Only accept non-negative numbers Java -

else if (bb.equals("dollar") && cc.equals("pound")){         input = joptionpane.showinputdialog("what amount convert?:");         double aa = double.parsedouble(input);         double output =(aa*rate);         joptionpane.showmessagedialog(null, string.format("your amount is: $%.2f",  output)); 

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how make if enters negative number message popup saying invalid input?

i assume number you're checking see if it's positive defined variable aa? prefer using bigdecimal when comes dealing monetary values, in case do:

bigdecimal aa = null; try {     aa = new bigdecimal(input); } catch (numberformatexception nfe) {     //not numerical value, throw error } if (aa.compareto(bigdecimal.zero) <= 0) {     //value negative }