Introduction
When beginners start learning Java, two topics often create confusion:
- Constructors
- Packages
Many students ask:
- What is a constructor?
- How many types of constructors exist?
- Is a default constructor the same as a no-argument constructor?
- How does Java automatically create constructors?
- Why do we need packages?
- How do packages work?
- How do we compile and run package-based programs?
This guide explains everything from scratch in a beginner-friendly way.
Part 1: Constructors in Java
Official Definition (Oracle Java Documentation)
According to Oracle Java Tutorials:
"A class contains constructors that are invoked to create objects from the class blueprint." ¹
In simple words:
A constructor is a special block of code that runs automatically whenever an object is created.
Think of a constructor as the "setup process" of an object.
Real-Life Example
Imagine buying a new mobile phone.
When the phone is turned on for the first time:
- Memory is allocated
- Default settings are loaded
- Language is selected
- System gets initialized
Similarly, when an object is created in Java:
Student s1 = new Student();
Java allocates memory and calls the constructor to initialize the object.
Characteristics of Constructors
A constructor:
✔ Has the same name as the class
✔ Has no return type
✔ Runs automatically when an object is created
✔ Can accept parameters
✔ Can be overloaded
✔ Cannot be static
✔ Cannot be abstract (TBD)
✔ Cannot be final (TBD)
Example
class Student {
Student() {
System.out.println("Constructor Called");
}
public static void main(String[] args) {
Student s = new Student();
}
}
Output:
Constructor Called
Notice:
Student()
has the same name as the class.
Why Do We Need Constructors?
Without constructors:
Student s = new Student();
s.name = "John";
s.age = 20;
You would need to initialize everything manually.
Constructors allow initialization during object creation itself.
Student s = new Student("John",20);
Much cleaner.
Types of Constructors in Java
Most beginner courses discuss:
- Default Constructor
- No-Argument Constructor
- Parameterized Constructor
Additionally, constructor overloading is an important concept.
Let's study each deeply.
1. Default Constructor
This is one of the most misunderstood topics in Java.
Official Definition
Oracle documentation explains that if a class contains no constructors, the compiler automatically provides one. ¹
What is a Default Constructor?
A default constructor is a constructor automatically created by the Java compiler when you do not write any constructor.
Example:
class Student {
}
You wrote:
class Student {
}
But Java internally behaves like:
class Student {
Student() {
}
}
The compiler secretly creates it.
Why Does Java Create It?
Imagine this code:
Student s = new Student();
Java needs some constructor to execute.
Since you didn't provide one, Java generates one automatically.
What Does the Default Constructor Do?
The generated constructor:
- Calls parent constructor using super()
- Initializes object fields with default values
Example:
class Student {
int id;
String name;
}
Object creation:
Student s = new Student();
Values become:
id = 0
name = null
These are Java's default field values.
Internal View
Compiler-generated constructor roughly looks like:
Student() {
super();
}
Important Rule
Java generates a default constructor ONLY IF NO CONSTRUCTOR EXISTS.
Example:
class Student {
Student(int id){
}
}
Now:
Student s = new Student();
Error!
Why?
Because Java sees that you already created a constructor.
Therefore it does not generate the default constructor.
2. No-Argument Constructor
Now comes the most confusing interview question.
What is a No-Argument Constructor?
A constructor that accepts zero parameters.
Example:
class Student {
Student() {
System.out.println("No Arg Constructor");
}
}
No parameters.
Hence:
Student()
is a no-argument constructor.
Is Default Constructor Equal to No-Argument Constructor?
Short Answer:
NO.
They are related but not identical.
Difference Table
| Feature | Default Constructor | No-Argument Constructor |
|---|---|---|
| Created By | Java Compiler | Programmer |
| Parameters | None | None |
| Written by User | No | Yes |
| Automatic | Yes | No |
Example 1: Default Constructor
class Student {
}
Compiler creates:
Student() {
}
This is a DEFAULT constructor.
Example 2: No-Argument Constructor
class Student {
Student() {
System.out.println("Created");
}
}
This is a NO-ARGUMENT constructor.
Easy Rule to Remember
Every default constructor is a no-argument constructor.
BUT
Not every no-argument constructor is a default constructor.
Why?
Because default constructors are compiler-generated.
No-arg constructors are programmer-written.
Interview Question
Question:
Is default constructor same as no-arg constructor?
Best Answer:
No.
A default constructor is automatically provided by the Java compiler when no constructor exists in the class.
A no-argument constructor is explicitly written by the programmer and takes no parameters.
3. Parameterized Constructor
Official Definition
W3Schools states that constructors can take parameters that initialize object attributes. ²
What is a Parameterized Constructor?
A constructor that receives values while creating objects.
