CS 1054 Lecture 3 Chapter 2 Understanding class

Example of a Ticket Machine Function: Prints a ticket when a customer inserts the

Storing Information … - - Price of the ticket Balance (amount of money entered

Operations on the machine n n Get the price of the ticket? What is

public class Ticket. Machine { private int price; private int balance; private int total;

Basic Elements n Here are three basic elements of class definitions: public class Class.

Methods n Methods have two parts: n n a header and a body Here

Types of Methods n Accessor Methods n n Methods that simply return values from

Accessor methods return type visibility modifier method name parameter list (empty) get. Price() public

Mutator methods visibility modifier return type (void) method name parameter public void insert. Money(int

Method vs. Constructor You can tell a method apart from a constructor from the

Local variables n Fields are one sort of variable. n n n They store

Local variables A local variable No visibility modifier public int refund. Balance() { int

Scope n n n The scope of a variable means where the variable can

Lifetime n n n The lifetime of a variable means how long a variable

Assignment n n n Values are stored into fields (and other variables) via assignment

Examples of assignment int A; int B; A = 10; B = 20; A

Reflecting on the ticket machines n Their behavior is inadequate in several ways: n

Making choices public void insert. Money(int amount) { if(amount > 0) { balance +=

Making choices ‘if’ keyword boolean condition to be tested - gives a true or

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CS 1054: Lecture 3, Chapter 2 Understanding class definitions 1. 0

Example of a Ticket Machine Function: Prints a ticket when a customer inserts the correct money for their fare. Note: A ticket machine may offer tickets of more than one type from which a customer chooses one.

Storing Information … - - Price of the ticket Balance (amount of money entered by customer) Total (amount of money collected by the machine so far)

Operations on the machine n n Get the price of the ticket? What is the balance so far? Insert Money What is the total amount of money collected?

public class Ticket. Machine { private int price; private int balance; private int total; public Ticket. Machine(int ticket. Cost) { price = ticket. Cost; balance = 0; total = 0; } public int get. Price() { return price; } public int get. Balance() { return balance; } public void insert. Money(int amount) { balance += amount; } } public void print () { total += balance; balance =0; }

Basic Elements n Here are three basic elements of class definitions: public class Class. Name { Fields Constructors Methods } The contents of a class

Methods n Methods have two parts: n n a header and a body Here is an example header /** * Return the price of a ticket */ public int get. Price() n

Types of Methods n Accessor Methods n n Methods that simply return values from the object Mutator Methods n Methods that change the state of the object

Accessor methods return type visibility modifier method name parameter list (empty) get. Price() public int { return price; } start and end of method body (block) return statement

Mutator methods visibility modifier return type (void) method name parameter public void insert. Money(int amount) { assignment statement balance += amount; } field being changed

Method vs. Constructor You can tell a method apart from a constructor from the signature for each public Ticket. Machine( int ticket. Cost ) public int get. Price() n n n The constructor can have no return type The constructor must match the name of the class

Local variables n Fields are one sort of variable. n n n They store values through the life of an object. They are accessible throughout the class. Methods can include shorter-lived variables. n n They exist only as long as the method is being executed. They are only accessible from within the method.

Local variables A local variable No visibility modifier public int refund. Balance() { int amount. To. Refund; amount. To. Refund = balance; balance = 0; return amount. To. Refund; }

Scope n n n The scope of a variable means where the variable can be accessed The scope of a parameter is only in the method that it has been passed to The scope of an instance variable is the entire class

Lifetime n n n The lifetime of a variable means how long a variable can be accessed The lifetime for a parameter to a method is only the duration of the method. The lifetime for an instance variable is the same as for the object

Assignment n n n Values are stored into fields (and other variables) via assignment statements: n variable = expression; n price = ticket. Cost; A variable stores a single value, so any previous value is lost. variable = variable + expression; Can be shortened to: variable += expression;

Examples of assignment int A; int B; A = 10; B = 20; A = B; What will be the value of A? What will be the value of B? A = A + B; What will be the value of A? What will be the Value of B? A -= B; What will be the value of A? What will be the value of B? A *= B; What will be the value of A? What will be the value of B?

Reflecting on the ticket machines n Their behavior is inadequate in several ways: n n No checks on the amounts entered. No refunds. No checks for a sensible initialization. How can we do better? n We need more sophisticated behavior.

Making choices public void insert. Money(int amount) { if(amount > 0) { balance += amount; } else { System. out. println("Use a positive amount: " + amount); } }

Making choices ‘if’ keyword boolean condition to be tested - gives a true or false result actions if condition is true if(perform some test) { Do the statements here if the test gave a true result } else { Do the statements here if the test gave a false result } ‘else’ keyword actions if condition is false