Core Java MCQ Questions – Java Online Challenge

In Iterator, hasMoreElements() method of Enumeration has been changed to:

TreeSet internally uses that one to store elements?

HashSet internally uses?

An attempt to add the null key to a TreeSet will result in:

TreeSet maintains which order?

Enumeration returned by ArrayList is

LinkedHashMap permits

Hashtable permits

How are you able to sort given HashMap on basis of values

The Comparator interface contains the method?

What will Collections.sort internally uses when number of elements are less than 7?

An attempt to add null key to a TreeMap will result in:

What will be the output of the following code –
import java.util.Map;
import java.util.TreeMap;

public class TreeMapTest {

public static void main (String args[])
{
Map<Integer,String> m= new TreeMap<Integer,String>();
m.put(11, “audi”);
m.put(null, null);
m.put(11,”bmw”);
m.put(null,”fer”);

System.out.println(m.size());
System.out.println(m);
}
}

Which of those is synchronized and does not allow null elements?

Which of these is synchronised and isn’t sorted by natural order?

What will be the output of the following code –
import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.ConcurrentSkipListMap;

public class ConcurrentSkipMapTest {

public static void main (String args[])
{
Map<Integer,String>concurrentSkipListMap= new ConcurrentSkipListMap<Integer,String>();
concurrentSkipListMap.put(11,”audi”);

Iterator<Integer> keyIterator = concurrentSkipListMap.keySet().iterator();
while (keyIterator.hasNext())
{
System.out.print(keyIterator.next());
concurrentSkipListMap.put(13,”bmw”);
}
}

}

What will be the output of the following code –
import java.util.EnumMap;
import java.util.Map;
public class EnumMapExample {
public enum Days
{
Monday;
}public static void main (String args[])
{
Map<Days,String> daysEnumMap = new daysEnumMap<Days,String>(Days.class);
daysEnumMap.put(Days.Monday,”Day1″);
daysEnumMap.put(Days.Tuesday,”Day2″);
System.out.println(“daysEnumMap.get(Days.Monday):”+ daysEnumMap.get(Days.Monday));
System.out.println(“daysEnumMap.containsKey(Days.Monday):”+
daysEnumMap.containsKey(Days.Tuesday)) ;
}

}

What will be the output of the following code –
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;

public class CopyOnWriteArrayListTest
{
public static void main (String args[])
{
List<String> copyOnWriteArrayList = new CopyOnWriteArrayList<String>();
copyOnWriteArrayList.add(” Ind “);
copyOnWriteArrayList.add(” usa “);
copyOnWriteArrayList.add(null);
for(String string : copyOnWriteArrayList)
{
System.out.print(string+””);
copyOnWriteArrayList.add(“newEle3”) ;
}
}
}

In ListIterator, when a call to next() or previous(), if a call is made to set(E element) followed by add() or remove() what happens?