1. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 1 Chapter 3 Classes and Objects

2. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 2 3.1Objects as Models A program can be thought of as a model of reality, with objects in the program representing physical objects. Properties of objects: –State (information stored within the object) –Behavior (operations that can be performed on the object)

3. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING MORE ON OBJECTS Objects are variables that are named instances of a class –the class is their type Objects have both data and methods Both the data items and methods of a class are members of the object Data items are also called fields or instance variables Invoking a method means to call the method, i.e., execute the method –Syntax for invoking an object's method: the dot operator objectVariableName.method() –objectVariableName is the calling object Copyright © 2000 W. W. Norton & Company. All rights reserved. 3

4. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 4 Example 1: Ball-point Pen The state of a ball-point pen with a retractable point can be represented by two values: –Is the point of the pen exposed? –How much ink remains in the pen? Operations on a pen include: –Press the button at the end of the pen. –Move the pen with the point held against a sheet of paper. –Replace the pen’s cartridge. –Determine how much ink remains in the pen.

5. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 5 Example 2: Bank Account A state of a bank account includes the account number, the balance, the transactions performed on the account since it was opened, and so forth. For simplicity, let’s assume that the state of a bank account consists of just the balance in the account. Operations on a bank account include: –Deposit money into an account. –Withdraw money from the account. –Check the balance in the account. –Close the account.

6. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 6 Example 3: Car The state of a car includes the amount of fluids in the car, the state of the tires, and even the condition of each part in the car. For programming purposes, we can focus on just a few elements of the state: –Is the engine on? –How much fuel remains in the car’s tank? Operations on a car include: –Start the engine. –Drive a specified distance.

7. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 7 Artificial Objects Nearly every “real-world” object can be modeled within a program. Programmers also work with artificial objects that don’t correspond to objects in the physical world. Like all objects, these artificial objects have state and behavior.

8. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 8 3.2Representing Objects Within a Program In Java, the state of an object is stored in instance variables (or fields). The behavior of an object is represented by instance methods.

9. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 9 Instance Variables Some instance variables will store a single value. Others may store entire objects. Instance variables needed for a ball-point pen: –pointIsExposed ( boolean ) –inkRemaining ( double ) Instance variables needed for a bank account: –balance ( double ) Instance variables needed for a car: –engineIsOn ( boolean ) –fuelRemaining ( double )

10. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 10 Instance Methods In Java, performing an operation on an object is done by calling one of the instance methods associated with the object. An instance method may require arguments when it’s called, and it may return a value. When asked to perform an operation on an object, an instance method can examine and/or change the values stored in any of the object’s instance variables.

11. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 11 Examples of Instance Methods Instance methods for ball-point pens: –pressButton :“Toggles” pointIsExposed. –write : Reduces value of inkRemaining. –replaceCartridge : Restores inkRemaining to its maximum value. –checkInkRemaining : Returns value of inkRemaining. Instance methods for bank accounts: –deposit : Adds an amount to balance. –withdraw : Subtracts an amount from balance. –getBalance : Returns value of balance. –close : Stores zero into balance.

12. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 12 Examples of Instance Methods Instance methods for cars: –startEngine : Stores true into engineIsOn. –stopEngine : Stores false into engineIsOn. –drive : Reduces fuelRemaining by an amount calculated by dividing the distance traveled by the expected fuel consumption. –addFuel : Increases fuelRemaining by a specified amount.

13. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Using Methods Methods are actions that an object can perform. To use a method you invoke or call it. Example of a method call: speciesOfTheMonth.writeOutput() Two basic kinds of methods: –methods that return a single value –void methods that do some action other than returning a value calling object—tells which object will do the action method name—tells which action the object will perform parameter list in parentheses—parameters give info to the method, but in this example there are no parameters

14. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Return Type of Methods All methods require that the return type be specified Return types may be: –a primitive data type, such as char, int, double, etc. –a class, such as String, SpeciesFirstTry, etc. –void if no value is returned You can use a method any place where it is legal to use its return type, for example the readLineInt() method of SimpleIO returns an integer, so this is legal: int next = SimpleIO.readLineInt();

15. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING void Method Example The definition of the writeOutput method of SpeciesFirstTry : Assuming instance variables name, population, and growthRate have been defined and assigned values, this method performs an action (writes values to the screen) but does not return a value public void writeOutput() { System.out.println("Name = " + name); System.out.println("Population = " + population); System.out.println("Growth = " + growthRate + "%"); }

16. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Return Statement Methods that return a value must execute a return statement that includes the value to return For example: public int getCount() { int count; // count is the number of … … return count; }

17. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Method and Class Naming Conventions Good Programming Practice Use verbs to name void methods –they perform an action Use nouns to name methods that return a value –they create (return) a piece of data, a thing Start class names with a capital letter Start method names with a lower case letter

18. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 18 3.3Classes The instance variables and instance methods that belong to a particular kind of object are grouped together into a class. Examples of classes: –BallpointPen –Account –Car

19. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Class Class—definition of a kind of object Like an outline or plan for constructing specific objects –see next slide or diagram in text Example: an Automobile class –Object that satisfies the Automobile definition instantiates the Automobile class Class specifies what kind of data objects of that class have –Each object has the same data items but can have different values Class specifies what methods each object will have –All objects of the same class have the exact same methods Copyright © 2000 W. W. Norton & Company. All rights reserved. 19

20. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 20 Declaring a Class A class declaration contains declarations of instance variables and instance methods. Most class declarations also contain declarations of constructors, whose job is to initialize objects. Form of a class declaration: public class class-name { variable-declarations constructor-declarations method-declarations } The order of declarations usually doesn’t matter.

21. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Class as an Outline Class Name: Automobile Data: amount of fuel ________ speed ________ license plate ________ Methods (actions): increaseSpeed: How: Press on gas pedal. stop: How: Press on brake pedal. First Instantiation: Object name: patsCar amount of fuel: 10 gallons speed: 55 miles per hour license plate: “135 XJK” Second Instantiation: Object name: suesCar amount of fuel: 14 gallons speed: 0 miles per hour license plate: “SUES CAR” Third Instantiation: Object name: ronsCar amount of fuel: 2 gallons speed: 75 miles per hour license plate: “351 WLF” Class Definition Objects that are instantiations of the class

22. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 22 Access Modifiers The declaration of an instance variable, a constructor, or an instance method usually begins with an access modifier ( public or private ). An access modifier determines whether that entity can be accessed by other classes ( public ) or only within the class itself ( private ). The most common arrangement is for instance variables to be private and constructors and instance methods to be public.

23. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 23 Declaring Instance Variables An instance variable declaration looks the same as the declaration of a variable inside a method, except that an access modifier is usually present: private double balance; The only access to balance will be through the instance methods in the Account class. The policy of making instance variables private is known as information hiding.

24. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Information Hiding and Encapsulation Information hiding protect data inside an object do not allow direct access Encapsulation Use classes and objects Objects include both data items and methods to act on the data Cornerstones of Object Oriented Programming (OOP) Both are forms of abstraction

25. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING public and private public any other class or program can directly access or change a public instance variable any other class or program can invoke a public method private only a method in the same class can access a private instance variable only a method in the same class can invoke a private method Instance variables should be private to prevent inappropriate changes.

26. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING A Well-Encapsulated Class Definition Programmer who uses the class Interface: Comments Headings of public methods Public defined constants Implementation: Private instance variables Private constants Private methods Bodies of public and private methods A programmer who uses the class can only access the instance variables indirectly through public methods and constants.

27. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Instance Variables SpeciesFirstTry class has three instance variables: name, population, and growthRate : public means that there are no restrictions on how these instance variables are used. Later we’ll see that these should be declared private instead of public. public String name; public int population; public double growthRate;

28. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Local Variables and Blocks A block (a compound statement) is the set of statements between a pair of matching braces (curly brackets) A variable declared inside a block is known only inside that block –it is local to the block, therefore it is called a local variable –when the block finishes executing, local variables disappear –references to it outside the block cause a compile error

29. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Local Variables and Blocks Some programming languages (e.g., C and C++) allow the variable name to be reused outside the local block –it is confusing and not recommended, nevertheless, it is allowed However, a variable name in Java can be declared only once for a method –although the variable does not exist outside the block, other blocks in the same method cannot reuse the variable's name

30. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING When and Where to Declare Variables Declaring variables outside all blocks but within the method definition makes them available within all the blocks Good programming Practice: declare variables at the beginning of the method initialize variables when you declare them do not declare variables inside loops –it takes time during execution to create and destroy variables, so it is better to do it just once for loops) it is ok to declare loop counters in the Initialization field of for loops, e.g. for(int i=0; i <10; i++)… –the Initialization field executes only once, when the for loop is first entered

31. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 31 Declaring Constructors When an object is created, its instance variables are initialized by a constructor. A constructor looks like an instance method, except that it has no result type and its name is the same as the name of the class itself. A constructor for the Account class: public Account(double initialBalance) { … } A class may have more than one constructor.

32. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Defining Constructors Constructor headings do not include the word void. In fact, constructor headings do not include a return type at all. A constructor with no parameters is called a default constructor. If no constructor is provided, Java automatically creates a default constructor. –If any constructor is provided, then no constructors are created automatically. Programming Tip Include a constructor that initializes all instance variables. Include a constructor that has no parameters. –Include your own default constructor.

33. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Constructor Example public class Pet { private String name; private int age; //in years private double weight; //in pounds... public Pet(String initialName) { name = initialName; age = 0; weight = 0; } Initializes three instance variables: name from the parameter and age and weight with default initial values. Sample use: Pet pet1 = new Pet("Eric");

34. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Example 2 Copyright © 2000 W. W. Norton & Company. All rights reserved. 34 class Test { int i; Test(int j) { i = j; } int get() { return i; } } A constructor is called when an instance of this class is first created. Here it is used to initialize a variable. The code for the constructor, Test(int j) { i = j; } shows that in the process of creating an instance of the class, an inital value for the instance variable is passed as an argument in the constructor. The code here shows a simple class with a constructor and one method. The above code for the constructor, Test(int j) { i = j; }

35. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Example 3 Copyright © 2000 W. W. Norton & Company. All rights reserved. 35 class Test { int i; Test() { i = 0; } int get() { return i; } } A valid class with no constructor explicitly defined. No constructor is defined so the JVM will use the default constructor: Test(){} Java does not actually require an explicit constructor in the class description. If you do not include a constructor, the Java compiler will create a default constructor in the byte code with an empty argument. This default constructor is equivalent to the explicit Test(){}

36. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Using Constructors Always use a constructor after new For example, using the Pet class in text: Pet myCat = new Pet("Calvin", 5, 10.5); –this calls the Pet constructor with String, int, double parameters If you want to change values of instance variables after you have created an object, you must use other methods for the object –you cannot call a constructor for an object after it is created –set (mutator) methods should be provided for this purpose

37. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Programming Tip: You Can Use Other Methods in a Constructor public Pet(String initialName, int initialAge, double initialWeight) { name = initialName; if ((initialAge < 0) || (initialWeight < 0)) { System.out.println("Error…"); } else { age = initialAge; weight = initialWeight; } shorter possibly more consistent with other constructors and methods avoids possible confusion about set parameters less method invocation overhead Use the one your instructor or supervisor prefers. public Pet(String initialName, int initialAge, double initialWeight) { set(initialName, initialAge, initialWeight); }

38. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING More Constructors Default constructor is created only if there are no constructors. If you define any constructor for your class, no default constructor is automatically created. Differences between methods and constructors. –There is no return type given in a constructor signature (header). The value is this object itself so there is no need to indicate a return value. –There is no return statement in the body of the constructor. –The first line of a constructor must either be a call on another constructor in the same class (using this), or a call on the superclass constructor (using super). If the first line is neither of these, the compiler automatically inserts a call to the parameterless super class constructor. Copyright © 2000 W. W. Norton & Company. All rights reserved. 38

39. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Constructors These differences in syntax between a constructor and method are sometimes hard to see when looking at the source. It would have been better to have had a keyword to clearly mark constructors as some languages do. Copyright © 2000 W. W. Norton & Company. All rights reserved. 39

40. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 40 Example: An Account Class Account.java public class Account { // Instance variables private double balance; // Constructors public Account(double initialBalance) { balance = initialBalance; } public Account() { balance = 0.0; }

41. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 41 // Instance methods public void deposit(double amount) { balance += amount; } public void withdraw(double amount) { balance -= amount; } public double getBalance() { return balance; } public void close() { balance = 0.0; }

42. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 42 3.4Creating Objects Once a class has been declared, it can be used to create objects (instances of the class). Each instance will contain its own copy of the instance variables declared in the class. A newly created object can be stored in a variable whose type matches the object’s class: Account acct; Technically, acct will store a reference to an Account object, not the object itself.

43. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Instantiating (Creating) Objects Syntax: className instanceName = new className(); Note the keyword new For example, the text defines a class named SpeciesFirstTry //instantiate an object of this class SpeciesFirstTry speciesOfTheMonth = new SpeciesFirstTry(); Public instance variables can be accessed using the dot operator: speciesOfTheMonth.name = “Klingon ox”; Copyright © 2000 W. W. Norton & Company. All rights reserved. 43

44. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 44 The new Keyword The keyword new, when placed before a class name, causes an instance of the class to be created. A newly created object can be stored in a variable: acct = new Account(1000.00); The acct variable can be declared in the same statement that creates the Account object: Account acct = new Account(1000.00); An object can also be created using the second constructor in the Account class: acct = new Account();

45. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 45 Declaring Instance Methods Parts of an instance method declaration: –Access modifier –Result type. If no value is returned, the result type is void. –Method name –Parameters –Body Outline of the deposit method: public void deposit(double amount) { … }

46. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 46 Method Overloading Java allows methods to be overloaded. Overloading occurs when a class contains more than one method with the same name. The methods must have different numbers of parameters or there must be some difference in the types of the parameters. Overloading is best used for methods that perform essentially the same operation. The advantage of overloading: Fewer method names to remember.

47. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 47 3.5Calling Instance Methods Once an object has been created, operations can be performed on it by calling the instance methods in the object’s class. Form of an instance method call: object. method-name ( arguments ) The parentheses are mandatory, even if there are no arguments.

48. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 48 Calling Account Instance Methods Suppose that acct contains an instance of the Account class. Example calls of Account instance methods: acct.deposit(1000.00); acct.withdraw(500.00); acct.close(); An object must be specified when an instance method is called, because more than one instance of the class could exist: acct1.deposit(1000.00); acct2.deposit(1000.00);

49. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 49 Using the Value Returned by an Instance Method When an instance method returns no result, a call of the method is an entire statement: acct.deposit(1000.00); When an instance method does return a result, that result can be used in a variety of ways. One possibility is to store it in a variable: double newBalance = acct.getBalance(); Another possibility is to print it: System.out.println(acct.getBalance());

50. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 50 How Instance Methods Work Sequence of events when an instance method is called: –The program “jumps” to that method. –The arguments in the call are copied into the method’s corresponding parameters. –The method begins executing. –When the method is finished, the program “returns” to the point at which the method was called.

51. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 51 3.6Writing Programs with Multiple Classes A program that tests the Account class: TestAccount.java public class TestAccount { public static void main(String[] args) { Account acct1 = new Account(1000.00); System.out.println("Balance in account 1: " + acct1.getBalance()); acct1.deposit(100.00); System.out.println("Balance in account 1: " + acct1.getBalance()); acct1.withdraw(150.00); System.out.println("Balance in account 1: " + acct1.getBalance());

52. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 52 acct1.close(); System.out.println("Balance in account 1: " + acct1.getBalance()); Account acct2 = new Account(); System.out.println("Balance in account 2: " + acct2.getBalance()); acct2.deposit(500.00); System.out.println("Balance in account 2: " + acct2.getBalance()); acct2.withdraw(350.00); System.out.println("Balance in account 2: " + acct2.getBalance()); acct2.close(); System.out.println("Balance in account 2: " + acct2.getBalance()); }

53. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 53 Output of the TestAccount program Balance in account 1: 1000.0 Balance in account 1: 1100.0 Balance in account 1: 950.0 Balance in account 1: 0.0 Balance in account 2: 0.0 Balance in account 2: 500.0 Balance in account 2: 150.0 Balance in account 2: 0.0

54. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 54 Compiling a Program with Multiple Classes The TestAccount class, together with the Account class, form a complete program. If the classes are stored in separate files, they could be compiled using the following commands: javac Account.java javac TestAccount.java As an alternative, both files can be compiled with a single command: javac TestAccount.java

55. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 55 Compiling a Program with Multiple Classes When a file is compiled, the compiler checks whether its dependent classes are up-to-date. If the.java file containing a dependent class has been modified since the.class file was created, javac will recompile the.java file automatically. When TestAccount.java is compiled, the javac compiler will look for Account.java and compile it if necessary.

56. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 56 Executing a Program with Multiple Classes Command to execute the TestAccount program: java TestAccount The Account class is not mentioned.

57. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 57 Using a Single File The Account and TestAccount classes can be put in the same file. –The file will need to be named TestAccount.java, because TestAccount contains the main method. –The public access modifier will have to be removed from the beginning of the Account class declaration. (Only one class in a file can be declared public.) Compiling TestAccount.java causes TestAccount.class and Account.class to be generated.

58. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 58 Using a Single File It’s often better to put only one class in each file. Advantages: –Classes are easier to locate. –Files are smaller and easier to edit. –If a class declaration is changed, only the class itself will have to be recompiled.

59. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 59 3.7How Objects Are Stored A variable of an ordinary (non-object) type can be visualized as a box: int i; Assigning a value to the variable changes the value stored in the box: i = 0;

60. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 60 Object Variables An object variable, on the other hand, doesn’t actually store an object. Instead, it will store a reference to an object. An object variable can still be visualized as a box: Account acct; Suppose that a new object is stored into acct : acct = new Account(500.00);

61. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 61 Object Variables The Account object isn’t stored in the acct box. Instead, the box contains a reference that “points to” the object: In many programming languages, including C++, a variable such as acct would be called a pointer variable.

62. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 62 The null Keyword To indicate that an object variable doesn’t currently point to an object, the variable can be assigned the value null : acct = null; When an object variable stores null, it’s illegal to use the variable to call an instance method. If acct has the value null, executing the following statement will cause a run-time error ( NullPointerException ): acct.deposit(500.00);

63. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 63 Object Assignment If i has the value 10, assigning i to j gives j the value 10 as well: j = i; Changing the value of i has no effect on j : i = 20; Assignment of objects doesn’t work the same way.

64. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 64 Object Assignment Assume that acct1 contains a reference to an Account object with a balance of $500. Assigning acct1 to acct2 causes acct2 to refer to the same object as acct1 : acct2 = acct1; acct1 and acct2 are said to be aliases, because both represent the same object.

65. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 65 Object Assignment An operation that changes the acct1 object will also change the acct2 object, and vice-versa. The statement acct1.deposit(500.00); will change the balance of acct2 to $1000.00:

66. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 66 Cloning Some classes allow the creation of a new object that’s identical to an existing object. The new object is said to be a clone of the old one. Clones are created by calling the clone method.

67. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 67 Garbage Objects can become “orphaned” during program execution. Consider the following example: acct1 = new Account(100.00); acct2 = new Account(200.00); acct1 = acct2; After these assignments, the object that acct1 previously referred to is lost. We say that it is garbage.

68. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 68 Garbage

69. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 69 Garbage Collection Java provides automatic garbage collection: as a Java program runs, a software component known as the garbage collector watches for garbage and periodically “collects” it. The recycled memory can be used for the creation of new objects. Garbage collection normally takes place when the program isn’t doing any other useful activity. Java is the first widely used programming language to incorporate garbage collection.

70. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 70 Memory Leaks Other popular languages rely on the program to explicitly release memory that’s no longer needed. This practice is potentially more efficient, but it’s also error-prone. Failing to recover garbage causes available memory to decrease (a memory leak). After a period of time, a program with a memory leak may run out of memory entirely. Releasing memory prematurely is even worse, often causing programs to crash.

71. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 71 3.8Developing a Fraction Class Fractions can be thought of as objects, so it’s not hard to develop a Fraction class. A Fraction object will need to store a numerator and a denominator. Both are integers. There are many potential operations on fractions, including adding, subtracting, multiplying, and dividing.

72. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 72 A First Attempt A first attempt at writing the Fraction class: public class Fraction { private int numerator; private int denominator; public Fraction(int num, int denom) { numerator = num; denominator = denom; } // Methods will go here } A Fraction object will be created as follows: Fraction f = new Fraction(4, 8);

73. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 73 Getters and Setters The Fraction class will need methods named getNumerator and getDenominator : public int getNumerator() { return numerator; } public int getDenominator() { return denominator; } An instance method that does nothing but return the value of an instance variable is said to be an accessor (or a getter).

74. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 74 Getters and Setters By convention, names of getters start with the word get. Sample calls of getNumerator and getDenominator : int num = f.getNumerator(); int denom = f.getDenominator(); An instance method that stores its parameter into an instance variable is said to be a mutator (or setter). Names of setters begin with the word set.

75. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 75 Getters and Setters Potential setters for the Fraction class: public void setNumerator(int num) { numerator = num; } public void setDenominator(int denom) { denominator = denom; } Sample calls of setNumerator and setDenominator : f.setNumerator(5); f.setDenominator(6);

76. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 76 Immutable Objects Setters can be useful, because they allow us to change data stored in private variables. In some cases, however, we may not want to allow changes to an object’s instance variables. Such an object is said to be immutable (unchangeable). The advantage of making objects immutable is that they can be shared without problems. Some of the classes in the Java API have this property, including the String class.

77. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 77 Writing the add Method A method that adds Fraction objects f1 and f2 would need to be called in the following way: Fraction f3 = f1.add(f2); add would have the following appearance: public Fraction add(Fraction f) { … } The parameter f represents the second of the two fractions to be added.

78. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 78 Writing the add Method A first attempt at writing the add method: public Fraction add(Fraction f) { int num = numerator * f.getDenominator() + f.getNumerator() * denominator; int denom = denominator * f.getDenominator(); Fraction result = new Fraction(num, denom); return result; } numerator and denominator refer to the numerator and denominator of the Fraction object that’s calling add.

79. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 79 Writing the add Method The add method can be shortened slightly by combining the constructor call with the return statement: public Fraction add(Fraction f) { int num = numerator * f.getDenominator() + f.getNumerator() * denominator; int denom = denominator * f.getDenominator(); return new Fraction(num, denom); }

80. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 80 Writing the add Method The add method can be further simplified by having it access f ’s numerator and denominator variables directly: public Fraction add(Fraction f) { int num = numerator * f.denominator + f.numerator * denominator; int denom = denominator * f.denominator; return new Fraction(num, denom); } Instance variables are accessed using a dot, just as instance methods are called using a dot.

81. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 81 Adding a toString Method The value stored in a Fraction object named f could be printed in the following way: System.out.println(f.getNumerator() + "/" + f.getDenominator()); The following method makes it easier to print fractions: public String toString() { return numerator + "/" + denominator; } In Java, the name toString is used for a method that returns the contents of an object as a string.

82. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 82 Adding a toString Method The toString method makes it easier to display the value stored in a Fraction object: System.out.println(f.toString()); The statement can be shortened even further: System.out.println(f); When given an object as its argument, System.out.println will automatically call the object’s toString method.

83. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 83 3.9Java’s String Class The Java API provides a huge number of prewritten classes. Of these, the String class is probably the most important. Instances of the String class represent strings of characters. The String class belongs to a package named java.lang. The java.lang package is automatically imported into every program. (No other package has this property.)

84. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 84 Creating Strings In Java, every string of characters, such as "abc", is an instance of the String class. String variables can be assigned String objects as their values: String str1, str2; String is the only class whose instances can be created without the word new : str1 = "abc"; This is an example of magic.

85. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 85 Visualizing a String A String object can be visualized as a series of characters, with each character identified by its position. The first character is located at position 0. A visual representation of the string "Java rules!" :

86. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 86 Common String Methods The String class has a large number of instance methods. Assume that the following variable declarations are in effect: String str1 = "Fat cat", str2; char ch; int index; The charAt method returns the character stored at a specific position in a string: ch = str1.charAt(0); // Value of ch is now 'F' ch = str1.charAt(6); // Value of ch is now 't'

87. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 87 Common String Methods One version of the indexOf method searches for a string (the “search key”) within a larger string, starting at the beginning of the larger string. Example: Locating the string "at" within str1 : index = str1.indexOf("at"); After this assignment, index will have the value 1. If "at" had not been found anywhere in str1, indexOf would have returned –1.

88. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 88 Common String Methods The other version of indexOf begins the search at a specified position, rather than starting at position 0. This version is particularly useful for repeating a previous search to find another occurrence of the search key. Example: Finding the second occurrence of "at" in str1 : index = str1.indexOf("at", index + 1); index will be assigned the value 5.

89. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 89 Common String Methods lastIndexOf is similar to indexOf, except that searches proceed backwards, starting from the end of the string. Example: Finding the last occurrence of "at" in str1 : index = str1.lastIndexOf("at"); The value of index after the assignment will be 5.

90. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 90 Common String Methods The second version of lastIndexOf begins the search at a specified position. Example: Finding the next-to-last occurrence of "at" : index = str1.lastIndexOf("at", index - 1); The value of index after the assignment will be 1. The String class has additional versions of indexOf and lastIndexOf, whose first argument is a single character rather than a string.

91. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 91 Common String Methods The length method returns the number of characters in a string. For example, str1.length() returns the length of str1, which is 7. The substring method returns a substring: a series of consecutive characters within a string. One version of substring selects a portion of a string beginning at a specified position: str2 = str1.substring(4); After the assignment, str2 will have the value "cat".

92. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 92 Common String Methods The other version of substring accepts two arguments: –The position of the first character to include in the substring –The position of the first character after the end of the substring Example: str2 = str1.substring(0, 3); After the assignment, str2 will have the value "Fat".

93. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 93 Common String Methods toLowerCase and toUpperCase will convert the letters in a string to lowercase or uppercase. After the assignment str2 = str1.toLowerCase(); the value of str2 is "fat cat". After the assignment str2 = str1.toUpperCase(); the value of str2 is "FAT CAT". Characters other than letters aren’t changed by toLowerCase and toUpperCase.

94. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 94 Common String Methods The trim method removes spaces (and other invisible characters) from both ends of a string. After the assignments str1 = " How now, brown cow? "; str2 = str1.trim(); the value of str2 will be "How now, brown cow?"

95. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 95 Chaining Calls of Instance Methods When an instance method returns an object, that object can be used to call another instance method. For example, the statements str2 = str1.trim(); str2 = str2.toLowerCase(); can be combined into a single statement: str2 = str1.trim().toLowerCase();

96. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 96 Using + to Concatenate Strings One of the most common string operations is concatenation: joining two strings together to form a single string. The String class provides a concat method that performs concatenation, but it’s rarely used. Concatenation is so common that Java allows the use of the plus sign ( + ) to concatenate strings: str2 = str1 + "s"; str2 now contains the string "Fat cats".

97. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 97 Using + to Concatenate Strings The + operator works even if one of the operands isn’t a String object. The non- String operand is converted to string form automatically: System.out.println("Celsius equivalent: " + celsius);

98. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 98 Using + to Concatenate Strings If the + operator is used to combine a string with any other kind of object, the object’s toString method is called. The statement System.out.println("Value of fraction: " + f); has the same effect as System.out.println("Value of fraction: " + f.toString());

99. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 99 Using + to Concatenate Strings In order for the + operator to mean string concatenation, at least one of its two operands must be a string: System.out.println("Java" + 1 + 2); // Prints "Java12" System.out.println(1 + 2 + "Java"); // Prints "3Java"

100. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 100 Using + to Concatenate Strings The + operator is useful for breaking up long strings into smaller chunks: System.out.println( "Bothered by unsightly white space? " + "Remove it quickly and\neasily with " + "the new, improved trim method!");

101. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 101 Using + to Concatenate Strings The += operator can be used to add characters to the end of a string: String str = "The quick brown fox "; str += "jumped over "; str += "the lazy dog."; The final value of str will be "The quick brown fox jumped over the lazy dog." Concatenating a number with an empty string will convert the number to string form. For example, if i contains 37, then i + "" is the string "37".

102. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 102 Program: Decoding a Vehicle Identification Number The manufacturer of a vehicle assigns it a unique identifying number, called the Vehicle Identification Number (VIN). A VIN packs a large amount of information into a 17-character string:

103. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 103 The Check Digit in a VIN The check digit in a VIN is computed from the other characters in the VIN; its purpose is to help detect errors. The check digit algorithm used in vehicle identification numbers will catch most common errors, such as a single incorrect character or a transposition of two characters.

104. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 104 The VIN Program The VIN program will split a VIN into its constituent pieces. The VIN is entered by the user when prompted: Enter VIN: JHMCB7658LC056658 World manufacturer identifier: JHM Vehicle description section: CB765 Check digit: 8 Vehicle identification section: LC056658

105. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 105 VIN.java // Displays information from a VIN entered by the user import jpb.*; public class VIN { public static void main(String[] args) { // Prompt the user to enter a VIN SimpleIO.prompt("Enter VIN: "); String vin = SimpleIO.readLine(); // Extract the parts of the VIN String manufacturer = vin.substring(0, 3); String description = vin.substring(3, 8); String checkDigit = vin.substring(8, 9); String identification = vin.substring(9);

106. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 106 // Display the parts of the VIN System.out.println("World manufacturer identifier: " + manufacturer); System.out.println("Vehicle description section: " + description); System.out.println("Check digit: " + checkDigit); System.out.println("Vehicle identification section: " + identification); }

107. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 107 A Condensed Version of the VIN Program VIN2.java // Displays information from a VIN entered by the user import jpb.*; public class VIN2 { public static void main(String[] args) { // Prompt the user to enter a VIN SimpleIO.prompt("Enter VIN: "); String vin = SimpleIO.readLine(); // Display the parts of the VIN System.out.println("World manufacturer identifier: " + vin.substring(0, 3)); System.out.println("Vehicle description section: " + vin.substring(3, 8)); System.out.println("Check digit: " + vin.substring(8, 9)); System.out.println("Vehicle identification section: " + vin.substring(9)); }

108. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 108 3.10Case Study: Checking an ISBN Number An ISBN (International Standard Book Number) is a unique number assigned to a book when it’s published, such as 0–393–96945–2. The number at the end is a check digit that’s calculated from the other digits in the ISBN. Our goal is to write a program named CheckISBN that calculates the check digit for an ISBN entered by the user: Enter ISBN: 0-393-96945-2 Check digit entered: 2 Check digit computed: 2

109. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 109 Design of the CheckISBN Program The CheckISBN program will have four steps: 1. Prompt the user to enter an ISBN. 2. Compute the check digit for the ISBN. 3. Display the check digit entered by the user. 4. Display the computed check digit. The ISBN will be stored as a string, and the other variables will be integers.

110. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 110 Computing the Check Digit The check digit is calculated by multiplying the first nine digits in the number by 10, 9, 8, …, 2, respectively, and summing these products to get a value we’ll call total. The check digit is now determined by the expression 10 – ((total – 1) mod 11) The value of this expression is a number between 0 and 10. If the value is 10, the check digit is X.

111. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 111 Computing the Check Digit Computation of the check digit for the ISBN 0– 393–96945–2: total= 0  10 + 3  9 + 9  8 + 3  7 + 9  6 + 6  5 + 9  4 + 4  3 + 5  2 = 0 + 27 + 72 + 21 + 54 + 30 + 36 + 12 + 10 = 262 Check digit: 10 – ((262 – 1) mod 11) = 10 – (261 mod 11) = 10 – 8 = 2

112. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 112 Extracting Digits from the ISBN In order to compute the check digit, the first nine digits in the ISBN must be extracted and converted to numeric form. Since the position of the first two dashes may vary, the program will need to search for them. Once the dashes have been found, the program can extract the language code, publisher, and book number and join these into a single string, the “reduced ISBN.” If the original ISBN is "0-393-96945-2", the reduced ISBN will be "039396945".

113. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 113 Extracting Digits from the ISBN Searching for the dashes can be done by calling the indexOf method. The substring method can extract a portion of the original ISBN. The + operator can put the pieces together to form the reduced ISBN. The following expression extracts a digit and converts it to a number: Integer.parseInt(reducedISBN.substring(i, i + 1)) i is the position of the digit in the reduced ISBN.

114. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 114 Displaying the Check Digit If the check digit is 10, the program will need to display the letter X instead of a normal digit. This problem can be solved by creating a string containing the digits from 0 to 9, plus the letter X: final String DIGITS = "0123456789X"; The value of the check digit can be used to select one of the characters in DIGITS. If the check digit is stored in the variable checkDigit, the expression will be DIGITS.charAt(checkDigit)

115. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 115 CheckISBN.java // Program name: CheckISBN // Author: K. N. King // Written: 1998-04-17 // Modified: 1999-02-11 // // Prompts the user to enter an ISBN number. Computes the // check digit for the ISBN. Displays both the check digit // entered by the user and the check digit computed by the // program. import jpb.*; public class CheckISBN { public static void main(String[] args) { // Prompt the user to enter an ISBN SimpleIO.prompt("Enter ISBN: "); String originalISBN = SimpleIO.readLine();

116. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 116 // Determine location of dashes int dashPos1 = originalISBN.indexOf("-"); int dashPos2 = originalISBN.indexOf("-", dashPos1 + 1); // Remove dashes from ISBN String reducedISBN = originalISBN.substring(0, dashPos1) + originalISBN.substring(dashPos1 + 1, dashPos2) + originalISBN.substring(dashPos2 + 1, 11); // Compute the check digit for the ISBN int total = 10 * Integer.parseInt(reducedISBN.substring(0, 1)) + 9 * Integer.parseInt(reducedISBN.substring(1, 2)) + 8 * Integer.parseInt(reducedISBN.substring(2, 3)) + 7 * Integer.parseInt(reducedISBN.substring(3, 4)) + 6 * Integer.parseInt(reducedISBN.substring(4, 5)) + 5 * Integer.parseInt(reducedISBN.substring(5, 6)) + 4 * Integer.parseInt(reducedISBN.substring(6, 7)) + 3 * Integer.parseInt(reducedISBN.substring(7, 8)) + 2 * Integer.parseInt(reducedISBN.substring(8, 9)); int checkDigit = 10 - ((total - 1) % 11);

117. Chapter 3: Classes and Objects Java Programming FROM THE BEGINNING Copyright © 2000 W. W. Norton & Company. All rights reserved. 117 // Display the check digit entered by the user System.out.println("Check digit entered: " + originalISBN.charAt(12)); // Display the computed check digit final String DIGITS = "0123456789X"; System.out.println("Check digit computed: " + DIGITS.charAt(checkDigit)); }