COMP 110: Introduction to Programming Tyler Johnson Mar 16, 2009 MWF 11:00AM-12:15PM Sitterson 014.

COMP 110:Introduction to Programming

Tyler JohnsonMar 16, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

Questions?


Program 3

Example solutions


Today in COMP 110

Constructors

Static Variables & Methods


Constructors

Section 6.1 in text


Creating Objects

Student jack = new Student();

Why does this look like a method call?

Because it is

This is a call to a special method called a constructor


Constructors

A constructor is a special method that is called when an object is created using new

The purpose of a constructor is to perform initializing actions

e.g. initialize the instance variables of an object


Constructors

The purpose of a constructor is similar to that of a mutator (setter)

Used to set the value of variable(s)

However, constructors create an object in addition to initializing it


Example: Pet class

public class Pet {

private String name; private int age; private double weight;

public Pet() { name = “No name yet.”; age = 0; weight = 0; }

public Pet(String initName, int initAge, double initWeight) { name = initName; age = initAge; weight = initWeight; }}


Constructors

A constructor must have the same name as its class

The constructor for the class Pet is Pet()The constructor for the class Student is Student()

Constructors do NOT specify a return type

Not even void


Constructors

The classes you have used up to this point use a constructor created automatically by Java

Gives default values to instance variables• May not be what you want

You can specify how instance variables should be initialized by creating your own constructors


Constructors w/ Parameters

Like methods, constructors can have parameters

public Pet(String initName, int initAge, double initWeight) { name = initName; age = initAge; weight = initWeight; }


Default Constructor

A constructor that takes no arguments is called a default constructor

public Pet() {name = “No name yet.”;age = 0;weight = 0;

}

Java automatically defines a default constructor if you do not define any constructors


Multiple Constructors

You can define multiple constructors

All have the same name, but different parameters

Group their definitions together


Constructors

You cannot call a constructor on an existing object

Pet myPet = new Pet();myPet.Pet("Roberto", 1, 150.0); //error

Must use mutators on objects that have already been created

myPet.setPet("Roberto", 1, 150.0); //ok


Calling Methods within Constructors

Just like calling methods within methods

/*constructor*/public Pet(String initName, int initAge, double initWeight) {

setPet(initName, initAge, initWeight); //have the mutator perform the set}

/*mutator*/public void setPet(String newName, int newAge, double newWeight) {

name = newName; age = newAge; weight = newWeight;}

16


Static Variables & Methods

Section 6.2 in text


The Keyword Static

The keyword static is a specifier that can be applied to instance variables and methods in Java

You are already familiar with some other specifiers such as public, private, final

public class Student {

private double gpa;

public double getGPA() {return gpa;

}}


The Keyword Static

The keyword static is used to indicate that only ONE copy of the instance variable or method should exist for the entire class

public class UnitsAndMeasures {

//static, all objects share the SAME copy of this variablepublic static final int FEET_PER_YARD = 3;

//NOT static, all objects have their OWN copy of this variableprivate int feet;

}


Example: Static Instance Variables

A class that counts the number of method calls to ALL of its objects

public class StaticExample {

//static, all objects share the SAME copy of this variableprivate static numberOfCalls = 0;

public void method() { numberOfCalls++;}

}

public class StaticExampleTester {

public static void main(String[] args) { StaticExample se = new StaticExample(); StaticExample se2 = new StaticExample();

se.method(); //changes numberOfCalls to 1 se2.method(); //changes numberOfCalls to 2}

}


The Keyword Static

The keyword static can also be used with methods

The main method

public static void main() { StaticExample se = new StaticExample(); StaticExample se2 = new StaticExample();

se.method(); //changes numberOfCalls to 1 se2.method(); //changes numberOfCalls to 2

}


Using the Keyword Static

When should you use the keyword static with a method?

When a method does not access instance variables

public int pow(int x, int y) {

int result = 1;for(int i = 0; i < y; i++) {

result *= x;}

return result;}

Does this method access any instance variables?

No. Should be declared static


Example

public class DimensionConverter {

public static final int INCHES_PER_FOOT = 12;

public static final double convertFeetToInches(double feet) {

return feet*INCHES_PER_FOOT;}

public static double convertInchesToFeet(double inches) {return inches / INCHES_PER_FOOT;

}}


Accessing Static Variables

From outside the class, static variables that are declared public can be accessed using the name of the class

int inchesPerFoot = DimensionConverter.INCHES_PER_FOOT;

Class Name Static Variable

Name

No Object is

Specified!


Calling Static Methods

From outside the class, static methods that are declared public can be accessed using the name of the class

int inches = DimensionConverter.convertFeetToInches(12);

Class Name Static Variable

Name

No Object is

Specified!


Restrictions on Static Methods

Static methods CANNOTAccess non-static instance variablesCall non-static methods

Static methods CANBe called by any method, static or non-static


Restrictions on Static Methods

Access Non-Static

Variables

Access Static Variabl

es

Call Non-Static

Methods

Call Static Method

s

Static methods X √ X √

Non-StaticMethods √ √ √ √


Example

public class Circle {

public static final double PI = 3.14159;private double area;

public static double area(double radius) { area = PI * (radius * radius); return area;}

} Will this code compile?

No.Cannot access non-static instance

variables in static methods


Example


public static final double PI = 3.14159;

private void printArea(double area) {System.out.println(area);

}

public static void area(double radius) {printArea(PI * (radius * radius));

}}

Will this code compile?

No.Cannot call non-static methods

inside static methods


Example


public static final double PI = 3.14159;

private void printArea() {System.out.println(area(3.0));

}

public static double area(double radius) {return PI * (radius * radius);

}}

Will this code compile?

Yes.CAN call static methods inside non-static methods


Programming Demo

Grade DistributionA class to display the distribution of letter grades in a classGiven the number of A,B,C,D, and F’s, compute the percentage of each type of grade• e.g. 15% A’s, 30% B’s, 30% C’s, 15% D’s, 10% F’s

Include accessors and mutators for each type of gradeDraw a bar graph of the grade distribution


Programming Demo

OutputEach * == 2 percent

0 10 20 30 40 50 60 70 80 90 100| | | | | | | | | | |****************************************************** A************** B*********C*****D***F


Wednesday

Math class

Wrapper class

Writing & Testing Methods

COMP 110: Introduction to Programming Tyler Johnson Mar 16, 2009 MWF 11:00AM-12:15PM Sitterson 014.

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Transcript of COMP 110: Introduction to Programming Tyler Johnson Mar 16, 2009 MWF 11:00AM-12:15PM Sitterson 014.