Java 20 | Types of constructors | Default constructor | No arg Constructor | Parameterized Construt

Types of constructors
Default constructor
No – arg Constructor
Parameterized Constructor
1 Default constructor
If you do not define any constructor in your class, java defines one for you by default. This constructor is known as default constructor.
class Demo {
}
after compilaton : -
class Demo {
Demo( ) { } // default constructor
}

Purpose of constructor is to initialize the instance variable
But default constructor has nothing in code so it initializes the instance variables with their default values ( like 0, 0.0, null ) depending on their type ( for integer 0, for string null)

class Demo {
int id;
String name;
int age;
void show (){
System.out.println(id + “ “+name+” “ +age);
}
public static void main( String args[]) {
Demo d =new Demo();
d.show();
}
}
output: 0 null 0

2 No-arg constructor
constructor with no argument is known as No-arg constructor.
the signature is same as default constructor, however body can have any code unlike default constructor where the body does nothing.
Ex:-
class Demo {
Demo ( ) {
System.out.println(“this is No_arg constructor”);
}
}

3 Parameterized Constructor
You can send parameter to the constructor just like a method.
class Demo{
Demo(int a, int b){
// statements
}
}

class Rectangle {
int length, width;
Rectangle (int l, int w){
length =l;
width =w;
}
}
Rectangle r =new Rectangle( ); wont compile no matching constructor
Rectangle rr= new Rectangle(5,4); will compile, arguments matched


Method :-
1) Method can be any user defined name
2) Method should have return type.
3) Method should be called explicitly either with objec reference or class reference
4) Method is not provided by compiler in any case.

Constructor: -
1) Constructor must be a class name.
2) It should not have any return type (not even void)
3) It will be called automatically when an object is created.
4) Java compiler provides a default constructor if we do not have any constructor