Deadlock in Java — Java Technical Interview Questions & Answers

Deadlock in Java

Deadlock in Java

  • Deadlock is part of Java concurrency. A thread is a path of work inside a process. Create with Thread or Runnable, then call start() — not run(). run() only runs on the same thread.
  • For Deadlock: if two threads share data, synchronize or the result can be wrong. sleep pauses. join waits for another thread to finish. daemon threads die when all user threads finish.
  • Deadlock deadlock: two threads wait for each other’s lock forever. Avoid by locking in the same order. Don’t guess thread order — it can change every run.
  • Let's take this on the board with one tiny Main class — no extra files. Deadlock in Java — output: 2000. Two threads each inc 1000 times. synchronized keeps the count exact — without it, you can get less than 2000. Start from main, go line by line, and stop at each print. That output is the proof for Deadlock.

Technical Interview Tip: Define deadlock with a 2-lock A/B story.

Code Examples for Deadlock in Java

Example

class Counter {
  private int n;
  synchronized void inc() { n++; }
  synchronized int get() { return n; }
}
public class Main {
  public static void main(String[] args) throws Exception {
    Counter c = new Counter();
    Thread t1 = new Thread(() -> { for (int i = 0; i < 1000; i++) c.inc(); });
    Thread t2 = new Thread(() -> { for (int i = 0; i < 1000; i++) c.inc(); });
    t1.start(); t2.start();
    t1.join(); t2.join();
    System.out.println(c.get()); // 2000
  }
}

Deadlock in Java — output: 2000. Two threads each inc 1000 times. synchronized keeps the count exact — without it, you can get less than 2000.

How to Explain Deadlock in Java to an Interviewer

What is Deadlock?

Deadlock — thread = one path of work. start() schedules it. run() does not. Deadlock — shared data needs synchronization. Deadlock — sleep = pause. join = wait for other thread.

Why do we use Deadlock in Java?

Makes the program clearer or safer Deadlock — thread = one path of work. start() schedules it. run() does not.

Write one small example of Deadlock.

Show syntax Say what prints

What mistake do beginners make with Deadlock?

Only the name, no example Deadlock — calling run() instead of start(). Deadlock = locks in wrong order.

Where is Deadlock used in a real program?

Day-to-day Java code