COMP 110 Constructors

COMP 110COMP 110ConstructorsConstructors

Luv KohliOctober 13, 2008

MWF 2-2:50 pmSitterson 014

AnnouncementsAnnouncementsProgram 3 assigned today, due October

31, 2pm

Ivan Sutherland talk today, 4pm, Sitterson Hall 011

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Questions?Questions?How is Lab 5 going?

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Today in COMP 110Today in COMP 110A couple of notes

Friday’s in-class exercise

Constructors

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Running a Java programRunning a Java programMake sure you are running the program

that has a main method in it

Otherwise, you will get this:

java.lang.NoSuchMethodError: main

Exception in thread "main"

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Local variablesLocal variablesWhat does the greet() method output?

public class Example{ private String str = “hello”; public void doStuff() { String str = “goodbye”; } public void greet() { doStuff(); System.out.println(str); }}

Outputs hello. Why?doStuff() uses local variable str, not instance variable str

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Local variablesLocal variablesWhat does the greet() method output?

public class Example{ private String str = “hello”; public void doStuff() { str = “goodbye”; } public void greet() { doStuff(); System.out.println(str); }}

Outputs goodbye. Why?Make sure you understand this for step 1 of Lab 5!

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Creating a new instanceCreating a new instancepublic class AnotherExample

{

private Student jack;

public void myMethod()

{

jack = new Student();

jack.setName(“Jack Smith”);

jack.setAge(19);

}

}

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Friday’s in-class exerciseFriday’s in-class exercise

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The perils of incorrectly initialized dataThe perils of incorrectly initialized data

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Spirit of KansasSpirit of Kansas crash, Feb. 23, 2008 crash, Feb. 23, 2008http://www.youtube.com/watch?v=_ZCp5h1gK2QAfter heavy rains, water affected air-data sensorsThese sensors feed angle of attack and yaw data to

flight-control systemWater distorted preflight readings in 3 of the plane’s 24

sensorsCaused flight-control system to make an erroneous

correction, making the plane stall and crash

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ConstructorsConstructorsCreate and initialize new objects

Special methods that are called when creating a new object

Student jack = new Student();

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Calling a constructor

Creating an objectCreating an objectCreate an object jack of class StudentStudent jack = new Student();

Scanner keyboard = new Scanner(System.in);

Create an object keyboard of class Scanner

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Create an object by calling a constructor

Return memory address of object

Assign memory address of object to variable

ConstructorsConstructorsCan perform any action you write into a

constructor’s definition

Meant to perform initializing actions◦For example, initializing values of instance

variables

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Similar to mutator methodsSimilar to mutator methodsHowever, constructors create an object

in addition to initializing it

Like methods, constructors can have parameters

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Example: Pet classExample: Pet classpublic 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; }}

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Default constructor

Example: Pet class, setPet methodExample: Pet class, setPet methodpublic void setPet(String newName, int newAge,

double newWeight)

{

name = newName;

age = newAge;

weight = newWeight;

}

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A closer lookA closer look

public Pet(String initName, int initAge, double initWeight)

{

name = initName;

age = initAge;

weight = initWeight;

}

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Same name as class name

No return type

ParametersBody

Initializing instance variablesInitializing instance variablesConstructors give values to all instance

variables

Even if you do not explicitly give an instance variable a value in your constructor, Java will give it a default value

Normal programming practice to give values to all instance variables

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Default constructorDefault constructorConstructor that takes no parameters

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

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

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Default constructorDefault constructorIf you define at least one constructor, a

default constructor will not be created for you

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Several constructorsSeveral constructorsYou can have several constructors per

class◦They all have the same name, just different

parameters

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Example: Pet classExample: Pet classpublic 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; }}

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Calling a constructorCalling a constructorPet myPet;

myPet = new Pet(“Frostillicus”, 3, 121.5);

You cannot use an existing object to call a constructor:

myPet.Pet(“Fang”, 3, 155.5); // invalid!

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Change an object using mutatorsChange an object using mutatorsmyPet.setPet(“Fang”, 1, 155.5);

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Calling methods from constructorsCalling methods from constructors

Just like calling methods from methods

public Pet(String initName, int initAge, double initWeight){ setPet(initName, initAge, initWeight);}

public void setPet(String newName, int newAge, double newWeight){ name = newName; age = newAge; weight = newWeight;}

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Calling methods from constructorsCalling methods from constructors

Can cause problems when calling public methods◦Problem has to do with inheritance, chapter 8◦Another class can alter the behavior of public

methods

Can solve problem by making any method that constructor calls private

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Example: Pet classExample: Pet classpublic class Pet{ private String name; private int age; private double weight;

public Pet(String initName, int initAge, double initWeight) { set(initName, initAge, initWeight); }

public void setPet(String newName, int newAge, double newWeight) { set(newName, newAge, newWeight); }

private void set(String newName, int newAge, double newWeight) { name = newName; age = newAge; weight = newWeight; }}

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WednesdayWednesdayTalk about Lab 5

More about constructors

Static variables and methods

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