You could use DecimalFormat
and just format the number
DecimalFormat formatter = new DecimalFormat("#,###,###");
String yourFormattedString = formatter.format(100000);
The result will be
1,000,000
for 1000000
10,000
for 10000
1,000
for 1000
Update 12/02/2019
This String.format("%,d", number)
would be a better(less hardcoded) solution as indicated in the comments below by @DreaminginCode so I thought I would add it here as an alternative
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