Constructors in Java

Author: kishanrajput23

42.Constructors/README.md

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+# Java Tutorial: Constructors in Java
+
+### Constructors in Java : 
+- Constructors are similar to methods,, but they are used to initialize an object.
+- Constructors do not have any return type(not even void).
+- Every time we create an object by using the new() keyword, a constructor is called.
+- If we do not create a constructor by ourself, then the default constructor(created by Java compiler) is called.
+
+### Rules for creating a Constructor :
+1. The class name and constructor name should be the same.
+2. It must have no explicit return type.
+3. It can not be abstract, static, final, and synchronized.
+
+### Types of Constructors in Java :
+- There are two types of constructors in Java :
+
+**1. Defaut constructor :** A constructor with 0 parameters is known as default constructor.
+- Syntax :
+
+```
+<class_name>(){
+//code to be executed on the execution of the constructor
+}  
+```
+- Example :
+
+```
+class CWH {
+    CWH(){
+        System.out.println("This is the default constructor of CWH class.");
+    }
+}
+public class CWH_constructors {
+    public static void main(String[] args) {
+        CWH obj1 = new CWH();
+    }
+}
+```
+
+```
+Output :
+This is the default constructor of CWH class.
+```
+
+- In the above code, CWH() is the constructor of class CWH The CWH() constructor is invoked automatically with the creation of object ob1.
+
+**2. Paramerterized constructor :** A constructor with some specified number of parameters is known as a parameterized constructor.
+- Syntax :
+
+```
+<class-name>(<data-type> param1, <data-type> param2,......){
+//code to be executed on the invocation of the constructor
+}
+```
+
+- Example :
+
+```
+class CWH {
+    CWH(String s, int b){
+
+        System.out.println("This is the " +b+ "th video of "+ " "+ s);
+    }
+}
+public class CWH_constructors {
+    public static void main(String[] args) {
+        CWH obj1 = new CWH("CodeWithHarry Java Playlist",42);
+    }
+}
+```
+
+```
+Output :
+This is the 42th video of  CodeWithHarry Java Playlist
+````
+
+- In the above example, CWH() constructor accepts two parameters i.e., string s and int b.
+
+### Constructor Overloading in Java :
+- Just like methods, constructors can also be overloaded in Java. We can overload the Employe constructor like below: 
+
+```
+public Employee (String n)
+           name = n;
+         }
+```
+
+**Note:**
+1. Constructors can take parameters without being overloaded 
+2. There can be more than two overloaded constructors 
+
+- Let's take an example to understand the concept of constructor overloading. 
+
+**Example :**
+
+- In the below example, the class Employee has a constructor named Employee(). It takes two argument,i.e., string s & int i. The same constructor is overloaded and then it accepts three arguments i.e., string s, int i & int salary. 
+
+```
+class Employee {
+// First constructor
+    Employee(String s, int i){
+        System.out.println("The name of the first employee is : " + s);
+        System.out.println("The id of the first employee is : " + i);
+    }
+//    Constructor overloaded
+    Employee(String s, int i, int salary){
+        System.out.println("The name of the second employee is : " + s);
+        System.out.println("The id of the second employee is : " + i);
+        System.out.println("The salary of second employee is : " + salary);
+    }
+
+}
+public class CWH_constructors {
+    public static void main(String[] args) {
+        Employee shubham = new Employee("Shubham",1);
+        Employee harry = new Employee("Harry",2,70000);
+
+
+
+    }
+}
+```
+
+```
+Output :
+
+The name of the first employee is : Shubham
+The id of the first employee is : 1
+The name of the second employee is : Harry
+The id of the second employee is : 2
+The salary of second employee is : 70000
+ ```
+
+**Quick quiz:** Overloaded the employee constructor to initialize the salary to Rs 10,000
+
+```
+class Employee {
+// First constructor
+    Employee(String s, int i){
+        System.out.println("The name of the first employee is : " + s);
+        System.out.println("The id of the first employee is : " + i);
+    }
+//    Constructor overloaded
+    Employee(String s, int i, int salary){
+        System.out.println("The name of the second employee is : " + s);
+        System.out.println("The id of the second employee is : " + i);
+        System.out.println("The salary of second employee is : " + salary);
+    }
+//  Overloaded constructor with salary 10,000
+    Employee(int salary){
+        System.out.println("The salary of third employee is : " + salary);
+    }
+
+}
+public class quick_quiz {
+    public static void main(String[] args) {
+        Employee shubham = new Employee("Shubham",1);
+        Employee harry = new Employee("Harry",2,70000);
+        Employee kishan = new Employee(10000);
+    }
+}        
+```
+
+### Code as described in the video:
+
+```
+class MyMainEmployee{
+    private int id;
+    private String name;
+
+    public MyMainEmployee(){
+        id = 0;
+        name = "Your-Name-Here";
+    }
+    public MyMainEmployee(String myName, int myId){
+        id = myId;
+        name = myName;
+    }
+    public MyMainEmployee(String myName){
+        id = 1;
+        name = myName;
+    }
+    public String getName(){
+        return name;
+    }
+    public void setName(String n){
+        this.name = n;
+    }
+    public void setId(int i){
+        this.id = i;
+    }
+    public int getId(){
+        return id;
+    }
+}
+
+public class cwh_42_constructors {
+    public static void main(String[] args) {
+        //MyMainEmployee harry = new MyMainEmployee("ProgrammingWithHarry", 12);
+        MyMainEmployee harry = new MyMainEmployee();
+        //harry.setName("CodeWithHarry");
+        //harry.setId(34);
+        System.out.println(harry.getId());
+        System.out.println(harry.getName());
+    }
+}
+```
+
+**Handwritten Notes: [Click to Download](https://api.codewithharry.com/media/videoSeriesFiles/courseFiles/java-tutorials-for-beginners-42/Chapter9.pdf)**
+
+**Ultimate Java Cheatsheet: [Click To Download](https://api.codewithharry.com/media/videoSeriesFiles/courseFiles/java-tutorials-for-beginners-42/UltimateJavaCheatSheet.pdf)**