Objects in Methods, Objects as Class Fields, Arrays of Objects

• We will examine the topic of methods that return objects, and also objects passed to methods as input parameters.

• We will design and implement a Fraction class, …
  • … and then we will write methods to add & subtract two Fraction objects.

• This is the way we want to be able to work with Fraction class objects:

     // Construct one fraction and print it
     Fraction f1;
     f1 = new Fraction(2, 3);
     System.out.println(f1);
     
     // Add two Fraction objects to get a sum Fraction object
     Fraction f2;
     f2 = new Fraction(1, 6);
     Fraction f3;
     f3 = addFraction(f1, f2);

     System.out.println(f1 + " + " + f2 + " = " + f3);

• In the code above, we use an addFraction method that takes two Fraction objects (f1, f2) as input and returns a Fraction object that we can assign to another Fraction object (f3).
• The output of the above System.out.println() is:

            2/3 + 1/6 = 5/6

• Notice that 5/6 is a reduced fraction, meaning that somewhere in the Fraction class, there needs to be some kind of reduce() method, so the reduced form of a fraction is stored in the class.

• The addFraction method is shown below.
  • In order to return a Fraction object, the addFraction method needs to call the Fraction constructor with the numerator and denominator of the Fraction that is the sum of Fractions one and two (last few lines of the code below):

     public static Fraction addFraction( Fraction one, Fraction two )
     {
        // Compute common denominator: 
        int dd = one.getDenominator() * two.getDenominator();
        
        // Cross multiply and add:
        int nn = one.getNumerator() * two.getDenominator() +
                 two.getNumerator() * one.getDenominator();
    
        Fraction sum = new Fraction( nn, dd );            
        return sum;
        //
        // or skip using a sum variable by doing this:
        //return new Fraction( nn, dd ); 
     }

• The subtractFraction method is not substantially different from addFraction.

• We will examine the topic of using an object of one class as a field in a different, but related class.
  • Objects in the real world are often composed of smaller related objects, for example, an account holder object within a bank account object…

• We will design and implement an AccountHolder class, …
• … and then we will use an AccountHolder class object as a field in a BankAccount class.
• … and then we will test the BankAccount class in a program.

• Here are the relevant sections of the AccountHolder class.

     public class AccountHolder
     {
         private String firstName;
         private String lastName;
         private int age;

         // Constructors:
         /**
          * Constructor #1
          * @param first name
          * @param last name
          * @param age
          */
         public AccountHolder( String fn, String ln, int ag ) {
            firstName = fn;
            lastName = ln;
            age = ag;
         }

         /*
          * Constructor #2 (default)
          */
         public AccountHolder()
         {
         }

         // Methods:
         // Write get and set methods for firstName, lastName, age below.
         ...
     } // end class

• The BankAccount class will use an AccountHolder object to represent an account holder:

    public class BankAccount
    {
        private int acctnum;
        private double balance;
        private AccountHolder customer;
        ...
        ...
    } // end class

• The BankAccount constructor will look like this:

    /**
     * Constructor#1 for objects of class BankAccount
     * @param acct
     * @param bal
     * @param firstname of customer
     * @param lastname of customer
     * @param age of customer
     */
    public BankAccount( int acct, double bal, String firstname, String lastname, int age )
    {
        // Write code to set account number, balance and customer name and age:
        acctNum = acct;
        balance = bal;
        // Since customer is an object, initializing it requires calling a constructor;
        // Use last 3 input parameters to construct the customer object.
        customer = new AccountHolder( firstname, lastname, age );
    }

• When a BankAccount object is constructed using the above constructor, a customer object will also be constructed.

    (Code in main() method of TestBankAccount.java)

    BankAccount myAccount;
    myAccount = new BankAccount( 1, 8000.00, "Joe", "Chung", 56 );
    // The last 3 constructor arguments, "Joe", "Chung", and 56 will
    // be passed to the AccountHolder constructor to construct a
    // customer object within the myAccount object.

