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Constructor
Last Updated : 26 May, 2026
title: Constructor in Java
OOPs188 words9 headingsTables includedExamples included
title: Constructor in Java description: Java mein constructors ka complete guide
Constructor ek special method hai jo object create hone par automatically call hota hai. Object ko initialize karne ke liye use hota hai.
Constructor ke Rules
- Class ke naam jaisa hi naam hona chahiye
- Return type nahi hota (void bhi nahi)
newkeyword se automatically call hota hai- Har class mein kam se kam ek constructor hota hai
Types of Constructors
1. Default Constructor
Jab hum koi constructor nahi likhte, compiler automatically ek default constructor add karta hai.
java exampleWoHoTech
class Car {
String brand;
int speed;
// Compiler yeh add karta hai automatically:
// Car() { }
}
Car c = new Car(); // Default constructor call2. No-arg Constructor (Explicit)
Hum khud bina parameter ka constructor likhein.
java exampleWoHoTech
class Car {
String brand;
int speed;
Car() {
brand = "Unknown";
speed = 0;
System.out.println("Car object bana");
}
}
Car c = new Car();
System.out.println(c.brand); // Unknown3. Parameterized Constructor
Parameters ke saath constructor — object banate time values dete hain.
java exampleWoHoTech
class Car {
String brand;
int speed;
Car(String brand, int speed) {
this.brand = brand;
this.speed = speed;
}
void show() {
System.out.println(brand + " - " + speed + " kmph");
}
}
Car c1 = new Car("Honda", 120);
Car c2 = new Car("Toyota", 150);
c1.show(); // Honda - 120 kmph
c2.show(); // Toyota - 150 kmph4. Constructor Overloading
Ek class mein multiple constructors alag-alag parameters ke saath.
java exampleWoHoTech
class Rectangle {
int width, height;
Rectangle() {
width = 1;
height = 1;
}
Rectangle(int side) {
width = side;
height = side;
}
Rectangle(int width, int height) {
this.width = width;
this.height = height;
}
int area() { return width * height; }
}
Rectangle r1 = new Rectangle(); // 1x1
Rectangle r2 = new Rectangle(5); // 5x5
Rectangle r3 = new Rectangle(4, 6); // 4x6
System.out.println(r1.area()); // 1
System.out.println(r2.area()); // 25
System.out.println(r3.area()); // 245. Copy Constructor
Ek existing object ki copy banana.
java exampleWoHoTech
class Student {
String name;
int marks;
Student(String name, int marks) {
this.name = name;
this.marks = marks;
}
// Copy Constructor
Student(Student s) {
this.name = s.name;
this.marks = s.marks;
}
}
Student s1 = new Student("Ram", 85);
Student s2 = new Student(s1); // Copy
s2.name = "Shyam"; // s1 pe koi effect nahi
System.out.println(s1.name); // Ram
System.out.println(s2.name); // ShyamConstructor Chaining — this()
Ek constructor doosre constructor ko call kare.
java exampleWoHoTech
class Employee {
String name;
String dept;
double salary;
Employee() {
this("Unknown", "General", 20000); // 3-arg constructor call
}
Employee(String name) {
this(name, "General", 20000); // 3-arg constructor call
}
Employee(String name, String dept, double salary) {
this.name = name;
this.dept = dept;
this.salary = salary;
}
}Constructor vs Method
| Constructor | Method | |
|---|---|---|
| -- | -- | -- |
| Naam | Class naam jaisa | Kuch bhi |
| Return type | Nahi hota | Hota hai (void bhi) |
| Call | Automatic (new se) | Manually call karte hain |
| Purpose | Object initialize | Operation perform |
| Inheritance | Inherit nahi hota | Inherit hota hai |
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Constructor.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this Java topic.
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