Calculate with numbers from different number systems
Main feedback has already been given by @AJNeufeld and this post is not about the performance of your program but rather other aspects.

You should try putting a check before you access an index in an array and slightly change your if block from this,

if (args[0].contains("help")) {


to,

if (args.length == 0 || args[0].contains("help") || args.length &lt; 5) {


as the former will run into ArrayIndexOutOfBoundsException if no argument is passed. Also it would be helpful to call the displayHelp() method in case no argument (args.length == 0) was passed so the user knows the usage of program.

Also, for safely accessing array indexes, you should put another OR condition args.length &lt; 5 which will ensure at least five parameters are passed, else again you may run into ArrayIndexOutOfBoundsException.

These checks should make the program a little more safer.