This is definitely a bit of a hack, but I don't know if the DecimalFormat
syntax allows for anything better. This simply checks to see if the number is real, and formats based on the spec you asked for.
double number = 25000.5;
DecimalFormat df;
if(number%1==0)
df = new DecimalFormat("##,###");
else
df = new DecimalFormat("##,###.00");
System.out.println(df.format(number));