Constructors in Java are used to initialize the values of the attributes of the object serving the goal to bring Java closer to the real world. We already have a default constructor that is called automatically if no constructor is found in the code. But if we make any constructor say parameterized constructor in order to initialize some attributes then it must write down the default constructor because it now will be no more automatically called.
Note: In Java, constructor of the base class with no argument gets automatically called in the derived class constructor.
Example:
Java
// Java Program to Illustrate// Invocation of Constructor// Calling Without Usage of// super Keyword // Class 1// Super classclass Base { // Constructor of super class Base() { // Print statement System.out.println( "Base Class Constructor Called "); }}// Class 2// Sub classclass Derived extends Base { // Constructor of sub class Derived() { // Print statement System.out.println( "Derived Class Constructor Called "); }}// Class 3// Main classclass GFG { // Main driver method public static void main(String[] args) { // Creating an object of sub class // inside main() method Derived d = new Derived(); // Note: Here first super class constructor will be // called there after derived(sub class) constructor // will be called }} |
Base Class Constructor Called Derived Class Constructor Called
Output Explanation: Here first superclass constructor will be called thereafter derived(sub-class) constructor will be called because the constructor call is from top to bottom. And yes if there was any class that our Parent class is extending then the body of that class will be executed thereafter landing up to derived classes.
But, if we want to call a parameterized constructor of the base class, then we can call it using super(). The point to note is base class constructor call must be the first line in the derived class constructor.
Implementation: super(_x) is the first line-derived class constructor.
Java
// Java Program to Illustrate Invocation// of Constructor Calling With Usage// of super Keyword// Class 1// Super classclass Base { int x; // Constructor of super class Base(int _x) { x = _x; }}// Class 2// Sub classclass Derived extends Base { int y; // Constructor of sub class Derived(int _x, int _y) { // super keyword refers to super class super(_x); y = _y; } // Method of sub class void Display() { // Print statement System.out.println("x = " + x + ", y = " + y); }}// Class 3// Main classpublic class GFG { // Main driver method public static void main(String[] args) { // Creating object of sub class // inside main() method Derived d = new Derived(10, 20); // Invoking method inside main() method d.Display(); }} |
x = 10, y = 20
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