Parameterized Constructor in Java

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Parameterized constructor can be made by the user if they want to assign some value to instance variable at time of creation of object.

Parameterized constructors are also a good way to perform some operations that are to be performed at the start such as Database connections, pre-checks on values of instance variables or certain operations on static variables.

Some features of parameterized constructor are:

  1. Default constructor will not be created if parameterized constructor is made by user.
  2. They can have any access specifier such as private, public or protected.
  3. Parameterized constructor can be used to initialize instance variables, pre-check on values of instance variables, static variable checks etc.
  4. Constructors cannot be inherited.

Let us see an example of parameterized constructor.

public class Tech4Humans {
	public static void main(String args[]) {
		//Creating object using parameterized constructor
		A obj = new A(29,"Robert");
		obj.printVariables();
		
	//	A obj1 = new A();   
	}
}


class A{
	int age;
	String name;
	
	//Parameterized Constructor
	A(int age, String name){
		this.age = age;
		this.name = name;
	}
	
	void printVariables() {
		System.out.println("name = "+name);
		System.out.println("age = "+age);
	}
}

Output

name = Robert
age = 29

Here we are creating the object using the parameterized constructor. Note here at Line 7. If we uncomment this line then compiler will throw the following error :

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
	The constructor A() is undefined

	at Tech4Humans.main(Tech4Humans.java:7)

This is because we have created a parameterized constructor, so compiler will not create a default constructor, so on calling ‘new A()‘ it threw an error since it cannot find the no-argument constructor.

We can create a no-argument constructor to avoid this error. Please see below code for reference:

public class Tech4Humans {
	public static void main(String args[]) {
		//Creating object using parameterized constructor
		A obj = new A(29,"Robert");
		obj.printVariables();
		
		A obj1 = new A();   
		obj1.printVariables();
	}
}


class A{
	int age;
	String name;
	
	//Parameterized Constructor
	A(int age, String name){
		this.age = age;
		this.name = name;
	}
	
	//no-argument Constructor. Constructor Overloading
	A(){
		this.age = 100;
		this.name = "Mona";
	}
	
	void printVariables() {
		System.out.println("name = "+name);
		System.out.println("age = "+age+"\n");
	}
}

Output

name = Robert
age = 29

name = Mona
age = 100

The above code is using the concept of overloading. So let us understand about Constructor Overloading.

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