRAP_CONTENT);
params.setMargins(10, 10, 10, 10);
textview.setLayoutParams(params); // note that textview would be your instanced TextView object