• If a class has an object instance variable, the accessor methods (get/set methods) may have to be written and used a bit differently.

• A BankAccount method to return the customer object's information might look like this:

    /**
     * Get method for customer's information
     * @return info
     */
    public String getCustomerInfo()
    {
        // Use get methods defined in the AccountHolder class:
        String info = "Name:\t" + customer.getFirstName() + " " + customer.getLastName() + "\n" +
                      "Birthday:\t" + costomer.getBirthDay() + "\n" +
                      "Age:\t" + customer.getAge() + "\n" +;
        return info;
    }

• A BankAccount method can be written to return the actual customer object like this:

    /**
     * Get method for customer
     * @return customer, an AccountHolder object
     */
    public AccountHolder getCustomer()
    {
        return customer;
    }

• The getCustomer method can then be used to access customer information using customer get methods.
  • This requires an additional dot (.) to be used to access AccountHolder methods.

    (Code in main() method of TestBankAccount.java)

    // Get bankaccount holder's first name only:
    System.out.println("Account holder's first name: " + myAccount.getCustomer().getFirstName());

    // myAccount.getCustomer() returns the customer object that's in the myAccount object, and then
    // .getFirstName() gets the customer object's firstname.

• We will examine the topic of arrays that contain objects.
• The arrays we have looked at before were arrays of simple, primitive types, such as int or double.
  • We have used an array of Strings in the MagicEightBall class example.
    • While Strings are objects in Java, they are “special,” i.e., don't require constructors to create Strings.

• We will design and implement a Cat class, …
• … and then we will use an array of Cat class objects as a field in a PetStore class.
• A PetStore represents a collection of Cats (and other animals).

• Here is the simple Cat class.

    public class Cat
    {
        // instance variable
        private String _name;
    
        /**
        * constructor #1
        * @param name
        */
        public Cat(String name)
        {
            _name = name;
        }
    
        // methods
        /**
        * Get method for _name
        * @return _name
        */
        public String getName()
        {
            return _name;
        }
    
    } // end class

• The PetStore class will have an array of Cats as an instance variable,

    public class PetStore
    {
        // instance variables
        // array of Cats
        private Cat [] cats;
        private final int MAXCATS = 15;
        ...

• The PetStore class constructor will need to “dimension” (set the length of) the array with the MAXCATS constant,

    public PetStore()
    {
        // construct array of cats
        cats = new Cat[MAXCATS];
    }

• Every element of the cats array currently contains null as a value, by default.
  • Every element of an int (or double) array would contain 0 (or 0.0) by default.
  • Every element of a boolean array would contain false by default.
  • Every element of an array of objects contains null by default.

• To add Cat objects to the cats array, we need to construct Cat objects and then assign the objects to the array elements.
  • We can do this in a addCats method,

    public void addCats()
    {
        // Add a new Cat using a Cat variable, c1
        Cat c1 = new Cat("Miu");
        cats[0] = c1;
        ...        
        // Add a new Cat without using a Cat variable
        cats[3] = new Cat("Skitty");
        ...
    }

• If we added Cat objects to the first 4 elements of the cat array, the remaining elements of the array would still contain the value null.
  • As with any array, the cats array is restricted to only Cat type objects.
    • We would not be able to add Dog class objects to the cats array.

• To access a Cat object as an array element, we need to use normal array syntax, i.e., array[index].
  • For example, to print the name of the 4th Cat object in the cats array, we would need to write,

    // Must call Cat object's getName method
    System.out.println(cats[3].getName());

• We will examine the ArrayList Class and its advantages over standard Arrays.

• allows for “expandable” arrays
• An ArrayList has these characteristics:
  • An ArrayList automatically expands as data is added.
  • An ArrayList has methods for inserting, deleting, and searching.
  • An ArrayList can be traversed using a foreach loop, iterators, or indexes.

