Thread: Calculating Fuel After Distance

I'm trying to calculate the amount of petrol that is left over given a particular distance and fuel efficiency. The 'drive' method is suppose to calculate this but I'm not getting the correct output.

Below is my Car class the main method and the tester class. 'testCar' and 'testAddPetrol' are passing but I can't get 'testDrive' to pass. Any hints as to what I'm doing wrong? Or the way I can calculate the petrol left over in the 'drive' method.

Thanks.

public class Car {	
 
	private double tankSize;		//capacity of tank	
	private double efficiency;		//litres per 100km
	private double gasInTank;		//litres in tank
 
	Car(int initTankSize_, double initEfficiency_)	{	
		tankSize = initTankSize_;
		efficiency = initEfficiency_;	
		gasInTank = 0;
	}
 
	public double getPetrol()	{
		return gasInTank;
	}	
 
	public double getEfficiency()	{		
		return efficiency;
	}
 
	public double getCapacity()	{		
		return tankSize;
	}	
 
	public void addPetrol(double litres)	{
		gasInTank = gasInTank + litres;			
		if(gasInTank > tankSize)	{
			gasInTank = tankSize;		
		}	
	}
 
//THIS IS WHERE I'M HAVING TROUBLE
	public void drive(double km){			
		double distance = (gasInTank * efficiency); 		
		if (distance > km)	{
			distance = km;
		}
		gasInTank = (gasInTank - (distance/efficiency));
 
 
	}
}

public class CarMain {
 
	public static void main(String[] args) {
 
 
		//TestCar
		Car c = new Car(60, 10);	
		c.getPetrol();  					//returns gasInTank from initial constructor
		c.getCapacity();					//returns tankSize from initial constructor
		c.getEfficiency();					//returns efficiency from initial constructor
 
		//testAddPetrol
		Car c2 = new Car(30,10);			
		c2.addPetrol(20);					
		c2.getPetrol();						
		c2.addPetrol(20);					
		c2.getPetrol();
 
		//testDrive
		Car c3 = new Car(30, 10);		
		c3.addPetrol(20);		
		c3.drive(100);		;	
		double petrolLeft = c3.getPetrol();
		System.out.println("This: " +petrolLeft);
		c3.drive(10000);
		System.out.println("After 10000: " +c3.getPetrol());
 
 
		//testSetEfficiency
		Car c4 = new Car(30,10);
 
 
 
 
	}
}

public class CarTest {
 
	/**
	 * Test method for {@link weeklytask4.Car#Car(double, double)}.
	 */
	@Test
	public final void testCar() {
		Car c = new Car(60,10);
		assertEquals("Petrol in constructor",0.0,c.getPetrol(),0.01);
		assertEquals("Capacity in constructor",60.0,c.getCapacity(),0.01);
		assertEquals("Efficiency in constructor",10.0,c.getEfficiency(),0.01);
	}
 
	/**
	 * Test method for {@link weeklytask4.Car#addPetrol(double)}.
	 */
	@Test
	public final void testAddPetrol() {
		Car c = new Car(30,10);
		c.addPetrol(20);
		assertEquals("After adding 20l of petrol", 20.,c.getPetrol(),0.01);
		c.addPetrol(20);
		assertEquals("After adding 20l more of petrol", 30.,c.getPetrol(),0.01);
	}
 
	/**
	 * Test method for {@link weeklytask4.Car#drive(double)}.
	 */
	@Test
	public final void testDrive() {
		Car c = new Car(30,10);
		c.setPetrol(20);
		c.drive(100);
		assertEquals("After 100 km", 10.,c.getPetrol(),0.01);
		c.drive(10000);
		assertEquals("After 10000 km", 0.,c.getPetrol(),0.01);
	}
 
	/**
	 * Test method for {@link weeklytask4.Car#setEfficiency(double)}.
	 */
	@Test
	public final void testSetEfficiency()
	{
		Car c = new Car(30,10);
		c.setEfficiency(5);
		assertEquals("Changing fuel efficiency",5.,c.getEfficiency(),0.01);
	}
 
	/**
	 * Test method for {@link weeklytask4.Car#setPetrol(double)}.
	 */
	@Test
	public final void testSetPetrol()
	{
		Car c = new Car(30,10);
		c.setPetrol(5);
		assertEquals("Changing fuel petrol",5.,c.getPetrol(),0.01);
		c.setPetrol(29);
		assertEquals("Changing fuel petrol",29.,c.getPetrol(),0.01);
		c.setPetrol(35);
		assertEquals("Changing fuel petrol",30.,c.getPetrol(),0.01);
	}
 
	/**
	 * Test method for {@link weeklytask4.Car#setCapacity(double)}.
	 */
	@Test
	public final void testSetCapacity()
	{
		Car c = new Car(30,10);
		c.setPetrol(25);
		c.setCapacity(35);
		assertEquals("Changing fuel capacity",25.,c.getPetrol(),0.01);
		assertEquals("Changing fuel capacity",35.,c.getCapacity(),0.01);
		c.setCapacity(20);
		assertEquals("Changing fuel capacity",20.,c.getPetrol(),0.01);
		assertEquals("Changing fuel capacity",20.,c.getCapacity(),0.01);
	}
 
}

Great. Thanks very much for the help Chris.