OCPJP Simulation Test 106

106

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Question: What is the result of the following program?

  1 import java.util.*;

  2 import java.util.concurrent.*;

  3 public class Mix {

  4   public static void main(String[] args) {

  5     List<String> shape = new ArrayList<>(Arrays.asList("circle","square","triangle"));

  6     List<String> collect1 = new CopyOnWriteArrayList<>(shape);

  7     List<String> collect2 = Collections.synchronizedList(shape);

  8     Set<String> collect3 = new ConcurrentSkipListSet<>(shape);

  9     synchronized(new Object()) {

 10       for(String s : collect1) collect1.add("star");

 11       for(String s : collect2) collect2.add("star");

 12       for(String s : collect3) collect3.add("star");

 13       System.out.println(collect1.size()+collect2.size()+collect3.size());

 14     }

 15   }}

Choice:
A. 663
B. 15
C. 666
D. The code does not compile
E. runtime exception at line 10
F. runtime exception at line 11
G. runtime exception at line 12
H. runtime exception at line 13



Next Question

Answer: F

Explain: The exception is:

Exception in thread "main" java.util.ConcurrentModificationException

at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)

at java.util.ArrayList$Itr.next(ArrayList.java:851)

at Mix.main(Mix.java:11)

You can modify concurrent collection inside its iterator, however, you can not modify the content of regular collections or synchronized collections inside its iterator. CopyOnWriteArrayList and ConcurrentSkipListSet are Concurrent collections. Collections.synchronizedList(xxx) are not regular collection wrapped as synchronized collection.

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