Java - Multithreading & Concurrency in Java

1️⃣ What is Multithreading?

Multithreading means running multiple parts of a program (called threads) at the same time.
A thread is a lightweight unit of execution inside a program.

Instead of executing tasks one by one, Java can execute several tasks simultaneously, improving performance and responsiveness.

Example:

Downloading a file while playing music
A game updating graphics while processing user input

Each task runs in its own thread.

2️⃣ What is Concurrency?

Concurrency refers to managing multiple tasks that make progress during overlapping time periods.
It doesn’t always mean they run exactly at the same instant (that’s parallelism), but they are handled efficiently together.

In Java:

Multithreading → creating multiple threads
Concurrency → controlling and coordinating them safely

3️⃣ Why is Multithreading Important?

Better CPU utilization
Faster program execution
Improved responsiveness (especially in GUIs/web apps)
Ability to handle multiple users/tasks

4️⃣ Creating Threads in Java

There are two common ways:

✅ Method 1: Extending Thread class

class MyThread extends Thread {

public void run() {

System.out.println("Thread running");

}

MyThread t = new MyThread();

t.start();

Method 2: Implementing `Runnable` interface

class MyTask implements Runnable {
    public void run() {
        System.out.println("Task running");
    }
}

Thread t = new Thread(new MyTask());
t.start();

Using Runnable is preferred because Java supports only single inheritance.

5️⃣ Thread Lifecycle (Stages)

A thread goes through different states:

New — thread created
Runnable — ready to run
Running — executing
Blocked/Waiting — paused temporarily
Terminated — finished execution

6️⃣ Synchronization (Very Important Concept)

When multiple threads access shared data, problems may occur (race conditions).

Synchronization ensures only one thread accesses critical code at a time.

Example:

synchronized void update() {

// critical section

}

This prevents data inconsistency.

7️⃣ Advantages of Multithreading

Efficient resource use
Faster applications
Better multitasking
Improved user experience

8️⃣ Challenges
- Debugging is difficult
- Risk of deadlocks
- Data inconsistency without synchronization
- More complex program design
  
  ✅ In Simple Words
  Multithreading allows Java programs to perform multiple tasks at once using threads, while concurrency ensures these threads work together safely and efficiently. It is essential for building fast and responsive applications.