Constructor in Java. What is a Constructor? Rules for create a Java Constructor

Size: px

Start display at page:

Download "Constructor in Java. What is a Constructor? Rules for create a Java Constructor"

Arron Watkins
5 years ago
Views:

1 Constructor in Java What is a Constructor? A constructor is a special method that is used to initialize a newly created object and is called just after the memory is allocated for the object. It can be used to initialize the objects,to required,or default values at the time of object creation. It is not mandatory for the coder to write a constructor for the class If no user defined constructor is provided for a class, compiler initializes member variables to its default values. numeric data types are set to 0 char data types are set to null character( \0 ) reference variables are set to null Rules for create a Java Constructor 1. It has the same name as the class 2. It should not return a value not even void Assignment 1: Create your First Constructor Java Step 1: Type following code in your editor Demo(){ value1 = 10; value2 = 20; System.out.println("Inside Constructor"); Demo d1 = new Demo();

2 Step 2) Save, Run & Compile the code. Observe the output. Constructor Overloading Constructor overloading is a technique in Java in which a class can have any number of constructors that differ in parameter lists.the compiler differentiates these constructors by taking into account the number of parameters in the list and their type Examples of valid constructors for class Account are Account(int a); Account (int a,int b); Account (String a,int b); Assignment 2: To understand Constructor Overloading Step 1) Type the code in editor /*Demo(){ value1 = 10; value2 = 20; System.out.println("Inside 1st Constructor"); */ Demo(int a){ System.out.println("Inside 2nd Constructor"); Demo(int a,int b){ value2 = b; System.out.println("Inside 3rd Constructor"); Demo d1 = new Demo(); Demo d2 = new Demo(30); Demo d3 = new Demo(30,40);

3 d2.display(); d3.display(); Step 2) Save, Compile & Run the Code. Step 3) Error =?. Try and debug the error before proceeding to next step. Step 4) Every class has a default Constructor. Default Constructor for class Demo is Demo(). In case you do not provide this constructor the compiler creates it for you and initializes the variables to default values. You may choose to override this default constructor and initialize variables to your desired values as shown in Assignment 1. But if you specify aparametrized constructor like Demo(int a),and want to use the default constructor Demo(), it is mandatory for you to specify it. In other words, in case your Constructor is overridden, and you want to use the default constructor, its need to be specified. Step 4) Uncomment line # 4-8. Save, Compile & Run the code. Constructor Chaining Consider a scenario where a base class is extended by a child.whenever an object of the child class is created, the constructor of the parent class is invoked first.this is called Constructor chaining. Assignment 3: To understand constructor chaining Step1 ) Copy the following code in the editor Demo(){ value1 = 1; value2 = 2; System.out.println("Inside 1st Parent Constructor"); Demo(int a){ System.out.println("Inside 2nd Parent Constructor");

4 DemoChild d1 = new DemoChild(); classdemochild extends Demo{ int value3; int value4; DemoChild(){ //super(5); value3 = 3; value4 = 4; System.out.println("Inside the Constructor of Child"); System.out.println("Value1 === "+value3); System.out.println("Value2 === "+value4); Step 2) Run the Code. Owing to constructor chaining, when object of child class DemoChild is created, constructor Demo() of the parent class is invoked first and later constructor DemoChild() of the child is created. Expected Output = Inside 1st Parent Constructor Inside the Constructor of Child Value1 === 1 Value2 === 2 Value1 === 3 Value2 === 4 Step3 ) You may observe the constructor of the parent class Demo is overridden. What is you want to call the overridden constructor Demo(int a) instead of the default constructor Demo() when your child object is created??? In such cases you can use the keyword "super" to call overridden constructors of the parent class. Syntax:- super(); --or--

5 super(parameter list); Ex: If your constructor is like Demo(String Name,int a) you will specify super("java",5) If used,the keyword super needs to be the first line of code in the constructor of the child class. Step 4) Uncomment Line # 26 and run the code. Observe the Output

C++ Important Questions with Answers

C++ Important Questions with Answers 1. Name the operators that cannot be overloaded. sizeof,.,.*,.->, ::,? 2. What is inheritance? Inheritance is property such that a parent (or super) class passes the characteristics of itself to children