A courier company uses the following Java class to record 
information about packages that they deliver around New Zealand.
Each package is uniquely identified by a barcode number.
They also record the weight of the package in kilograms, its destination
city and customer, and a contents indicator that is either 
N for normal, F for fragile, or E for explosive.


public class ShippingItem {

   private int barcode;
   private String customer;
   private String city;
   private char contents;
   private double weight;

   public ShippingItem(int code, char cont, double wt) {

      barcode = code;
      customer = "Chh";
      city = "Akl";
      contents = cont;
      weight = wt;
    }

    void printSummary() {
      
      System.out.println(barcode + ":" + contents + "-" +
                         weight +  "-" + city);
    }

    void setCity(String ct) {
      city = ct;
    }
}

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1. Understanding Classes and Objects [10 marks]

What output does the following test class produce?

public class ShippingTest {

   public static void main(String[] args) {

      ShippingItem c1 = new ShippingItem(3145, 'N', 67.5);
      c1.printSummary();
      c1.setCity("Wel");
      c1.printSummary();
      c1 = new ShippingItem(4096, 'F', 310.0);
      c1.printSummary();
   }
}

ANSWER:

3145:N-67.5-Akl
3145:N-67.5-Wel
4096:F-310.0-Akl

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2. Accessor methods: getters and setters [10 marks]

Implement a method ``getBarcode'' that returns the barcode of a
ShippingItem, and a method ``setBarcode'' that allows to set the
barcode of a ShippingItem:

ANSWER

    public int getBarcode() { return barcode;}

    public void setBarcode(int bc) { barcode = bc;}


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3. Text Output  [10 marks]

Implement a method ``printFull'' that outputs the full information
about a ShippingItem on a single line, where all data member values
are separated by commas (","). So e.g. the 
ShippingItem(3145, 'N', 67.5) of question 1 should print as:

3145,Chh,Akl,N,67.5

ANSWER

    public void printFull() {
	System.out.println(barcode + "," + customer + "," + 
			   city + "," + contents + "," + weight);
    }


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4. Constructor magic [25 marks]

What output does the following set of class definitions produce?


public class Test {
    public static void main(String[] args) {
        A a = new A(0);
        a.show();
        B b = new B(3);
        b.show();
        b = new B();
        b.show();
    }
}

public class A {

    protected int aValue;
    public A() {aValue = 1;}
    public A(int n) {aValue = n;}
    public void show() {
        System.out.println("A.aValue = " + aValue);
    }
}

public class B extends A {

    protected int bValue;
    public B() {super(2); bValue = 1;}
    public B(int n) {bValue = n;}
    public void show() {
        System.out.println("B.aValue = " + aValue);
        System.out.println("B.bValue = " + bValue);
    }
}

ANSWER

A.aValue = 0
B.aValue = 1
B.bValue = 3
B.aValue = 2
B.bValue = 1

======================================================================

5. Debugging [20 marks]

Mark all errors in the following class definition.
Always explain what is wrong in a short single sentence each.

ANSWERS are comments!

public class Buggy {

    public static final int magicNumber = 42;

    private int privateMagicNumber = 0;

    public void Buggy() {      
// "void"  constructors have no return type
// or if somebody claims that this is a method: should be
//  lowercase "buggy()" then

        System.out.println("Creating Buggy instance");
        magicNumber = magicNumber + 1;
// cannot change a final variable
    }

    public getPrivateMagicNumber() {
// no return type: should be "public int getPrivateMagicNumber() {"

        return privateMagicNumber;
    }

    public setPrivateMagicNumber(int i) {
// no return type: should be "public int setPrivateMagicNumber() {"
// given the code

        privateMagicNumber = i;
        return i;
    }

    public static void main(String[] args) {

        Buggy buggy = new Buggy();
        buggy.setPrivateMagicNumber("4711");
// wrong argument type "4711", should be an int!
// or: no method setPrivateMagicNumber(String v) is defined

        System.out.println( buggy.privateMagicNumber + "/" +
                            Buggy.magicNumber);
    }
}


======================================================================

6. Command Line stuff [25 marks]

Implement a class ``CountingEcho'' that allows for an arbitrary number
  of command line 
  arguments. It should print out each command line argument on a
  separate line, prefixing each output line with the size of the
  respective string. Finally output the total number of command-line
  arguments supplied, and the sum of their string sizes.

  [Hint: the class String supplies a public method "length()" that
  returns the size of a string.]

Here is the output of some example runs of the program:

>java CountingEcho good day
4: good
3: day
2 arguments, 7 characters

>java CountingEcho 
0 arguments, 0 characters

>java CountingEcho wreck a nice beach
5: wreck
1: a
4: nice
5: beach
4 arguments, 15 characters

ANSWER


public class CountingEcho {

    public static void main(String[] args) {
	int nChars = 0;

	for(int i = 0; i < args.length; i++) {
	    int n = args[i].length();
	    nChars += n;
	    System.out.println(n + ": " + args[i]);
	}

	System.out.println(args.length + " arguments, "
			   + nChars + " characters");
    }
}