WHAT IS A CONSTRUCTOR
"It is a special type of method same name as class name that determines how an object is initialized when it's created".
what is constructor in java
Like other methods, we can also define constructor Method in our java program but unlike other methods, we cannot call a constructor directly; Java called constructor automatically when an object has created. When we use new keyword to create an object of a class, java does three thing;
Allocates memory for the object.
Initialize that object instance variable, with their initial value or to a default.
Call the constructor Method of the class.
If a class doesn't have defined any constructor method, we will still create an object of that class but we have to set instance variable or call other methods that object needs to initialize itself to that object afterward.
By defining constructor method in our own classes, we can set initial values of instance variable, call method based on those variable or call methods on other objects, or calculate initial properties of our object. We can also overload constructor, as we would regular methods, to create an object that has specific properties based on the argument we give to new.
BASIC CONSTRUCTOR
by defining a constructor looks like a regular method, with 2 basic difference.
Constructor and class name are always same.
It doesn't have any return type
For example, in the below table a simple class person, with a constructor that initializes it's instance variable based on the argument to new. The class also includes a method for the object to introduce itself, and a main() method to test each of these class.
class Person
{
String name;
int age;
Person (String n, int a)
{
name = n;
age = a;
}
void printPerson ()
{
System.out.print("Hi, I am " +name);
System.out.println(" I am "+ age + " years old.");
}
public static void main(String args[])
{
Person p;
p = new Person ("Ajab", 20);
p.printPerson();
p = new Person ("Rizwan", 30);
p.printPerson();
The output of the program is given below:
Hi, I am Ajab. I am 20 years old.
Hi, I am Rizwan. I am 30 years old
CONSTRUCTOR OVERLOADING
like other methods, constructor can also take different number and types of parameters, enabling us to create our objects with exactly the properties you want it to have, or for it to be able to calculate properties from different kinds of input.
constructor overloading in Java
For example, the MyRectone class in the given table creates a MyRectone Constructor and passing different parameter instead of creating different methods for the given arguments.
class MyRectone
{
int x1 = 0;
int y1 = 0;
int x2 = 0;
int y2 = 0;
MyRectone ( int x1, int x2, int x2, int y2)
{
this.x1 = x1;
this.y1 = y1;
this.x2 = x2;
this.y2 = y2;
}
MyRectone (Point topLeft, Point bottomRight)
{
x1 = topLeft.x;
y1 = topLeft.y;
x2 = bottomRight.x;
y2 = bottomRight.y;
}
MyRectone ( Point topLeft, int w, int h)
{
x1 = topLeft.x;
y1 = top left.y;
x2 = ( x1 + w);
y2 = (y1 + h);
}
void printRect ()
{
System.out.print ("MyRectone: ");
}
public static void main (String args [] )
{
MyRectone rect;
System.out.println ("Calling MyRectone with coordinates 35,35 70,70");
rect = new MyRectone (35,35,70,70);
rect.printRect();
System.out.println ("Calling MyRectone with coordinates (15,15) (30,30)");
rect = new MyRectone (15,15,30,30);
rect.printRect();
System.out.print (" Calling buildRect w/1 point (10,10),");
System.out.println ("width (50) and height (50)");
rect = new MyRectone ( new Point (10,10), 50, 50);
rect.printRect();
Output
Calling MyRectone with coordinates 35,35 70,70:
MyRectone:
Calling buildRect w/1 points (15,15), (30,30):
MyRectone:
Calling buildRect w/1 point (10,10), width (50) and height (50):
MyRectone:
CALLING ANOTHER CONSTRUCTOR
Some constructor may be a superset of another constructor defined in your class; that is, they might have the same behavior plus a little bit more. Rather than duplicating identical behavior in multiple constructor Methods in our class, it makes sense to be able to just call that first constructor from inside the body of the second constructor. Java provides a special syntax for doing this. To call a constructor defined on the current class, use this form:
this (arg1, arg2, arg3 …..);
The arguments to this are, of course, the arguments to the constructor.
Overloading a constructor means typing in multiple versions of the constructor, each having a different argument list, like the following examples:
class Car {
Car() { }
Car(String s) { }
}
The preceding Car class has two overloaded constructors, one that takes a string, and one with no arguments. Because there's no code in the no-arg version, it's actually identical to the default constructor the compiler supplies, but remember-since there's already a constructor in this class (the one that takes a string), the compiler won't supply a default constructor. If you want a no-arg constructor to overload the with-args version you already have, you're going to have to type it yourself, just as in the Car example.
Overloading a constructor is typically used to provide alternate ways for clients to instantiate objects of your class.
overloading is the ability to define more than one method with the same name in the same class with different method signatures is part of the method declaration. It is the combination of the method name and the parameter list. class overloadDemo { int c;
public int add(int a,int b) { c=a+b; System.out.println("add="+c);
}
public int add(int a, int b, int d);
{ this.add(5,6);
c=a+b+d; System.out.println("add="+c);
}
public static void main(String arg[]) { int a=5,b=6,d=10;
overloadDemo as=new overloadDemo(); as.add(5,6,10);
}
}
Method overriding is a concept where a method written in one class is overshadowed or masked by a method with the same name and signature in one of the child classes.