• Programmers are frequently faced with the choice of using a simple array or an ArrayList.
• If the data has a known number of elements or small fixed size upper bound, or where efficiency in using primitive types is important, arrays are often the best choice.
• However, many data storage problems are not that simple, and ArrayList (or one of the other Java Collections classes) might be the right choice.

• Use ArrayList when there will be a large variation in the amount of data that you would put into an array.
• Arrays should ideally be used only when there is a constant amount of data.
  • For example, storing information about the days of the week should use an array because the number of days in a week is constant.
  • Use an ArrayList for your email contact list because there is no upper bound on the number of contacts.

• A possible disadvantage of ArrayList is that it holds only object types and not primitive types (e.g., int, double, character).
• To use a primitive type in an ArrayList, put it inside an object or use of the wrapper classes (eg, Integer, Double, Character, …).
  • The wrapper classes are immutable, so if you use, eg, Integer, you will not be able to change the integer value.

• Here are some of the most useful ArrayList methods.
  • Note that E is the notation for the type of an element in a collection such as in an ArrayList.

• Constructors:

    new ArrayList<E>()  	Creates ArrayList of Class E objects with
                            initial default capacity of 10.

    new ArrayList<E>(cap)  	Creates ArrayList with initial int
                            capacity cap.

• Adding elements:

    a.add(e)      adds element e to the end of ArrayList a
    a.add(i, e)   Inserts e at index i, shifting elements up as necessary.

• Replacing an element:

    a.set(i,e)    Sets the element at index i to e.

• Getting the elements:

    a.get(i)      Returns the object at index i.
    a.toArray()   Returns elements in a as an array of objects.

• Removing elements:

    a.clear()             Removes all elements from ArrayList a
    a.remove(i)           Removes the element at position i.
    a.removeRange(i, j)   Removes the elements from positions i thru j.

• Other:

    a.size()   Returns the number of elements in ArrayList a.

    ArrayList<E> a = new ArrayList<E>();  // Construct default size ArrayList a

    E s = new E();     // Construct s object of Class E.

    . . .

    a.add(s);     // Add object s to the end of the ArrayList a

    . . .

    s = a.get(i); // Assigns i-th element from a to object s.

• 1. Use special “foreach” loop.
  • This is fast and works for all kinds of lists, but is not entirely flexible (can only go sequentially forward with no deletions, additions, or multiple references).
  • foreach loops should be the first choice for ArrayLists

    ArrayList<String> a = new ArrayList<String>();
    . . .
    for ( String s : a ) {
       System.out.println( s );
    }

• 2. Use standard for loop with index.
  • This is fast.
  • It does allow orders other than sequentially forward by one.

    // Assume a is an ArrayList:
    for ( int i = 0; i < a.size(); i++ ) {
       System.out.println( a.get( i ) );
    }

• If the data in your ArrayList has a natural sorting order you can simply call the static Collections.sort() method.

    Collections.sort(yourArrayList);

• Example: sort words in an ArrayList:

    // Purpose: An exercise using Scanner, ArrayList, and
    //          Collections.sort.  Read words, sort them, print
    //          them.

    import java.util.Scanner;
    import java.util.ArrayList;
    import java.util.Collections; // for sorting method    

    public class Alphabetize
    {
       public static void main(String[] args)
       {
          //... Declare variables.
          Scanner in    = new Scanner(System.in);
          ArrayList<String> words = new ArrayList<String>();

          //... Read input one word at a time.
          System.out.println("Enter words.  End with EOF (ex. CTRL-Z then Enter, or CTRL-D)");

          //... Read input one word at a time, adding it to an array list.
          while (in.hasNext()) {
             words.add(in.next());
          }

          //... Sort the words.
          Collections.sort(words);

          //... Print the sorted list.
          System.out.println("\n\nSorted words\n");

          // foreach loop:
          for (String word : words) {
             System.out.println(word);
          }
            
       } // end main
    } // end class