Java Constructor Chaining with example

Constructor Chaining is nothing but calling one Constructor from another. We will be using this keyword and super keyword in calling a constructor. this can be used to call a constructor within the same class whereas super can be used to call the constructor of the Parent class.

How Constructor Chaining works ?

A constructor can be called automatically or explicitly.

• If you didn’t call the Parent class Constructor then the compiler will be automatically calling the Parent class no-args constructor by itself.
• this() will call the Default Constructor of the same class whereas this(parameter) will call a parameterized constructor of the same class.
• Same way super() will call the Default constructor of the Parent class whereas super(parameter) will call a parameterized constructor of the Parent class.

Constructor Chaining Example in Java

Below code is the example of constructor chaining in Java. In this Java program we will show how to call constructor using both this and super keyword.

package com.javainterviewpoint;
class Parent
{
    public Parent()
    {
        System.out.println("Parent class no-args constructor called");
    }
    public Parent(String name)
    {
        System.out.println("Parent class Parameterized constructor called by "+name);
    }
}
public class Child extends Parent
{
    public Child()
    {
        this("JIP");
        System.out.println("Child class no-args constructor called");
    }
    public Child(String name)
    {
        super("JIP");
        System.out.println("Child class Parameterized constructor called by "+name);
    }
    public static void main(String args[])
    {
        Child c = new Child();
    }
}

Output :

Parent class Parameterized constructor called by JIP
Child class Parameterized constructor called by JIP
Child class no-args constructor called

Important Points to remember :

1. There is a lot of difference between Constructor Overloading and constructor chaining don’t get confused with those. Constructor Overloading is nothing but having more than one constructor in the same class whereas Constructor Chaining involves calling of both Parent class and Child class.

2. Call to another constructor of the same class or the parent class should be the first line, If not we will be getting the error like “Constructor call must be the first statement in a constructor”. 

package com.javainterviewpoint;
class Parent
{
    public Parent()
    {
        System.out.println("Parent class no-args constructor called");
    }
    public Parent(String name)
    {
        System.out.println("Parent class Parameterized constructor called by "+name);
    }
}
public class Child extends Parent
{
    public Child()
    {
        System.out.println("Child class no-args constructor called");
        this("JIP");
    }
    public Child(String name)
    {
        System.out.println("Child class Parameterized constructor called by "+name);
        super("JIP");
    }
    public static void main(String args[])
    {
        Child c = new Child();
    }
}

Output :

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
	Constructor call must be the first statement in a constructor
	Constructor call must be the first statement in a constructor

	at com.javainterviewpoint.Child.(Child.java:18)
	at com.javainterviewpoint.Child.main(Child.java:27)

3. If you didn’t call any constructor of the Parent or Child class then the compiler will automatically call the Default Constructor or no-args Constructor of the Parent class.

package com.javainterviewpoint;
class Parent
{
    public Parent()
    {
        System.out.println("Parent class no-args constructor called");
    }
    public Parent(String name)
    {
        System.out.println("Parent class Parameterized constructor called by "+name);
    }
}
public class Child extends Parent
{
    public Child()
    {
        System.out.println("Child class no-args constructor called");
    }
    public Child(String name)
    {
        System.out.println("Child class Parameterized constructor called by "+name);
    }
    public static void main(String args[])
    {
        Child c1 = new Child();
        Child c2 = new Child("JIP");
    }
}

Output : 

Parent class no-args constructor called
Child class no-args constructor called
Parent class no-args constructor called
Child class Parameterized constructor called by JIP

As in the above code we can see that we are not calling any constructor, then the compiler will automatically calls the default constructor of the Parent class.

4. If you didn’t declare any constructor in the Child class then compiler will declare one default constructor for you and Parent class default constructor will be called. But if a Parent class doesn’t have a default constructor then we will be getting “Implicit super constructor Parent() is undefined. Must explicitly invoke another constructor”

package com.javainterviewpoint;
class Parent
{
    public Parent(String name)
    {
        System.out.println("Parent class Parameterized constructor called by "+name);
    }
}
public class Child extends Parent
{
    public Child()
    {
        System.out.println("Child class no-args constructor called");
    }
    public Child(String name)
    {
        System.out.println("Child class Parameterized constructor called by "+name);
    }
    public static void main(String args[])
    {
        Child c1 = new Child();
        Child c2 = new Child("JIP");
    }
}

Output :

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
	Implicit super constructor Parent() is undefined. Must explicitly invoke another constructor
	Implicit super constructor Parent() is undefined. Must explicitly invoke another constructor

	at com.javainterviewpoint.Child.(Child.java:11)
	at com.javainterviewpoint.Child.main(Child.java:21)