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

Solutions to Chapter 18 | Threads and Locks
18.6 You are given a class with synchronized method A, and a normal method C. If you
have two threads in one instance of a program, can they call A at the same time? Can
they call A and C at the same time?
SOLUTION
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Java provides two ways to achieve synchronization: synchronized method and synchronized
statement.
Synchronized method: Methods of a class which need to be synchronized are declared with
“synchronized” keyword. If one thread is executing a synchronized method, all other threads
which want to execute any of the synchronized methods on the same objects get blocked.
Syntax: method1 and method2 need to be synchronized
1 public class SynchronizedMethod {
2 // Variables declaration
3 public synchronized returntype Method1() {
4 // Statements
5 }
6 public synchronized returntype method2() {
7 // Statements
8 }
9 // Other methods
10 }
Synchronized statement: It provides the synchronization for a group of statements rather
than a method as a whole. It needs to provide the object on which these synchronized statements
will be applied, unlike in a synchronized method.
Syntax: synchronized statements on “this” object
1 synchronized(this) {
2 /* statement 1
3 * ...
4 * statement N */
5 }
i) If you have two threads in one instance of a program, can they call A at the same time?
Not possible; read the above paragraph.
ii) Can they call A and C at the same time?
Yes. Only methods of the same object which are declared with the keyword synchronized
can’t be interleaved.
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