Example:
class Student {
String name;
int age;
Student(String name,int age){
this.name = name;
this.age = age;
}
}
Object Creation
Student s1 = new Student("John",20);
Values become:
name = John
age = 20
Why Use Parameterized Constructors?
Without constructor:
Student s = new Student();
s.name="John";
s.age=20;
With constructor:
Student s = new Student("John",20);
Cleaner and safer.
Constructor Overloading
Java allows multiple constructors.
Example:
class Student {
Student() {
}
Student(String name) {
}
Student(String name,int age) {
}
}
Java selects constructor based on arguments.
new Student();
new Student("John");
new Student("John",20);
This is constructor overloading.
Constructor Chaining (TBD)
One constructor can call another constructor.
Example:
class Student {
Student() {
this("Unknown");
}
Student(String name){
System.out.println(name);
}
}
Output:
Unknown
This improves code reuse.
Summary of Constructors
- Constructor initializes objects.
- Constructor name must match class name.
- No return type.
- Runs automatically.
- Default constructor is compiler-generated.
- No-arg constructor is programmer-written.
- Parameterized constructor accepts values.
- Constructors can be overloaded.
Part 2: Packages in Java
Now let's move to another important topic.
What is a Package?
Official Definition
According to Oracle:
"A package is a namespace that organizes a set of related classes and interfaces." ³
Beginner-Friendly Definition
A package is a folder-like structure used to organize Java classes.
Think about your computer.
Without folders:
1000 files in one place
Very messy.
Folders solve this problem.
Packages do the same in Java.
Why Do We Need Packages?
Imagine a company with:
5000 Java classes
Without packages:
Student.java
Employee.java
Manager.java
Account.java
Admin.java
Everything mixed together.
Hard to manage.
Packages provide organization.
Advantages of Packages
1. Organization
Classes stay grouped.
Example:
com.company.student
com.company.employee
com.company.finance
2. Avoid Name Conflicts
Two developers may create:
Student
class.
Packages allow:
college.Student
and
school.Student
Both can exist.
3. Access Control
Packages help control visibility.
Keywords:
public
protected
default
private
work together with packages.
Package Syntax
package mypackage;
Must be the first statement.
Example:
package mypackage;
public class Student {
}
Creating Your First Package
Folder structure:
project
|
|-- mypackage
|
Student.java
Student.java
package mypackage;
public class Student {
public void show(){
System.out.println("Hello Package");
}
}
Using Package Class
import mypackage.Student;
public class Main {
public static void main(String[] args){
Student s = new Student();
s.show();
}
}
Import Keyword (TBD)
Used to access classes from other packages.
Syntax:
import packageName.ClassName;
Example:
import mypackage.Student;
Import All Classes
import java.util.*;
The * means all classes.
Example:
ArrayList
Scanner
HashMap
can be used.
Common Java Packages
java.lang
Imported automatically.
Contains:
String
System
Math
Object
java.util
Contains:
ArrayList
Scanner
HashMap
Collections
java.io
Contains:
File
BufferedReader
PrintWriter
How Java Stores Packages
Package:
package com.company.student;
creates structure:
com
|
company
|
student
Java expects folder names to match package names.
Compiling Packages
This is where beginners get confused.
Suppose:
package mypackage;
public class Student {
}
File:
Student.java
Compile:
javac -d . Student.java
What Does -d Mean?
-d means destination directory.
javac -d . Student.java
means:
"Create package folders in current directory."
Output:
mypackage
|
Student.class
Running Package Programs
Compile:
javac -d . Student.java
Run:
java mypackage.Student
Notice:
java Student
❌ Wrong
java mypackage.Student
✅ Correct
Example
File:
package mypackage;
public class Student {
public static void main(String[] args){
System.out.println("Hello Package");
}
}
Compile:
javac -d . Student.java
Run:
java mypackage.Student
Output:
Hello Package
Package Naming Convention
Usually:
com.company.project
Examples:
com.google
com.microsoft
com.amazon
org.apache
Reverse domain naming prevents conflicts.
Real Industry Example
Spring Framework:
org.springframework
Hibernate:
org.hibernate
JUnit:
org.junit
Large projects may contain hundreds of packages.
Final Summary
Constructors:
- Special methods used for initialization.
- Run automatically during object creation.
Types:
- Default Constructor
- No-Argument Constructor
- Parameterized Constructor
- Constructor Overloading
Important:
- Default constructor is generated by compiler.
- No-arg constructor is written by programmer.
- Every default constructor is a no-arg constructor.
- Not every no-arg constructor is a default constructor.
Packages:
- Organize Java classes.
- Prevent naming conflicts.
- Improve maintainability.
- Support access control.
- Created using package keyword.
- Accessed using import keyword.
Compile package:
javac -d . Student.java
Run package:
java mypackage.Student
References
Oracle Java Tutorials – Constructors:
Oracle Java Tutorials – Packages:
Java Language Specification (JLS):
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