Android : Checkbox selection issue with multiple selection

i have 8 check boxes tat is weekdays sunday to saturday and one all days. the problem is if i select all days it is coming. But i check all and uncheck some days means it is not reflecting in my result , the result remains all days

here is my code

public void onClick(View v) {
        if (v == btnSave) {
            days.clear();
            if (all.isChecked()) {

                days.add("Su");
                days.add("Mo");
                days.add("Tu");
                days.add("We");
                days.add("Th");
                days.add("Fr");
                days.add("Sa");
            } else {
                if (sun.isChecked()) {
                    days.add("Sun");
                }
                if (mon.isChecked()) {
                    days.add("Mon");
                }
                if (tue.isChecked()) {
                    days.add("Tue");
                }
                if (wed.isChecked()) {
                    days.add("Wed");
                }
                if (thur.isChecked()) {
                    days.add("Thu");
                }
                if (fri.isChecked()) {
                    days.add("Fri");
                }
                if (sat.isChecked()) {
                    days.add("Sat");
                }
            }

            Log.d("my days are ", "size is " + days.size());
            myPreference.setNumberOfPractiseDays("TOTAL_NUMBER_OF_DAYS",
                    days.size());
            for (int i = 0; i < days.size(); i++) {
                myPreference.setPractiseDays("DAYS" + i, days.get(i));
            }
            finish();

        } else if (v == all) {
            if (all.isChecked()) {
                sun.setChecked(true);
                mon.setChecked(true);
                tue.setChecked(true);
                wed.setChecked(true);
                thur.setChecked(true);
                fri.setChecked(true);
                sat.setChecked(true);

            } else {
                sun.setChecked(false);
                mon.setChecked(false);
                tue.setChecked(false);
                wed.setChecked(false);
                thur.setChecked(false);
                fri.setChecked(false);
                sat.setChecked(false);
            }

        }
    }

Thanks in advance