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# Thread Fundamentals
A **thread** is the smallest unit of execution in a program. In Java, every program runs on at least one thread, known as the **main thread**.
## **1. What is a Thread?**
* A **thread** is a lightweight process that runs independently.
* Java supports **multithreading**, meaning multiple threads can run simultaneously.
* Each thread has its own **execution path** but shares resources with other threads in the same process.
### **Analogy**
Imagine a **thread** as a train running on a track.
* **A single-threaded program** is like one train running on a single track, completing one task at a time.
* **A multi-threaded program** is like multiple trains running on parallel tracks, performing tasks simultaneously.
Here’s a visual representation of threads:
```
Main Thread -----> Task 1 -----> Task 2 -----> Task 3 (Sequential Execution)
Multi-Threading:
Thread 1 -----> Task A -----> Task B
Thread 2 -----> Task X -----> Task Y
(Thread 1 and Thread 2 run in parallel)
```
### **Java Thread Package**
Threads in Java are part of the `java.lang` package, primarily using the following classes and interfaces:
* `java.lang.Thread` → The main class for creating and managing threads.
* `java.lang.Runnable` → Functional interface for defining thread tasks.
* `java.util.concurrent` package → Provides higher-level concurrency utilities.
## **2. Why Use Threads?**
* Improves application performance by utilizing **multiple CPU cores**.
* Allows multiple tasks to execute **simultaneously** (e.g., UI responsiveness, background tasks).
* Enables **parallel execution** for tasks like data processing, computations, and network requests.
## **3. Thread Lifecycle**
A thread goes through multiple states:
1. **New** → Thread is created but not started.
2. **Runnable** → Ready to run but waiting for CPU.
3. **Running** → Actively executing.
4. **Blocked/Waiting** → Paused, waiting for a resource.
5. **Terminated** → Execution is completed or stopped.
## **4. Thread Priorities**
Each thread in Java has a priority (range **1 to 10**). Default priority is `5 (NORM_PRIORITY)`.
* `Thread.MIN_PRIORITY` → 1
* `Thread.NORM_PRIORITY` → 5
* `Thread.MAX_PRIORITY` → 10
## **5. Daemon vs User Threads**
* **User Threads** → Standard application threads.
* **Daemon Threads** → Background threads that terminate when no user thread is running.
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