Classes and Methods: Classes

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1 Class declaration Syntax: [<import_statements>] Classes and Methods: Classes [<modifiers>] class <identifier> [<instance_variable_declarations>] [<constructor_declarations>] [<method_declarations>] When writing a program, must decide what classes you need Once this is decided, must decide for each class What instance variables (data members) are required What methods are required What their visibility should be Classes represent things that have properties (instance variables) and can perform actions (methods) In general, nouns become classes and instance variables Verbs become methods Class names start with capital letters There is a file for each class The file nanme must be the same as the class name: Class MyClass will be saved in file MyClass.java 1

2 Classes and Methods: Instance Variables Declaration syntax (for single declaration): [<visibility modifier>] [final] <data type> <identifier> [= <expression>]; Represent variables whose values are specific to a particular instance of the class Memory representation: 2

3 Represent actions a class can take Classes and Methods: Methods For example, a savings account class would have methods for making deposits and withdrawals General syntax for a method: [<modifiers>] <return_type> <identifier> ([<parameter_list>]) [<body>] Body is a sequence of Java statements May include declarations Example: public void (double amt) balanace = balance - amt; Variables declared within the method body are called local variables They are only accessible from within the method body The return type can be any Java type The return type void means that this method does not return a value A method returns a value with the return statement Syntax: return <expression> Example: public double getbalance() return balance; 3

4 Methods are executed by calling them Classes and Methods: Method Calls Syntax: [<object or class id>.]<method id>([<argument list>]) Semantics: Arguments are evaluated and their values copied into the AR Control passes to first statement in body Statements in body executed If a return statement is executed, it ends the execution of the method If it returns a value, it is sent back to the point of the call Sample calls: amount = account.getbalance(); account.withdraw(50.0); 4

5 Classes and Methods: Visibility Modifiers Visibility modifiers determine what can be accessed where Programs should be designed so that all access to the instance variables of a class are controlled by the programmer A program that uses a class should not be able to change instance variables at will Modifier private means that a class component can only be accessed from within the class itself Modifier public means that a class component can only be accessed from outside In general, instance variables are declared private, most methods declared public 5

6 Classes and Methods: Accessors and Mutators (Getters and Setters) Since instance variables are usually declared private, how can a program see or change their values? The answer is that the creator of a class creates methods to do these tasks This way, the creator of the class is able to control how a program can access the instance variables There are two types: 1. Accessors Method that returns the value of an object s instance variable Usually named get<value> Often called get methods Example: public double getbalance() return balance; 2. Mutators Method that changes the value of a method s instance variable Usually named set<value> Often called set methods Example: public void setbalance(double amt) balance = amt ; 6

7 Classes and Methods: Constructors Constructor is a method that creates an instance of a class Every class must have at least one constructor Syntax: [<modifiers>] <class_id> ([<parameter_list>]) body Note that syntax same as for regular methods except No return type class id must be the name of the class Important task of constructor is to initialize instance variables Could use a mutator, but they allow setting of variable multiple times Initialization is usually intended to occur one time only By performing initialization from within the constructor, will only happen once - when object created Example: public Account (int acctnum, double startingamount) accountnumber = acctnum; balance = startingamount; Default constructors If programmer does not provide a constructor, Java will generate a default Default is always of form <class_id> () Instance variables are initialized to zero (or equivalent) If the programmer provides a constructor, Java will NOT create a default 7

8 public class Account private int accountnumber; private double balance; Classes and Methods: A Complete Class public Account (int acctnum, double startingamount) accountnumber = acctnum; balance = startingamount; public void deposit (double amt) balance = balance + amt; public void withdraw (double amt) balance = balance - amt; public double getbalance () return balance; public int getaccountnumber () return accountnumber; 8

9 Classes and Methods: Complete Programs 1. A complete program consists of one or more classes, one of which is runnable A runnable class is one which has a main method 2. Example: public class Runner public static void main(string args[]) Account acct = new Account(117292, 150.0); Account acct2 = new Account(7755, 800.0); System.out.println("Starting balance for account " + acct.getaccountnumber() + " = " + acct.getbalance()); System.out.println("Deposit $25.31"); acct.deposit(25.31); System.out.println("Withdraw $30.00"); acct.withdraw(30.0); System.out.println("Withdraw $72.95"); acct.withdraw(72.95); System.out.println("End balance for account " + acct.getaccountnumber() + " = " + acct.getbalance()); System.out.println(); System.out.println("Starting balance for account " + acct2.getaccountnumber() + " = " + acct2.getbalance()); System.out.println("Withdraw $425.00"); acct2.withdraw(425.0); System.out.println("Deposit $325.50"); acct2.deposit(325.50); System.out.println("Deposit $95.00"); acct.deposit(95.0); System.out.println("End balance for account " + acct2.getaccountnumber() + " = " + acct2.getbalance()); 9

class declaration Designing Classes part 2 Getting to know classes so far Review Next: 3/18/13 Driver classes:

class declaration Designing Classes part 2 Getting to know classes so far Review Next: 3/18/13 Driver classes: Designing Classes part 2 CSC 1051 Data Structures and Algorithms I Getting to know classes so far Predefined classes from the Java API. Defining classes of our own: Driver classes: Account Transactions