ACO 101:Introduction to Computer Science

Anatomy Part 2: Methods

Structure of a Class// A Bank Account with deposit/withdraw

public class BankAccount {private long balance;private String accountNumber;private String accountName;private int homeBranch;

//create a new bank account object

public BankAccount( String number, String name ) { }

/** deposit or credit to account */public void credit( long amount ) {

assert amount>=0 : "Error: negative

deposit";balance += amount;

}

comment describes the class

Beginning of class definition, including accessibility

Attributes of the class

A constructor

A method with a parameter

BankAccount Class (continued)// BankAccount class continued...

public long getBalance( ) {return balance;

}public void setBalance(long balance) {

if ( balance >= 0 )this.balance =

balance;else

ErrorMessage("invalid ...");}

An accessor method: returns value of an attribute.

A mutator method: changes value of an attribute; also performs data checking.

What is a Method?Programming view: a method is a function. It can return a value or not.

Design view: methods define the behavior of objects. Methods are the way by which objects communicate

// define a "deposit" behavior for a bank account.public class BankAccount {

private long balance; // acct balance attribute public void deposit( long amount ) {

balance = balance + amount;}

//... more methods and variables (attributes)}

Invoking a MethodTo invoke the deposit method, you must use is as a behavior of a particular BankAccount object.Example:

// define a "deposit" behavior for a bank account.BankAccount myAcct = new BankAccount(“1111111");

// read some deposit datastring s = this.txtDepositAmount.Text;decimal money = Convert.ToDecimal(s);

// method: deposit the money in my accountmyAcct.deposit( money );

call "deposit" method of a BankAccount object

Writing a Method (1)A method consists of these parts.

// return the maximum of 2 double valuespublic double Max ( double a, double b ) {

if ( a > b ) return a;else return b;

}

Access Control:who can use this method?

Type of value returned by this method. "void" if nothing is returned.

Method Name

ParametersMethod

Body, with returned

value.

Scope of Methods• All methods (functions) must be part of a class. • This avoids name conflicts. The max( ) method of the MyMath class

does not conflict with the max( ) method of an other class.

/** My own Math library */public class MyMath {

/** a max method */public static double max( double x, double y)

{ if ( x >= y ) return x;else return y;

}//... more methods and attributes

}

Scope of Methods (2)• From inside of the class, you can refer to a method using just its name.• From outside of the class, you must use a class name (static methods)

or object name (instance method) to call a method. (We will go over the difference between static and instance methods later)

public class TestMyMath {public static void main( String [] args ) {

Scanner console = new Scanner(System.in);double x = console.nextDouble( );double y = console.nextDouble( );// call "max" of MyMath class:double r1 = MyMath.Max( x, y );// call "max" of C# Math class:double r2 = Math.Max( x, y );

}

Visibility (Accessibility) of MethodsYou determine what objects or parts of the

program can access an object's methods.• private: method can only be invoked by objects of this class.• protected: method can be invoked by other code in the same package,

and by objects of this class and its subclasses.• public: method can be invoked by any program.

public void deposit( long amount ) { /* body of the method */

}

visibility type of returned value. "void" if method doesn't return any value.

Returned Value of a Method

class BankAccount {

public void deposit(long amount) {balance += amount;

}public long getBalance( ) { return balance; }

A method may return a value. The type of returned value must be declared in the method header.

A method which doesn't return any value should have a return type of "void".

In the method body, use "return <expression>".

void means this method does not return a value.