Week 5 Answers

1. Write a "Car" class, which contains fields for fuel tank size and mpg, and which has a method "getRange()" to compute range based on mpg/tank capacity.

Here's one way to do it:

public class Car {
    float tankSize;
    float mpg;

    public Car(float tankSize, float mpg) 
    {
        this.tankSize = tankSize;
        this.mpg = mpg;
    }
    
    public float getRange() 
    {
        return (mpg * tankSize);
    }
    public toString() 
    {
        return "Car with mpg=" + mpg + ", tankSize=" + tankSize;
    }
}

2. Write a "Garage" class that contains two "Car" objects - one for Carl and one for Mrs. Carl. Provide a method for the "Garage" class which tells you the largest range of any car in the Garage.

public class Garage 
{
    Car husbandsCar;
    Car wifesCar;
    public Garage(Car husband, Car wife) 
    {
	husbandsCar = husband;
	wifesCar = wife;
    }
    
    public float maxRange() 
    {
	float h = husbandsCar.getRange();
	float w = wifesCar.getRange();
	if (h > w) {
	    return h;
	}
	return w;
    }
    
    public toString() 
    {
        return "Garage with " + husbandsCar + " and " + wifesCar;
    }
}

3. Write a "PeopleCarrier" subclass of Car which modifies "range" to be determined partly by number of people being carried.

public class PeopleCarrier extends Car {
    int numPassengers;
    
    public PeopleCarrier(float tankSize, float mpg, int numPassengers) 
    {
        // We can use the constructor in "Car" by using the "super" keyword
        super(tankSize,mpg);
        this.numPassengers = numPassengers;
    }
    public float getRange() 
    {
        // Work out the "standard" range, use the "super" keyword again...
        float carRange = super.getRange();
        
        // If there are a lot of passengers, halve it
        if (numPassengers > 4) {
            carRange = carRange / 2;
        }
        return carRange;
    }

    public toString() 
    {
        return "PeopleCarrier with mpg=" + mpg + ", tankSize=" + tankSize
               + " with " + numPassengers + " passengers";
    }
}

4. Write another class which creates a new "Garage" and puts two PeopleCarriers in it. Prove that when you ask "Garage" which has the largest range, the "PeopleCarrier.getRange()" method is called.

public class UseGarage 
{
    public static void main(String[] args) 
    {
        PeopleCarrier p1 = new PeopleCarrier(10,20,4);
        System.out.println("p1 has a range of " + p1.getRange());

        PeopleCarrier p2 = new PeopleCarrier(10,20,8);
        System.out.println("p2 has a range of " + p2.getRange());

        Garage g = new Garage(p1,p2);

        System.out.println("The biggest range in the garage is " + 
                           g.maxRange());
    }
}

One way to confirm that the correct "getRange" method is called is to put a "println" in it...
5. Make the definition of your "Garage" class look like this:

          public abstract class Garage {
              . . .
          }
   

   What difference does this make?
You won't be able to create a new "Garage" object, e.g. if you have:

        Garage g = new Garage(p1,p2);

then you'll get an error saying "Garage is abstract; cannot be instantiated"
6. Try and think of a better example than mine!
7. What method have you implemented in all your classes (assuming you followed the advice above)?
toString()!
8. If all classes are extensions of "Object", what methods do you think should be part of Object? Look in the documentation and see what there actually is.
See java documentation.
9. especially for Julian Assuming you have modified your "Person" class to include a "toString" method, so that the following code:

       Person me = new Person("nick",21);             // 1
       System.out.println("the person is " + me);     // 2
   

   displays something like:

       the person is nick, who is 21 and was created on xxxxx
   

   Can you come up with a way to change the code at "2" (you can change the line or add extra lines, but not change any of the "Person" class methods) so that the output will now be:

      the person is Person@1bab50a
   

   i.e. bypass the "Person.toString()" method?
I couldn't see how to do this, but well done to Martin for cracking it!
10. especially for Martin The following code:

            int x = 123;
    	System.out.println("x is -->" + x + "<--");
    

    will by default output:

    x is -->123<--
    

    What can you do (if anything) to make the integer value be padded up to eight digits with leading zeroes? i.e.:

    x is -->00000123<--
    

Here's how you can get the number with leading zeroes, but I didn't find a way to make this format the default for all integers - maybe someone else did?

public class Martin 
{
    public static void main(String[] args) 
    {
        int x=123;
        System.out.println("x is -->" + x + "<--");

        java.text.DecimalFormat df = new java.text.DecimalFormat();

        df.setMinimumIntegerDigits(8);
        df.setGroupingSize(0);
        System.out.println("with leading zeros, x is -->" + df.format(x)
                           + "<--");
        
    }
}