Threads Types

Short answer: 2 main types But like most things in Java, there’s a deeper layer if you peek under the hood 👀

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🧠 1. User Threads (a.k.a. Non-Daemon Threads)

These are the main actors on stage 🎭

• Created by your application
• JVM waits for them to finish
• Keep the program alive

Thread t = new Thread(() -> {
    System.out.println("User thread running");
});
t.

start();

👉 Examples:

• main() thread
• Worker threads
• Business logic threads

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🌙 2. Daemon Threads

These are the background workers 🛠️

• Run in background
• JVM does NOT wait for them
• Die automatically when all user threads finish

Thread t = new Thread(() -> {
    while (true) {
        System.out.println("Daemon working...");
    }
});
t.setDaemon(true);
t.start();

👉 Examples:

• Garbage Collector thread
• JVM housekeeping threads

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⚔️ User vs Daemon

Feature	User Thread 🧑	Daemon Thread 🌙
Purpose	Main work	Background work
JVM waits?	✔ Yes	❌ No
Lifecycle	Independent	Depends on user threads

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🧠 Internal JVM Threads (bonus insight)

Even if you don’t create threads, JVM already runs some:

• Garbage Collector
• JIT Compiler
• Signal Dispatcher

👉 Most of these are daemon threads

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⚠️ Important Rule

t.setDaemon(true);

👉 Must be called before start() Otherwise → IllegalThreadStateException

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🎯 Interview Answer

Java has two main types of threads: User threads and Daemon threads. User threads perform main application tasks and keep JVM alive, while daemon threads run in the background and terminate when all user threads finish.

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🧠 Memory Trick

User Thread = Life of program 🧑 Daemon Thread = Support system 🌙

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If you want deeper:

• Thread lifecycle (NEW → RUNNABLE → BLOCKED...)
• Or how threads actually map to OS threads (important for interviews)