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Software Engineering
Module: Core of Java Topic: Constructors
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The content in this presentation is aimed at teaching learners to:
Classes and Objects
• Define Constructor
• Explain the need for Constructor
• Create Constructors and Parameterized Constructors
• Use “this” keyword
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Classes and Objects
Using Objects
Dice diceOne = new Dice();
diceOne.roll();
int value =
diceOne.getFaceValue(); Constructor
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Example : class Car{
public Car(){ // this is constructor. . . }
}
Classes and Objects
ConstructorConstructor
Is a function that is implicitly invoked when a new object is created.
Performs the necessary actions in order to initialize the object.
Is a special function with the same name as the class.
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Classes and Objects
Program JVM Memory
class Car{ String name;
public Car(){ name=“My Car”;
}}class MainClass{ public static void main(String args[]){
Car c ; c = new Car(); }
}
Stack memory
Heap memory
Reference variable (Declaration)
Reference variable (Declaration)
Allocates memory for the object
(for Instantiation)
Allocates memory for the object
(for Instantiation)
Calls the constructor
Calls the constructor
Constructor of the Car ClassConstructor of the Car Class
c
Name My Car
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Classes and Objects
ConstructorConstructor
Associating a variable name with an object type.
Is done by the new keyword, a JAVA operator that creates the object.
A call to the constructor follows this new operator. This in turn, initializes the new object.
DeclarationDeclaration
InstantiationInstantiation
InitializationInitialization
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Classes and Objects
Program JVM Memory
class Car{ String name; public Car(){
name=“My First Car”; }}class MainClass{ public static void main(String args[]){ Car c1,c2; c1 = new Car(); c2 = new Car(); }}
Stack memory
Heap memory
c1
c2
Does it sound meaningful to have the same name for both the cars: “My First Car”?
Does it sound meaningful to have the same name for both the cars: “My First Car”?
Name My First Car
Name My First Car
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Classes and Objects
Program JVM Memory
class Car{ String name; public Car(String carName){
name=carName; }}class MainClass{ public static void main(String args[]){ Car c1,c2; c1 = new Car(“My First Car”); c2 = new Car(“My Second Car”); }}
Stack memory
Heap memory
c1
c2
Parameterized Constructor
Name My Second Car
Name My First Car
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Classes and Objects
Program JVM Memory
class Car{ String name;
public Car(){ name=“My First Car”; }
public Car(String carName ){ name=carName;
}}class MainClass{
public static void main(String args[]){ Car c 1,c2; c1 = new Car(); c 2 = new Car(“My Second Car”);
}}
Stack memory
Heap memory
c1
c2 Name My Second Car
Name My First Car
It is possible for a class to have more than one constructor? It is possible for a class to have more than one constructor?
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Classes and Objects
Program JVM Memory
class Car{ String name; public Car(){
name=“My Car”; }}class MainClass{ public static void main(String args[]){ Car c 1,c2; c1 = new Car(); c2 = c1; }}
Stack memory
Heap memory
c1
c2Name My Car
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Classes and Objects
Program JVM Memory
class Car{ String name; public Car(){
name=“My Car”; }}class MainClass{ public static void main(String args[]){ Car c 1,c2; c 1= new Car(); c 2= new Car(); c2=c1; }}
Stack memory
Heap memory
c1
c2
Name My Car
Name My Car
Can a class perform an action on itself?Can a class perform an action on itself?
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Classes and Objects
Program
class Car{ String name;
public Car(){ name=“My First Car”;
} public Car(String name ){
this.name=name; }
}class MainClass{ public static void main(String args[]){ Car c 1,c2; c1 = new Car(); c 2 = new Car(“My Second Car”);
}}
Refers to the current object Refers to the
current object
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Classes and Objects
“this” keyword
• Is a reference to the current object within an instance method or constructor.
• Explicit constructor invocation -- call another constructor in the same class.
Example for explicit constructor invocation : public class MyClass { private int x, y, a, b; public MyClass() { this(0, 0, 0, 0); } public MyClass(int a, int b) { this(0, 0, a, b); } public MyClass(int x, int y, int a, int b) { this.x = x; this.y = y; this.a = a; this.b = b; } ... }
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Classes and Objects
Will this work?
Class Test{ // no constructor int value=10; void display(){ System.out.println(“Value is : ”+value); }}class MainClass{ public static void main(String args[]){ Test test = new Test(); // creating object with constructor Test() test.display(); }}
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Classes and Objects
What if no constructor is defined in a class ?
At the time of compilation the Java Compiler will
insert an empty Definition Of Default Constructor to
the java class and then it will compile the program.
The empty Definition Of Default Constructor of any
class will be as follows:
public class-name(){}
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Classes and Objects
Create the following class:
Class Test{int sum(int valueOne, int valueTwo){ return valueOne + valueTwo;}
}
Write a main method that instantiates this class, calls sum method and prints the return value. Compile and run the program.
TRY ITEXERCISE
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Classes and Objects
