This is a simple text watcher to resolve "How to set thousands separator in Android?" quiz.

ThousandSeparatorWatcher.java

package com.chootdev.myapplication;

import android.text.Editable;
import android.text.TextWatcher;
import android.util.Log;
import android.widget.EditText;

import java.text.DecimalFormat;
import java.text.NumberFormat;
import java.util.Locale;

/**
 * Created by Choota on 12/8/17.
 */

public class ThousandSeparatorWatcher implements TextWatcher {

    private static ThousandSeparatorWatcher thousandSeparator;
    private static String separatorThis = ",";
    private static EditText editTextThis;

    public static ThousandSeparatorWatcher init(EditText editText) {
        editTextThis = editText;

        if (thousandSeparator == null)
            thousandSeparator = new ThousandSeparatorWatcher();

        return thousandSeparator;
    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {

    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {

    }

    @Override
    public void afterTextChanged(Editable s) {
        if (editTextThis != null) {
            editTextThis.removeTextChangedListener(this);

            try {
                String originalString = s.toString();

                Long longval;
                if (originalString.contains(separatorThis)) {
                    originalString = originalString.replaceAll(separatorThis, "");
                }
                longval = Long.parseLong(originalString);

                DecimalFormat formatter = (DecimalFormat) NumberFormat.getInstance(Locale.US);
                formatter.applyPattern("#,###,###,###");
                String formattedString = formatter.format(longval);

                //setting text after format to EditText
                editTextThis.setText(formattedString);
                editTextThis.setSelection(editTextThis.getText().length());

            } catch (NumberFormatException nfe) {
                nfe.printStackTrace();
            }

            editTextThis.addTextChangedListener(this);
        } else {
            Log.e("ThousandWatcher", "EditText must not be null");
        }
    }

    public static String resolveWithoutSeparator(EditText editText) throws ThousandSeparatorException {
        if (editText != null && editText.getText() != null && !editText.getText().toString().trim().isEmpty()) {
            return editText.getText().toString().replaceAll(separatorThis, "");
        } else {
            if (editText == null)
                throw new ThousandSeparatorException("EditText must not be null");
            else
                return "";
        }
    }

    public static class ThousandSeparatorException extends Exception {
        ThousandSeparatorException(String s) {
            super(s);
        }
    }
}