Constructors

✔ Definition

Constructors are special methods whose name is the same as the class name
Constructors do not have a return type (not even void)
Constructors are invoked automatically by JVM at the time of object creation
Constructors are mainly used to initialize instance variables

Lab329.java

class Lab329 {

  public static void main(String[] args) {
    Student stu1 = new Student();
    stu1.show();

    Student stu2 = new Student();
    stu2.show();
  }
}

class Student {

  int sid;
  String sname;

  void show() {
    System.out.println(sid + "\t" + sname);
  }
}

✔ Output:

0   null
0   null

👉 Default values:

int → 0
String → null

Lab330.java

class Lab330 {

  public static void main(String[] args) {
    Student stu = new Student();
    stu.sid = 99;
    stu.sname = "Midhun";
    stu.show();
  }
}

class Student {

  int sid;
  String sname;

  Student() {
    System.out.println("Student Default Constructor");
  }

  void show() {
    System.out.println(sid + "\t" + sname);
  }
}

✔ Output:

Student Default Constructor
99   Midhun

Lab331.java

class Lab331 {

  public static void main(String[] args) {
    Student stu = new Student();
    stu.sid = 99;
    stu.sname = "Midhun";
    stu.show();
  }
}

class Student {

  int sid;
  String sname;

  Student() {
    System.out.println("Student Default Constructor");
  }

  void show() {
    System.out.println(sid + "\t" + sname);
  }
}

✔ Output:

Student Default Constructor
99   Midhun

Lab332.java

class Lab332 {

  public static void main(String[] args) {
    Student stu = new Student(); //  ERROR
    stu.sid = 99;
    stu.sname = "Midhun";
    stu.show();
  }
}

class Student {

  int sid;
  String sname;

  Student(int id, String sn) {
    System.out.println("Student 2-Arg Constructor");
    sid = id;
    sname = sn;
  }

  void show() {
    System.out.println(sid + "\t" + sname);
  }
}

Result:

Compilation Error

👉 Reason:

No default constructor exists
Only parameterized constructor is defined
So new Student() is invalid

🧠 Key Concepts

🔹 Default Constructor

Provided by JVM only if no constructor is written
Initializes variables with default values

🔹 Parameterized Constructor

Used to initialize variables with custom values

Student(int id,String name){
    sid=id;
    sname=name;
}

🔹 Important Rule

👉 If you write any constructor, JVM will NOT provide default constructor

🚀 Quick Summary

Concept	Meaning
Constructor	Special method for initialization
No return type	Not even `void`
Auto execution	At object creation
Default constructor	Given only if none defined
Parameterized constructor	Custom initialization

Common Interview Point

👉 “Why compilation error in Lab332?”

Answer:

Because constructor requires arguments
But object is created without arguments

Lab333.java

class Lab333 {

  public static void main(String[] args) {
    Student stu1 = new Student(88, "Midhun");
    stu1.show();

    Student stu2 = new Student(99, "Balan");
    stu2.show();
  }
}

class Student {

  int sid;
  String sname;

  Student(int id, String sn) {
    System.out.println("Student 2-Arg Constructor");
    sid = id;
    sname = sn;
  }

  void show() {
    System.out.println(sid + "\t" + sname);
  }
}

✔ Output:

Student 2-Arg Constructor
88    Midhun
Student 2-Arg Constructor
99    Balan

Lab334.java

class Lab334 {

  public static void main(String[] args) {
    Student stu1 = new Student();
    stu1.show();

    Student stu2 = new Student(99, "Balan");
    stu2.show();
  }
}

class Student {

  int sid;
  String sname;

  Student() {
    System.out.println("Student Default Constructor");
  }

  Student(int id, String sn) {
    System.out.println("Student 2-Arg Constructor");
    sid = id;
    sname = sn;
  }

  void show() {
    System.out.println(sid + "\t" + sname);
  }
}

✔ Output:

Student Default Constructor
0    null
Student 2-Arg Constructor
99    Balan

Lab335.java

class Lab335 {

  public static void main(String[] args) {
    Student stu1 = new Student(99, "Midhun", "sri@JavaWorld", 99999);
    stu1.show();

    Student stu2 = new Student(88, "Vas", "vas@JavaWorld");
    stu2.show();

    Student stu3 = new Student(77, "SD");
    stu3.show();

    Student stu4 = new Student();
    stu4.show();
  }
}

class Student {

  int sid;
  String sname;
  String email;
  long phone;

  Student(int id, String sn, String em, long ph) {
    System.out.println("Student 4-Arg Constructor");
    sid = id;
    sname = sn;
    email = em;
    phone = ph;
  }

  Student(int id, String sn, String em) {
    System.out.println("Student 3-Arg Constructor");
    sid = id;
    sname = sn;
    email = em;
  }

  Student(int id, String sn) {
    System.out.println("Student 2-Arg Constructor");
    sid = id;
    sname = sn;
  }

  Student() {
    System.out.println("Student Default Constructor");
  }

  void show() {
    System.out.println(sid + "\t" + sname + "\t" + email + "\t" + phone);
  }
}

✔ Output:

Student 4-Arg Constructor
99   Midhun   sri@JavaWorld   99999

Student 3-Arg Constructor
88   Vas   vas@JavaWorld   0

Student 2-Arg Constructor
77   SD   null   0

Student Default Constructor
0   null   null   0

🧠 Key Concepts

🔹 Constructor Overloading

Multiple constructors in the same class with different parameters
JVM decides which constructor to call based on arguments

🔹 Default Values

Type	Default
int	0
String	null
long	0

🔹 Execution Flow

Constructor runs first
Then object is ready → methods can be called

🔹 Important Rules

Constructor name = class name
No return type
Can overload constructors
If no constructor → JVM provides default
If any constructor exists → JVM does NOT provide default

🚀 Quick Summary

Constructor Type	Purpose
Default	Initialize with default values
Parameterized	Initialize with custom values
Overloaded	Provide multiple ways to create object

Interview Insight

👉 “Which constructor will be called?”

