1. A Second Look at Classes and Objects - 2
Chapter 9
A Second Look at Classes and Objects - 2

2. Contents Passing Objects As Arguments to Methods
Returning Objects from Methods
The toString Method
Writing an equals Method
Methods That Copy Objects

3. I. Passing Objects As Arguments to Methods
When a variable is passed as an argument to a method, it is said to be passed by value.
This means that a copy of the variable's value is passed into the method's parameter.
When the method changes the contents of the parameter variable, it does not affect the contents of the original variable that was passed as an argument.

4. I. Passing Objects As Arguments to Methods
To pass an object as a method argument, we pass an object reference.
Reference variable holds the memory address of an object.
The method has access to the object that the variable reference.
When writing a method that receives the value of a reference as an argument, we must take care not to accidentally modify the contents of the object that is referenced by default.

6. I. Passing Objects As Arguments to Methods
Passing a reference as an argument
A Rectangle object
changeRectangle(box);
public static void changeRectangle(Rectangle r) {
r.setLength(0.0);
r.setWidth(0.0); }
length: 12.0 width: 5
address

7. I. Passing Objects As Arguments to Methods
Both box and r reference the same object
A Rectangle object
length: 12.0 width: 5
The box variable holds the address of a Rectangle object.
address
The r variable holds the address of a Rectangle object.
address

8. II. Returning Objects from Methods
A method can return a reference to an object.
The address of the object is then returned from the method.

10. II. Returning Objects from Methods
account = getAccount();
A BankAccount object
balance:
address
public static BankAccount getAccount() { (Statements appear here) return new BankAccount(balance); }
The getAccount method returns a reference to a BankAccount object.

11. III. The toString Method
Most classes can benefit from having a method named toString:
Is implicitly called under certain circumstances
Typically, the method toString returns a string that represents the state of an object.
An object's state is simply the data that is stored in the object's fields at any given moment.
For example, the value of the balance field represents the BankAccount object's state at that moment.

12. III. The toString Method
An example of a class, which holds data about a company's stock:
symbol: holds the trading symbol for the company's stock
sharePrice: holds the current price per share of the stock.
UML diagram for Stock class
Stock
- symbol : String - sharePrice : double
+ Stock(sym : String, price : double) + getSymbol() : String + getSharePrice() : double + toString : String

13. III. The toString Method

14. III. The toString Method

15. III. The toString Method

16. III. The toString Method
Evey class automatically has a toString method that returns a string containing:
the object's class name, and
symbol, and
an integer that is usually based on the object's memory address.

17. IV. Writing an equals Method
How to determine whether two objects contain the same data?
We cannot compare them with == operator.
Instead, the class must have a method such as equals for comparing the contents of objects.
Stock com1 = new Stock(“XYZ”, 9.62); Stock com2 = new Stock(“XYZ”, 9.62);
if(com1 == com2) System.out.println(“Both objects are the same.”); else System.out.println(“The objects are different.”);
This is a mistake.

18. IV. Writing an equals Method
symbol: “XYZ” sharePrice: 9.62
A Stock object
The com1 variable holds the address of a Stock address.
address
The if statement compares these two address.
symbol: “XYZ” sharePrice: 9.62
A Stock object
The com2 variable holds the address of a Stock address.
address

19. IV. Writing an equals Method
The equals method accepts a Stock object as its argument.
public boolean equals(Stock object2) { boolean status; //Determine whether this object's symbol and //sharePrice fields are equal to object2's //symbol and sharePrice fields if(symbol.equals(object2.symbol) && sharePrice == object2.sharePrice) status = true; else status = false;
return status; }

20. IV. Writing an equals Method
This code uses the Stock class's equals method to compare two Stock object.
Stock com1 = new Stock("XYZ", 9.62); Stock com2 = new Stock("XYZ", 9.62); if(com1.equals(com2)) System.out.println("Both objects are the same."); else System.out.println("The objects are different.");

21. V. Methods That Copy Objects
How to make a copy of an object?
Stock com1 = new Stock(“XYZ”, 9.62); Stock com2 = com1;
This is a mistake.
- This does not make a copy of the object referenced by com1.
- It makes a copy of address that is stored in com1 and stores that address in com2.

22. V. Methods That Copy Objects
Stock com1 = new Stock(“XYZ”, 9.62); Stock com2 = com1;
symbol: “XYZ” sharePrice: 9.62
A Stock object
The com1 variable holds the address of a Stock address.
address
The com2 variable holds the address of a Stock address.
address
Both variables reference the same object.
This type of assignment operation is called a reference copy because only the address is copied, not the actual object itself.

23. V. Methods That Copy Objects
We should write a method in the class Stock named copy, which returns a copy of a Stock object:
public Stock copy() { //Create a new Stock object and initialize it //with the same data held by the calling object Stock copyObject = new Stock(symbol, sharePrice); //Return a reference to the new object return copyObject; }

24. V. Methods That Copy Objects
Copy Constructors
Another way to create a copy of an object is to use a copy constructor.
A copy constructor is simply a constructor that accepts an object of the same class as an argument.
It makes the object that is being created a copy of the object that was passed as an argument.

25. V. Methods That Copy Objects
The Copy Constructor of the Stock class:
public Stock(Stock object2) { symbol = object2.symbol; sharePrice = object2.sharePrice; }
How to use the copy constructor?
Stock com1 = new Stock(“XYZ”, 9.62); //Create another Stock object that is //a copy of the com1 object Stock com2 = new Stock(com1);