Scientech Easy Java Constructor overloading Tutorial with Program | Scientech Easy

Wednesday, July 11, 2018

Java Constructor overloading Tutorial with Program

In this tutorial, we will learn the Java Constructor overloading technique with the help of examples. Constructor overloading is an interesting topic whose technique is almost similar to method overloading in Java. Let's start one by one concept of constructor overloading in easy ways and step by step program

Constructor Overloading in Java

Before we proceed further, let's understand the concept of overloading. The main concept of overloading is "Same name, Different parameters or different signature". Thus we can define as "Constructor overloading is a technique in which a class can have more than one constructor having the same name with different parameters list". i.e When a class has more than one constructor having the same name but different parameters within the same class, we say, the constructor is overloaded. 
Simply Overloaded constructor means you have more than one constructor in your class and to compile each constructor must have the different argument list. (Remember) 

When the multiple constructors are executed after creating an object, The compiler differentiates these constructors by taking the order, number, and the types of parameters used in the list. See below image
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Java Constuctor Overloading

Let's see a simple example to clear the concept.
Program Source Code 1: Here we are creating three objects of class 'School'. One with default constructor with initialization value of the instance variables, second with one parameter, and third with two parameters constructor.
    package scientech; public class School { // Declare the instance Variables. String scName; int estYear; // Constructor overloading begins from here. // Create default constructor and Initialize values. If You don't initialize values then default values null and 0 will print as output provided by default const. School(){ scName="RSVM"; estYear=1975; } // Create one parameter const. // Here we are setting the name of parameter different from the name of the variable because we are not using this reference. School(String name){ scName=name; } // Create two parameters const. and set the name of parameters different from the name of variables. School(String name, int year){ scName=name; estYear=year; } // Create one method to print output. You can also print output in const. void display(){ System.out.println(scName+ " " +estYear); } // Main method declaration. public static void main(String[] args) { // Create the first object with reference variable sc. It would call the default const. School sc=new School(); // Create second object with reference variable sc1. It would call one parameterized const. School sc1=new School("RSVM"); // Create the third object with rf var. sc2 and it would call two parameterized const. School sc2=new School("RSVM",1975); // Now call the method using reference variables sc, sc1 and sc2 one by one to print output. sc.display(); sc1.display(); sc2.display(); } }
    Output: RSVM 1975 RSVM 0 // Here 0 is the default value provided by one default constructor because we did not pass any value from one parameter constructor for second instance variable. RSVM 1975
In the program source code 1, we called default and parameterized constructors using different object creation. You can suppose any number of constructor within a class but the concept will always be the same. 

this REFERENCE

All instance methods receive an implicit argument called 'this' which refers to the current object. The current object is the object on which the method was called and ' this' reference can be used inside any method to refer to the current object. 
In the body of the method or constructor, the 'this' reference can be used like any other object reference to access instance variables, instance methods, and constructor. 
The main situation where the 'this' reference can be used are:

➲ When you set the name of the parameters different from the name of the instance variables inside the constructor or method, in this case, we can use 'this' reference to access the instance variables and you must use the following syntax:
                 this.instance_variable_name=instance_variable_name;
➲ When you need to pass a reference to the current object as an argument to another method.
➲ When you need to call to call a constructor from another constructor. you can use this() syntax. The argument to this() must match with target constructor and this() must be the first line inside the constructor. we will learn more details about this() in the separate tutorial. 
By using this(), the duplicate code can be avoided in multiple constructors, especially when the initialization routine is complex. 

Let us take an example of Student class in which constructor Student() has an argument "name" which is similar to the name of the instance variable "name". Now to assign the value of formal argument(Parameter) "name" to the instance variable "name", we must use dot(.) before the name of the instance variable.
Program Source Code 2: Follow all the above steps to make overloading.

    package scientech; public class Student { // Instance variable. String name; String schoolName; int rollNo; Student(String name, String sName){ this.name=name;// here 'this' reference is used. schoolName=sName; } Student(String name, String sName, int rollNo){ this.name=name; // this reference. schoolName=sName; this.rollNo=rollNo; } void details(){ System.out.println(name+ " " +schoolName+ " " +rollNo); } public static void main(String[] args){ // Create object and pass the arguments. Student st=new Student("Riddhi", "DPS"); Student st1=new Student("Siddhi", "RSVM", 05); st.details(); st1.details(); } }
    Output: Riddhi DPS 0 Siddhi RSVM 5
Now practice the program for more clear the concepts.
Program Source Code 3: In this program, we will use Access modifier, Encapsulation, Const. overloading, and this reference to make the program more interesting.
    package practice; public class Number { // Declare the variable as private. private int num=300; // Encapsulated code. // Declaration of Overloading constructor. public Number(){ // here you can also initialize the value of instance variable. } Number(int num){ this.num=num; } // Create getter method for private variable. public int getNum(){ return num; } // Create the public setter method for variable. public void setNum(int num){ this.num=num; // this reference used in method. } }
Create another class NumberTest and call the class Number.
    package practice; public class NumberTest { public static void main(String[] args) { Number n1=new Number(); //Call with 0 argument. Number n2=new Number(); // Call with 0 argument. Number n3=new Number(500); // Call with 1 argument. Number n4=new Number(600); // Call with 1 argument. n2.setNum(400); // assign new value. System.out.println(" First Number: " +n1.getNum()); System.out.println(" Second Number: " +n2.getNum()); System.out.println(" Third Number: " +n3.getNum()); System.out.println(" Fourth Number: " +n4.getNum()); } }
    Output: First Number: 300 Second Number: 400 Third Number: 500 Fourth Number: 600
If you have any problem to understand the Encapsulation concepts then you can learn Encapsulation in very easy ways and step by step from the Core Java menu. Practice all the programs of Encapsulation to clear the concepts. 

Program Source Code 4: 
    package fraction; public class Fraction { private int num; // Numerator is enacpsulated. So you cannot access directly from outside the class. private int deno; // Denominator is encapsulated. // Parameterized Constructor. Fraction(int n, int d){ // Declaration of values of Instance variables. num=100; deno=200; } // Default const. Fraction() { this(1,1);// Here this reference with parameters will call parameterized const. } // Create the getter method to read the values because we cannot read values directly from outside the class. public int getNum(){ return num; } public int getDeno(){ return deno; } }
Now create another class and create the object of the class Fraction.
    package fraction; public class FractionTest { // Main method. public static void main(String[] args) { //Default constructor will be called after object creation. Fraction obj=new Fraction(); // Call the gettter method using object reference variable obj to read the value of variables. System.out.println(+obj.getNum()); System.out.println(+obj.getDeno()); } }
    Output: 100 200
I hope this tutorial will help to know about the basic concept of Constructor Overloading in Java with programs. If you practice all programs, I am assure that you will not have any problem to understand the concepts.  


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