Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Chapter 18 | Threads and Locks
How to Approach:
In a Microsoft, Google or Amazon interview, it’s not terribly common to be asked to implement
an algorithm with threads (unless you’re working in a team for which this is a particularly
important skill). It is, however, relatively common for interviewers at any company to
assess your general understanding of threads, particularly your understanding of deadlocks
Deadlock Conditions
In order for a deadlock to occur, you must have the following four conditions met:
1. Mutual Exclusion: Only one process can use a resource at a given time.
2. Hold and Wait: Processes already holding a resource can request new ones.
3. No Preemption: One process cannot forcibly remove another process’ resource.
4. Circular Wait: Two or more processes form a circular chain where each process is waiting
on another resource in the chain.
Deadlock Prevention
Deadlock prevention essentially entails removing one of the above conditions, but many of
these conditions are difficult to satisfy. For instance, removing #1 is difficult because many
resources can only be used by one process at a time (printers, etc). Most deadlock prevention
algorithms focus on avoiding condition #4: circular wait.
If you aren’t familiar with these concepts, please read http://en.wikipedia.org/wiki/Deadlock.
A Simple Java Thread
1 class Foo implements Runnable {
2 public void run() {
3 while (true) beep();
4 }
5 }
6 Foo foo = new Foo ();
7 Thread myThread = new Thread(foo);
8 myThread.start();
8 5
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