Answer:

Depends on number and type of arguments

Lab336.java

class Lab336 {

  public static void main(String[] args) {
    Student stu = new Student();
    stu.Student(77, "SD"); //  trying to call constructor like a method
    stu.show();
  }
}

class Student {

  int sid;
  String sname;

  Student(int id, String sn) {
    System.out.println("Student 2-Arg Constructor");
    sid = id;
    sname = sn;
  }

  Student() {
    System.out.println("Student Default Constructor");
  }

  void show() {
    System.out.println(sid + "\t" + sname);
  }
}

Result:

Compilation Error
Reason: Constructor cannot be called like a method

Lab337.java

class Lab337 {

  public static void main(String[] args) {
    Student stu = new Student();
    stu.Student(77, "SD"); //  method call
    stu.show();
  }
}

class Student {

  int sid;
  String sname;

  void Student(int id, String sn) {
    //  This is NOT a constructor
    System.out.println("Student 2-Arg Constructor");
    sid = id;
    sname = sn;
  }

  Student() {
    System.out.println("Student Default Constructor");
  }

  void show() {
    System.out.println(sid + "\t" + sname);
  }
}

⚠️ Output:

Student Default Constructor
77    SD

🧠 Explanation:

void Student(...) → method, NOT constructor
Because constructor should not have return type
Default constructor is executed first
Then method is called manually

Lab338.java

class Lab338 {

  public static void main(String[] args) {
    Student stu = new Student(77, "SD"); //  No matching constructor
    stu.show();
  }
}

class Student {

  int sid;
  String sname;

  void Student(int id, String sn) {
    //  This is a method, not constructor
    System.out.println("Student 2-Arg Constructor");
    sid = id;
    sname = sn;
  }

  void show() {
    System.out.println(sid + "\t" + sname);
  }
}

Result:

Compilation Error

🧠 Reason:

No actual constructor Student(int, String) exists
Only a method exists → compiler cannot find constructor

🚀 Key Concepts from this Page

🔹 1. Constructor Rules

Constructor name must match class name
No return type (not even void)

🔹 2. Common Mistakes

Calling constructor like a method

stu.Student(77,"SD");// WRONG

Adding return type

void Student(...)// This becomes a method, NOT constructor

Missing constructor

If you define no constructor → JVM gives default
If you define any constructor → JVM does NOT give default

🔹 3. Constructor vs Method

Feature	Constructor	Method
Name	Same as class	Any name
Return type	Not allowed	Required
Call	Automatic	Manual
Purpose	Initialize object	Perform operation

Interview Traps from This Page

👉 Q: Why is void Student() not a constructor?

Because constructor must not have return type

👉 Q: Can we call constructor using object?

No — only via new

👉 Q: What happens if constructor is missing?

✔ JVM provides default constructor (only if no other constructor exists)

✔ Definition​

Lab329.java​

✔ Output:​

Lab330.java​

✔ Output:​

Lab331.java​

✔ Output:​

Lab332.java​

Result:​

🧠 Key Concepts

🔹 Default Constructor​

🔹 Parameterized Constructor​

🔹 Important Rule​

🚀 Quick Summary

Common Interview Point​

Lab333.java​

✔ Output:​

Lab334.java​

✔ Output:​

Lab335.java​

✔ Output:​

🧠 Key Concepts

🔹 Constructor Overloading​

🔹 Default Values​

🔹 Execution Flow​

🔹 Important Rules​

🚀 Quick Summary

Interview Insight​

Lab336.java​

Result:​

Lab337.java​

⚠️ Output:​

🧠 Explanation:​

Lab338.java​

Result:​

🧠 Reason:​

🚀 Key Concepts from this Page

🔹 1. Constructor Rules​

🔹 2. Common Mistakes​

Calling constructor like a method​

Adding return type​

Missing constructor​

🔹 3. Constructor vs Method​

Interview Traps from This Page

✔ Definition

Lab329.java

✔ Output:

Lab330.java

✔ Output:

Lab331.java

✔ Output:

Lab332.java

Result:

🔹 Default Constructor

🔹 Parameterized Constructor

🔹 Important Rule

Common Interview Point

Lab333.java

✔ Output:

Lab334.java

✔ Output:

Lab335.java

✔ Output:

🔹 Constructor Overloading

🔹 Default Values

🔹 Execution Flow

🔹 Important Rules

Interview Insight

Lab336.java

Result:

Lab337.java

⚠️ Output:

🧠 Explanation:

Lab338.java

Result:

🧠 Reason:

🔹 1. Constructor Rules

🔹 2. Common Mistakes

Calling constructor like a method

Adding return type

Missing constructor

🔹 3. Constructor vs Method