If you are a B.Tech student in India, especially from CSE or IT branch, then you have definitely heard this word in your 2nd or 3rd year — Multithreading. And honestly, the first time I saw it in Java, I was confused.
Like… what exactly is a thread? How is it different from a process? Why do we even need it?
When I was preparing for my Java exam and later while building projects, things slowly started making sense. So in this blog, I’ll explain Multithreading in Java in very simple words — like one student explaining to another.
Let’s start from the basics.
Introduction
In real life, we do multiple things at the same time.
For example:
- Listening to music while coding.
- Downloading a movie while browsing Instagram.
- Writing an assignment while chatting with friends (okay maybe not a good idea before exams 😅).
Similarly, in programming, sometimes we want our application to perform multiple tasks simultaneously.
That’s where Multithreading in Java comes into the picture.
In simple words, multithreading allows a program to execute multiple parts of code at the same time, making it faster and more efficient.
What is Thread?
Before understanding multithreading, we need to understand what a thread is.
🔹What is a Process?
When you open:
- Chrome
- VS Code
- IntelliJ
- Spotify
Each one is a separate process.
A process is basically a program that is currently running.
🔹What is a Thread?
A thread is a smaller unit of a process.
In simple words:
A thread is a path of execution inside a program.
If a process is like your college project, then threads are like team members working on different tasks.
For example:
Imagine you are building a Train Ticket Reservation System project:
- One thread handles user login
- One thread checks seat availability
- One thread processes payment
All of them are part of the same application (process), but they can run independently.
🔹 Why Not Just Use One Thread?
- Tasks run one by one.
- If one task takes time, everything else waits.
Example: If your app is downloading data from an API and it takes 5 seconds, the UI might freeze.
Many students get confused here They think multithreading means multiple programs.
No. It means multiple threads inside the same program.
Thread Life Cycle in Java
This is very important for exams. Almost every university asks this question.
A thread in Java goes through different states during its lifetime.
Let’s understand one by one.
Using New keyword
When you create a thread object using:
Thread t = new Thread();
It is in the New state.
It is created but not started yet.
Runnable
When you call:
t.start();
The thread moves to Runnable state.
Now it is ready to run, but CPU will decide when to actually execute it.
Running
When the thread starts executing its code, it is in the Running state.
This is when the run() method is actually being executed.
Blocked / Waiting
Sometimes a thread has to wait:
- For user input
- For another thread
- For a resource like file or database
Then it goes to Blocked or Waiting state.
Example: In a banking app, one thread might wait for payment confirmation.
Terminated (Dead)
After the thread completes its task, it moves to the Terminated state.
It cannot be restarted again.
Thread Life Cycle Summary Table
| State | Description |
|---|---|
| New | Thread object created |
| Runnable | Ready to run |
| Running | Currently executing |
| Blocked | Waiting for resource |
| Terminated | Execution completed |
When I first learned this, I tried to memorize it. Later I realized, just imagine it like a student:
- Admission taken → New
- Sitting in class → Runnable
- Writing exam → Running
- Waiting for teacher → Blocked
- Course completed → Terminated
Much easier to remember!
Creating Threads in Java
Now comes the practical part how do we create threads?
There are mainly two ways to create threads in Java.
By Extending Thread Class
This is the most basic and common way taught in college.
Example:
class MyThread extends Thread {
public void run() {
System.out.println("Thread is running...");
}
}
public class Main {
public static void main(String[] args) {
MyThread t1 = new MyThread();
t1.start();
}
}
Explanation:
- We extend the Thread class.
- Override the run() method.
- Call start() to begin execution.
⚠ Important: Never call run() directly. Always call start().
Because:
- start() creates a new thread.
- run() works like a normal method call.
Many students lose marks here in exams.
By Implementing Runnable Interface
This method is more flexible and preferred in real projects.
Example:
class MyRunnable implements Runnable {
public void run() {
System.out.println("Thread using Runnable");
}
}
public class Main {
public static void main(String[] args) {
MyRunnable obj = new MyRunnable();
Thread t1 = new Thread(obj);
t1.start();
}
}
🔹 Why Runnable is Better?
Because Java does not support multiple inheritance.
If you extend Thread, you cannot extend another class.
But if you implement Runnable, you can still extend another class.
In real projects (especially in Spring Boot backend), Runnable or Executor framework is preferred.
🔹Comparison Table
| Feature | Extending Thread | Implementing Runnable |
|---|---|---|
| Inheritance | Cannot extend another class | Can extend another class |
| Flexibility | Less | More |
| Recommended | For beginners | For real-world projects |
Multithreading Example from College Life
Let’s say you are preparing for semester exams.
You:
- Revise Java
- Practice DSA
- Complete assignment
- Prepare lab record
If you do everything one by one, it takes a lot of time.
But if:
- You revise theory in morning
- Practice coding in afternoon
- Write assignment at night
Multiple tasks progress together.
That’s exactly what multithreading does in programs.
Benefits of Multithreading
Better Performance
Multiple tasks can run at the same time.
- Web server handling multiple users.
- Banking app processing multiple transactions.
Improved Responsiveness
Very important in UI applications.
- Java Swing app
- Android app
You don’t want UI to freeze while data loads.
Multithreading keeps UI smooth.
Efficient CPU Utilization
- Dual core
- Quad core
- Even 8 cores
Multithreading allows programs to use multiple cores effectively.
Resource Sharing
Threads share memory of the same process.
This makes communication between them easier and faster.
But careful — this can also cause problems like:
- Race condition
- Deadlock
(Advanced topics, but good to know.)
Used Everywhere in Real Projects
Web servers use threads.
Spring Boot handles requests using threads.
Even simple applications use background threads.
So this is not just exam theory — it’s practical.
Common Confusion Among Students
Many students think:
Multithreading automatically makes program faster.
Not always.
If:
- Threads are not managed properly
- Synchronization is not handled
Then it can cause bugs which are very hard to debug.
Trust me, debugging thread issues is painful 😅
When Should You Use Multithreading?
Use it when:
- You have independent tasks.
- You need background processing.
- You want better performance.
Avoid it when:
- Your program is simple.
- Tasks depend heavily on each other.
Frequently Asked Questions
What is the difference between process and thread ?
A process is an independent program in execution. A thread is a smaller unit inside a process.
Threads share memory. Processes do not.
Why do we call start() instead of run()?
Because start() creates a new thread and then calls run() internally.
Calling run() directly does not create a new thread.
Can we restart a thread after it is terminated?
No. Once a thread is completed, it cannot be restarted.
You must create a new thread object.
Which method is better: Thread or Runnable?
Implementing Runnable is better because it allows more flexibility and supports multiple inheritance.
Is multithreading important for interviews?
Yes. Especially for:
- Java backend roles
- Spring Boot developers
- System design discussions
Basic understanding is very important.
Conclusion
So, in simple words, Multithreading in Java allows a program to perform multiple tasks at the same time.
We learned:
- What is a thread
- Thread life cycle
- How to create threads
- Benefits of multithreading
When I first studied this topic, it felt complicated. But once I started thinking in real-life examples like college projects, exam preparation, and backend applications everything became much clearer.
If you are a beginner, don’t try to learn everything at once. First understand:
- Thread
- start() vs run()
- Life cycle
Then slowly move to advanced topics like:
- Synchronization
- Deadlock
- Executor Framework
Multithreading is not just an exam topic. It is a core concept in real world Java development.
Keep practicing. Try small programs. Break things. Debug them. That is how you will actually understand it.
Happy Coding 🚀
