Constructor in Java

Constructor in Java

Constructor is used to initialize the property.

Constructor must be the class name.

Constructor must have an access modifiers.

Constructor never have any non-access modifiers.

Constructor always call by the new keyword.

Constructor never have any return type.

Constructor always return same type of object.

There are two type of Constructors :

• Default Constructor or Non-Parameterized Constructor
• Parameterized Constructor

1. Default Constructor or Non-Parameterized Constructor

This Constructor provided by the compiler when you dont define any type of Constructor in your class.

class A{

}

//save and compile
//then using javap tool you can see default constructor like below :
        
C:\Users\Deep Singh\Desktop>javap A
        
Compiled from "A.java"
        
class A {
    A();// this is a default constructor
}

//its access modifier is default 
                
    

Default Constructor access modifiers is depends on class access modifiers.

public class A{

}

//save and compile
//then using javap tool you can see default constructor like below :
        
C:\Users\Deep Singh\Desktop>javap A
        
Compiled from "A.java"
        
public class A {
      public A();// this is a default constructor
}

    

As we know Top class can never make private or protected so if you want to change the constructor access modifiers except the public or default then you need to define no-parameterized constructor in your class.

class A{

    //no-parameterized constructor
    private A(){

    }

}
    

If your class contain any type of define constructor then compiler never provide default constructor.

class A{
            
    protected A(){

    }

}

//save and compile
//then using javap tool you can see default constructor like below :
        
C:\Users\Deep Singh\Desktop>javap A
        
Compiled from "A.java"
        
class A {
      protected A();// there is no defualt constructor
}
                
    

Default constructor or non-parameterized constructor is used to initialize variabled with default values.

class A{
            
    int i;

    public static void main(String [] h){
        A a=new A();
        System.out.println(a.i); // 0
    }

}
    
/*If i does not contain any value so its
default value is based on its type so 
it can be initialize by 0.*/

OR

class A{
            
    int i=10;

    public static void main(String [] h){
        A a=new A();
        System.out.println(a.i); // 10
    }

}
    
/*If i contain any value so
it can be initialize by 10.*/
    

2. Parameterized Constructor

Parameterized constructor is used to get input from user for initializing user specific value.

class A{
                    
    int i;

    A(int j){
        i=j;
    }
        
    public static void main(String [] h){
        A a=new A(20);
        System.out.println(a.i); // 20
    }
        
}
            

For Example :

class Student{
    String name;
    int roll;

    Student(String n,int r){
        name=n;
        roll=r;
    }

}

class Test{

    public static void main(String [] h){
        Student s=new Student("rahul",123);
        System.out.println(s.name);
        System.out.println(s.roll);
    }

}

Output is : rahul
Output is : 123