Part 1
Interview questions on Collections in java for experienced covered in this post:
What are Legacy Classes?
- A legacy class means old classes.
- Earlier versions of Java didn’t include the Collection
framework.
- From java version 1.2 started using these Legacy classes.
These classes were reengineered to support the collection interface. These
classes are known as Legacy classes.
- Following are the Legacy classes.
1. Vector.
2. Stack.
3. Dictionary.
4. HashTable.
5. Properties.
What is the purpose of Arrays utility class?
- Array utility class contained in java.util.Arrays
package.
- This class provides some utility methods for manipulating
the arrays like sorting, searching, converting an array into a collection
etc.
 
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What is the purpose of Collections Utility class? What are the utility methods are available in Collections Class?
- Collections is a utility class for the collection
framework.
- Collections utility class has some methods which are as
follows.
1. addAll()
2. binarySearch()
3. checkedCollection()
4. copy()
5. fill()
6. min()
7. max()
8. replaceAll()
9. reverse()
10. shuffle()
11. synchronizedCollection()
12. unmodifiableCollection()
What is the difference between Collection and Collections?
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Collection |
Collections |
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1. |
The Collection is parent Interface. |
Collections is a utility class. |
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2. |
It extends the Object class. |
It extends the Iterable interface. |
Is Collection object by default thread-safe?
- All collections are not thread-safe by default.
- Following collection objects are thread-safe rest are not
thread-safe.
1. Vector.
2. HashTable.
3. ConcurrentHashMap.
4. CopyOnWriteArryList.
5. CopyOnWriteArraySet.
How will you convert collection object to thread-safe collection object?
- We have a utility class that has some utility methods to
convert non-thread-safe collections to the thread-safe collection.
- Each collection has its utility method to convert
non-thread-safe to thread-safe.
- Following are the collections and its
utility methods.
1. Collection has Collections.synchronizedCollection().
2. Map has Collections.synchronziedMap().
3. SortedMap has Collections.synchronizedSortedMap().
4. NavigableMap has Collections.navigableMap().
5. List has Collections.synchronizedList().
6. Set has Collections.synchronizedSet().
7. NavigableSet has
Collections.synchronizedNavigableSet().
8. SortedSet has Collections.synchronizedSortedSet().
How to remove the duplicates from list object without stream API?
- Couple of ways you can remove duplicates from the list.
1. By using HashSet.
2. By using LinkedHashSet.
- Please refer to the below example.
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package simplifiedjava.crackedInterview;
import
java.util.ArrayList;
import
java.util.HashSet;
import
java.util.LinkedHashSet;
import
java.util.List; import java.util.Set;
public class
RemoveDuplicatesFromList {
public static void
main(String[] args) {
List<Integer> list
= new
ArrayList<Integer>();
list.add(10);
list.add(20);
list.add(30);
list.add(30); list.add(10);
System.out.println("----- Using HashSet-----");
Set<Integer> listHashSet
= new
HashSet<Integer>(list);
for(Integer i
: listHashSet) {
System.out.println(i); }
System.out.println("----- Using LinkedHashSet-----");
Set<Integer> listLinkedHashSet
= new
LinkedHashSet<Integer>(list);
for(Integer i
: listLinkedHashSet) {
System.out.println(i);
}
}
}
Output:
----- Using HashSet-----
20
10
30
----- Using LinkedHashSet-----
10
20
30 |
Is it possible to remove the duplicates and arrange all objects in
sorting order in single step?
- Yes, It is possible to remove the duplicates and arrange
all objects in sorting order using LinkedHashSet.
- Please refer to the below example.
|
package simplifiedjava.crackedInterview;
import
java.util.ArrayList;
import
java.util.LinkedHashSet;
import
java.util.List; import java.util.Set; public class RemoveDuplicatesFromList {
public static void
main(String[] args) {
List<Integer> list
= new
ArrayList<Integer>();
list.add(10);
list.add(20);
list.add(30);
list.add(30); list.add(10);
System.out.println("----- Using LinkedHashSet-----");
Set<Integer> listLinkedHashSet
= new
LinkedHashSet<Integer>(list);
for(Integer i
: listLinkedHashSet) {
System.out.println(i);
}
}
}
Output:
----- Using LinkedHashSet-----
10
20
30 |
Can you make collection object ready only? If yes then how will you
make sure your collection object is read only?
- We can make collection read-only by unmodifiableList()
method which exists in the Collections utility class.
- If you apply the unmodifiableList() method on any list
object then you are not allowed to modify the list object. Still, you try
to modify it then you will get UnSupportedOperationException.
- Make sure once you invoked to unmodifiableList() method on
the list then you have to assign the same list to the old reference
variable.
- If you don’t assign then you won’t get
UnSupportedOperationException and you will be able to modify the list.
- Please refer to the below example.
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package simplifiedjava.crackedInterview;
import
java.util.ArrayList;
import
java.util.Collections; import java.util.List; public class UnmodifiableListDemo { public static void main(String[] args) {
List<Integer> list
= new
ArrayList<Integer>();
list.add(10);
list.add(20);
list.add(30);
list.add(30); list.add(10); System.out.println("Before Unmodified "+ list); list = Collections.unmodifiableList(list); list.remove(2);
System.out.println("After Unmodified "+ list);
}
}
Output:
Exception in thread "main" Before Unmodified [10, 20, 30, 30, 10]
java.lang.UnsupportedOperationException
at
java.util.Collections$UnmodifiableList.remove(Unknown
Source)
at
simplifiedjava.crackedInterview.UnmodifiableListDemo.main(UnmodifiableListDemo.java:21) |
When you will get UnSupportedOperationException?
- UnSupportedOperationException is a runtime exception.
- Java interview questions and answers all MNC - Click here
- Basic core java interview questions and answers for freshers - Click here
- Core java interview questions for 3 years experience - Click here
- Core java interview questions and answers for 3-5 years exp - Click here
- Core java interview questions and Answers for 5 - 7 Years exp - Click here
- Basic Java Interview Questions and Answers - Click here
- Java interview questions and answers on oops - Click here
- Java interview questions and answers on Strings - Click here
- Java interview questions on exception handling - Click here
- Interview questions on multithreading in java for experienced - Click here
- Interview questions on serialization in java for experienced - Click here
- Interview questions on inner class in java for experienced - Click here
- Interview questions on Collections in java for experienced - Click here
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