Looking inside classes

Choices

Week 4

• Class bodies contain fields, constructors and methods.

• Fields store values that determine an object’s state.

• Constructors initialize objects.• Methods implement the behavior of objects.• Fields persist (remain) for the lifetime of an

object.• Parameters are used to receive values into a

constructor or method.

Java Class DefinitionsWHAT WE LOOKED AT LAST WEEK

• Conditional Statements

• Local Variables

Java Class Definitions CONCEPTS COVERED THIS WEEK

Java Class DefinitionsREFLECTING ON OUR TICKET MACHINES

Their behavior is inadequate in several ways:

– No checks on the amounts entered.– No refunds.– No checks for a sensible initialization.

How can we do better?

– We need more sophisticated behavior.

Java Class DefinitionsMAKING CHOICES

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

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}

Java Class DefinitionsMAKING CHOICES

‘if’ keywordboolean condition to be tested - gives a true or false result

actions if condition is true

actions if condition is false‘else’ keyword

Making decisions in Java • This is the code to test whether a

student has passed or failed a module, assuming 40% is the pass mark:

if ( studentMark >= 40 ) {      result = "pass";}

else {      result = "fail";} // end if

public class Person{ private String name; private int age; private String job; private float salary; 

public Person(String newName, int newAge) { name = newName; age = newAge;

salary = 0.0f; }

Java Class DefinitionsEXAMPLE CLASS CONSTRUCTOR

if (mark.getAge() == 65) {

mark.changeJob(“RETIRED”);

}

//flow-of-control carries on from here …

Graphically - this is an example of a flow chart

Program - SelectionSELECTION CONSTRUCT: if

yes

nois Mark 40 years old?

retire Mark

//carry on from here …

yes

nois Mark 40 years old?

retire Mark

//carry on from here …

public void payRise (float percentage)

{

if ( getAge() >= 65 ) {

System.out.println("No pay rise for you!");

}

else {

salary = salary + (salary * percentage)

}

//flow-of-control carries on from here …

}

Program - SelectionSELECTION CONSTRUCT: if

Program - SelectionSELECTION CONSTRUCT: if … else

yes noIs Mark retired?

Increase Mark’s Salarydisplay message

flow-of-control carries on from here!

yes noIs Mark retired?

Increase Mark’s Salarydisplay message

flow-of-control carries on from here!

Improved printTicket()

public void printTicket() { if(balance >= price) { // Simulate the printing of a ticket. System.out.println("##################"); System.out.println("# The BlueJ Line"); System.out.println("# Ticket"); System.out.println("# " + price + " cents."); System.out.println("##################"); System.out.println(); // Update the total collected with the price. total = total + price; // Reduce the balance by the price. balance = balance - price; } else { System.out.println("You must insert at least: " + (price - balance) + " more cents."); } }

Uses if else to check if there are sufficient funds to buy a ticket

EQUALITY AND RELATIONAL OPERATORS

Example Meaningbalance == price balance equal to pricebalance != price balance not equal to pricebalance > price balance greater than pricebalance < price balance less than pricebalance >= price balance greater or equal to pricebalance <= price balance less or equal to price

Examplesint janet = 44, john = 12;

janet == john john != 12

john <= 12 7 < john

janet > john

janet >= 44

• The == operator is not a valid way make a comparison of two String objects. It does not compare Strings the way we would expect.

• The String class contains a method that is appropriate. The method returns a boolean, which takes the value true when the string executing the method has exactly that same characters in the same order as the string that is passed as a parameter, and false otherwise. For example

• "hello".equals("hello") returns true• "hello".equals("goodbye") returns false

Logical Operators Logical operator Java Logical operatorcond-1 AND cond-2 &&cond-1 OR cond-2 ||cond-1 EXCLUSIVE OR cond-2 ^ NOT cond-1 !

e.g. if (gender == 1 && age >= 65)

Note the use of the round brackets which MUST surround the whole condition. Extra sets of brackets CAN be used for clarity if desired.

Java Class DefinitionsNESTED SELECTION: if … else

Note that else links with the preceding nearest if statement, from the deepest level of nesting outwards, e.g.

if ( condition1 ) if ( condition2 ) statement1; else statement2;else

statement3;

Q1. If condition1 is true, and condition2 is false, which statement will be executed?

Q2. If condition1 is false, and condition2 is true, which statement will be executed?

Java Class DefinitionsLOCAL VARIABLES

public int refundBalance(){ return balance; balance = 0;}

Why won’t this method compile?

unreachable statement!

Java Class DefinitionsLOCAL VARIABLES

Fields are only one sort of variable.– They store values through the life of an object.– They are accessible by all methods throughout the

class.

Methods can include other (shorter-lived) variables.– They exist only as long as the method is being

executed.– They are only accessible from within the specific

method.

Java Class DefinitionsLOCAL VARIABLES

public int refundBalance(){ int amountToRefund; amountToRefund = balance; balance = 0; return amountToRefund;}

A local variable

No visibilitymodifier

Required ReadingObjects First With Java – A Practical Introduction using

BlueJ

Reading for this week’s lecture

• Chapter 2

Reading for next week’s lecture

• Chapter 3 (pages 56 – 75)