Here is a small example for method overriding
first create a class
class alpha{
void m1(){
// please look second class void name both are same
System.out.println("This is class alpha's comment");
}
}
then create your second class
class zero extends alpha{
// extends is a inbuilt java keyword
void m1(){
System.out.println("This is class zero's comment and this is the output");
}
}
you must save this file with your second class name like this
zero.java
then output should be - This is class zero's comment and this is the output
this is the simple example
That means that you have two or more methods with the same name, but with a different parameter list - either the number of parameters varies, or the type of at least one of the parameters.
when overriding of a class or a method is necessary, they can be declared as abstract
Here are some of the most common differences between both of them. If you are working in Java for more than 1 year, you might be familiar with all of them but any way its good revision: 1) First and major difference between Overloading and Overriding is that former occur during compile time while later occur during runtime. 2) Second difference between Overloading and Overriding is that, you can overload method in same class but you can only override method in sub class. 3) Third difference is that you can overload static method in Java but you can not override static method in Java. In fact when you declare same method in Sub Class it's known as method hiding because it hide super class method instead of overriding it. 4) Overloaded methods are bonded using static binding and Type of reference variable is used, while Overridden method are bonded using dynamic bonding based upon actual Object. 5) Rules of Overloading and Overriding is different in Java. In order to overload a method you need to change its method signature but that is not required for overriding any method in Java.
False. A method with the same signature in both the superclass and its subclass is known as method overriding, and is a valid concept in Java.
In C++, overriding and function, method, or operator is a different thing than (dynamic) polymorphism, so overriding a polymorphic method is almost entirely possible.
Method overriding is similar to method overloading, with a small difference. In overriding, a method in a parent class is overridden in the child class. The method in the child class will have the same signature as that of the parent class. Since the method in the child class has the same signature & name as the method of its parent class, it is termed as overriding. In situations where you may have to explicitly call the parent class method you can use the "super" keyword and for explicitly calling the current objects method you can use the "this" keyword.
when overriding of a class or a method is necessary, they can be declared as abstract
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Overloading is the means by which we can provide two or more different definitions of the same method in the same namespace. Overriding is the means by which a derived class may redefine the meaning of a base class method.
Here are some of the most common differences between both of them. If you are working in Java for more than 1 year, you might be familiar with all of them but any way its good revision: 1) First and major difference between Overloading and Overriding is that former occur during compile time while later occur during runtime. 2) Second difference between Overloading and Overriding is that, you can overload method in same class but you can only override method in sub class. 3) Third difference is that you can overload static method in Java but you can not override static method in Java. In fact when you declare same method in Sub Class it's known as method hiding because it hide super class method instead of overriding it. 4) Overloaded methods are bonded using static binding and Type of reference variable is used, while Overridden method are bonded using dynamic bonding based upon actual Object. 5) Rules of Overloading and Overriding is different in Java. In order to overload a method you need to change its method signature but that is not required for overriding any method in Java.
Method overloading is when you have multiple methods in a class that have the same name but a different signature. Method overriding is similar to method overloading, with a small difference. In overriding, a method in a parent class is overridden in the child class. The method in the child class will have the same signature as that of the parent class. Since the method in the child class has the same signature & name as the method of its parent class, it is termed as overriding. In situations where you may have to explicitly call the parent class method you can use the "super" keyword and for explicitly calling the current objects method you can use the "this" keyword.
False. A method with the same signature in both the superclass and its subclass is known as method overriding, and is a valid concept in Java.
Java does not support object overriding. It does support operator overloading by means of the "+" symbol which is used for both numeric addition as well as string concatenation.
In C++, overriding and function, method, or operator is a different thing than (dynamic) polymorphism, so overriding a polymorphic method is almost entirely possible.
The dictionary definition of polymorphism refers to a principle in biology in which an organism or species can have many different forms or stages. This principle can also be applied to object-oriented programming and languages like the Java language. Subclasses of a class can define their own unique behaviors and yet share some of the same functionality of the parent class.Overriding and Overloading are two techiques to achive polymorphism in Java.Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading. Method overriding : When a method in a class having the same method name with same arguments is said to be method overriding.
Method overriding is similar to method overloading, with a small difference. In overriding, a method in a parent class is overridden in the child class. The method in the child class will have the same signature as that of the parent class. Since the method in the child class has the same signature & name as the method of its parent class, it is termed as overriding. In situations where you may have to explicitly call the parent class method you can use the "super" keyword and for explicitly calling the current objects method you can use the "this" keyword.
There is no such thing as overlording in Java.
Hiding means a class cannot see the definition. Overriding implies that a class must see that to "override"