Design and implement a class Car that models a car traveling on a horizontal x-axis, consuming gasoline as it moves. Provide methods for construction, to drive by a given number of miles, to add a given number of gallons to the tank, and to get the current distance from the origin and the fuel level both separately and together. Specify the fuel efficiency in miles/gallon. Should this be an immutable class? Indicate why or why not in the header comments of your program.
Supply a tester class called Test car that implements at least two cars and exercise their constructors (there should be a zero parameter and a one-parameter constructor) and methods.