Programming: Java From the Beginning
Chapter 3: Classes and Objects
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Objects
Represent a physical object Have two properties State
Contains one or more items of information Fields or variables
Behavior
Responds to operations that are performed on it May change the state of the object
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Example of an Object
Retractable ball-point pen State Variables
Pen exposed (boolean: true or false) Ink supply (float)
Behavior
Operations (methods)
Press button to expose/not expose pen Replace ink (add to ink supply) Determine how much ink remains Write (use ink)
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Bank Loan Object
State
Variables
Principal Duration Interest Rate Current Balance Interest Collected (to date)
Behavior
Operations (methods)
Initiate a loan (construct a loan) including computation of payment Charge interest Make a payment (amortization schedule) Print totals
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Objects
Write a class definition Write the methods The class/methods are not
executed until "called" from another program
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Class definition
public class ClassName { variables constructors methods }
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Class Definition of a LoanObject
public class LoanObject { // variables double principal; double interest; int months; double balance; double totalInterest; . . . }
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Class Definition of a LoanObject
Constructors Creates an Instance of the Object
Creating a loan A program calls the constructor when a "loan" is
created
Usually passes values of the parameters Java Creates a Default Constructor if no parameters are passed Default Constructor can be overridden in the Class
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Default Constructor
public LoanObject () // () indicates no parameters are passed { principal = 0.0; interest = 0.0; months = 0; balance = 0.0; totalInterest = 0.0 payment = 0.0 }
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Other Constructors
Generally receive parameters from the
calling program
Creating/Constructing a Loan
Parameters:
double principal double interest (yearly rate) int years
Parameters are also called arguments
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Other Constructors
public LoanObject (double argPrin, double argInterest, int argYears) // (arguments go inside the parenthesis) { principal = argPrin; interest =(argInterest / 12.0) * .01; months = argYears * 12 ; balance = argPrin; totalInterest = 0.0 payment = principal * interest / (1 Math.pow(... }
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Create an Instance of the Object
Code goes in the Calling Program
Do NOT put this code in the CLASS
double interest; double principal; int years; String strPrincipal = JOptionPane.showInputDialog ("enter principal") principal = Double.parseDouble(strPrincipal); String strInterest = JOptionPane.showInputDialog ("interest rate?"); interest = Double.parseDouble(strInterest); String strYrs = JopetionPane.showInputDialog ("Number of years?"); years = Integer.parseInt(strYrs); LoanObject LO1 = new LoanObject (principal, interest, years); // yearly interest rate
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Explanation of the Constructor Call
LoanObject LO1 = new LoanObject (principal, interest, years); /* LoanObject is the name of the class LO1 is an instance of the Loan Object -- it is ONE Loan LO1 is a variable name that I created new == calls the constructor named LoanObject
three parameters -- double double integer --> calls the constructor with three parameters double double integer --> error or default constructor if there is no constructor with arguments double double integer */
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Method/Constructor Overloading
Generally there are multiple constructors Different argument lists Known as Constructor Overloading
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Class/Instance Methods
Back to the Class Definition LoanObjects Methods (refer back to slide 4)
Charge Interest Make a Payment (amortization schedule) Print Loan Statistics
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Charge Interest Method
public double interest () { double newBalance = (1 + Interest) * balance; int integerBalance (int) = Math.round(newBalance * 100); newBalance = integerBalance / 100.00; return newBalance; }
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Charge Interest Method
public double interest () { double newBalance = (1 + Interest) * balance; . . return newBalance; } Public double interest () --> no parameters are passed --> () --> one parameter is returned to the calling program. It is a double. Return newBalance
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Other Methods
makePayment printTotals getPayment setPayment
The last two methods were used 1) as an example of a getter and setter method and 2) without them the payment that would print would be the last payment which is generally only a partial payment (see if statement)
are available on the class handout also available at my web site.
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Loan Example
The Class Definition does not execute anything. But it
must be compiled The java program calls the Class constructor and methods.
Only one loan is created in this example. Value of the Object/Class definition is not fully realized until we get into arrays and/or data files
If you do not compile the class definition, the java
program will automatically compile it when you compile the calling program.
Syntax errors received from the calling program can be confusing because the error may be from the class. I suggest you compile the class separately.
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How Objects Are Stored
LoanObject LO1; LO1
LoanObject LO1 = new LoanObject(150000, 12, 30); Loan Object LO1
Principal = 150000. Interest = .01 Months = 360 Balance = 150000. totalInterest = 0
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LoanObjects
LO1stores the location/address of the
object A pointer variable (in C/C++) AKA a reference variable (in Java)
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How Objects Are Stored
LoanObject LO1 = new LoanObject(150000, 12, 30); LoanObject LO2 = LO1
Loan Object LO1
Principal = 150000. Interest = .01 Months = 360 Balance = 150000.
LO2
totalInterest = 0
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LoanObjects
Previous Slide: LO1 and LO2 are two
references/pointers They point to the same object There is only one object, but two pointers to it. To create two objects you need two calls to the constructor
LoanObject LO1 = new LoanObject(...) LoanObject LO2 = new LoanObject(...)
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LoanObjects
LoanObject LO1 = new LoanObject(150000.0, 12.0, 30) LoanObject LO2 = new LoanObject(10000.0, 24.0, 20)
Loan Object
Principal = 150000. Interest = .1 Months = 360 Balance = 150000. totalInterest = 0
LO1
Loan Object
Principal = 10000. Interest = .2
LO2
Months = 240 Balance = 10000. totalInterest = 0
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LoanObjects
LoanObject LO1 = new LoanObject(150000.0, 12.0, 30) LoanObject LO2 = new LoanObject(10000.0, 24.0, 20) LO1 = LO2 Loan Object
Principal = 150000. Interest = .1 Months = 360 Balance = 150000. totalInterest = 0
LO1
Loan Object
Principal = 10000. Interest = .2
LO2
Months = 240 Balance = 10000. totalInterest = 0
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Notes on Previous Slide
The top LoanObject is garbage Once an object has no pointer to it it becomes garbage Will not be able to be referenced again Java has automatic garbage collection Object will be deleted.Memory will be reused.
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Section 3.8 Developing a Fraction Class
A nice example of another class file Please read it
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Getters and Setters
These two instance methods are widely used. A getter method is called from the calling program to set one (or more) of the instance variables. It changes the value of a class variable. A setter method returns a value for one of the instance variables so that the calling program can use the variable.
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Printing an Object
Print the class variables Include an instance method named
toString to print the object Example:
Public String toString () { return "Original Principal: " + principal; }
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Printing an Object
To print an Object In the calling program
System.out.println(LO1);
This statement will call the toString method in the LoanObject Class.
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