Interview questions on Collections in java for experienced

January 28, 2023

 Part 10

Interview questions on Collections in java for experienced covered in this post:

    Interview questions on Collections classes in java for experienced

        Can we store objects in descending order in SortedMap? If yes then, how can I store objects in descending order in SortedMap?

    -          We can store objects in descending order in SortedMap.

    -          There are a couple of ways we can do it in the case of SortedMap.

    1.       The first way is to implement Comparator in TreeMap Constructor.

    2.       The second way is you can use the Collections class reverseOrder() method.

    -          Please refer to the below two examples.

    1. Using Comparator Interface. 

    package simplifiedjava.crackedInterview; 

    import java.util.Collections;

    import java.util.Comparator;

    import java.util.SortedMap;

    import java.util.TreeMap; 

    public class SortedMapDemo { 

          public static void main(String[] args) {

          SortedMap<Integer, String> sMap = new TreeMap<>(new DecendingOrder());

                sMap.put(1000, "Thousand");

                sMap.put(1, "One");

                sMap.put(10000, "Ten Thousand");

                sMap.put(100, "Hundred");

                sMap.put(10, "Ten");         

                System.out.println(sMap);

          }

    } 

    class DecendingOrder implements Comparator<Integer>{ 

          @Override

          public int compare(Integer i1, Integer i2) {

                return i2.compareTo(i1);

          }    

    }

    Output: {10000=Ten Thousand, 1000=Thousand, 100=Hundred, 10=Ten, 1=One}

     

    2. Using reverseOrder() of Collections class. 

    package simplifiedjava.crackedInterview; 

    import java.util.Collections;

    import java.util.Comparator;

    import java.util.SortedMap;

    import java.util.TreeMap; 

    public class SortedMapDemo { 

          public static void main(String[] args) {

                SortedMap<Integer, String> sMap = new TreeMap<>(Collections.reverseOrder());

                sMap.put(1000, "Thousand");

                sMap.put(1, "One");

                sMap.put(10000, "Ten Thousand");

                sMap.put(100, "Hundred");

                sMap.put(10, "Ten");         

                System.out.println(sMap);

          }

    }

    Output: {10000=Ten Thousand, 1000=Thousand, 100=Hundred, 10=Ten, 1=One}

     

     

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        What is NavigableMap? What is the purpose of NavigableMap?

    -          NavigableMap is a child class of SortedMap.

    -          NavigableMap has added some new methods to navigate throughout the collection object.

    -          Please refer to the below example.

    package simplifiedjava.crackedInterview; 

    import java.util.NavigableMap;

    import java.util.TreeMap; 

    public class NavigableMapDemo { 

          public static void main(String[] args) {

             NavigableMap<Integer, String> nMap = new TreeMap<Integer, String>();

                nMap.put(50,"Fifty");

                nMap.put(10,"Ten");

                nMap.put(30,"Thirty");

                nMap.put(20,"Twenty");

                nMap.put(40,"Forty");        

                System.out.println("Actual nMap : " + nMap);         

                System.out.println("Lower value of 30 using lower()   = "+ nMap.lowerKey(30));

                System.out.println("Higher value of 30 using floor()  = "+ nMap.floorKey(30));

                System.out.println("Lowest value of 30 using ceiling()      = "+ nMap.ceilingKey(30));

                System.out.println("Lowest value of 30 using higher()       = "+ nMap.higherKey(30));

                System.out.println("Lowest value of 30 using pollFirst()    = "+ nMap.pollFirstEntry());

                System.out.println("Lowest value of 30 using pollLast()     = "+ nMap.pollLastEntry());          

                NavigableMap<Integer, String> reverse = nMap.descendingMap();           

                System.out.println("Reverse nMap : " + reverse);     

          }

    }

    Output:

    Actual nMap : {10=Ten, 20=Twenty, 30=Thirty, 40=Forty, 50=Fifty}

    Lower value of 30 using lower()         = 20

    Higher value of 30 using floor()        = 30

    Lowest value of 30 using ceiling()     = 30

    Lowest value of 30 using higher()      = 40

    Lowest value of 30 using pollFirst()   = {10=Ten}

    Lowest value of 30 using pollLast()      = {50=Fifty}

    Reverse nMap : {40=Forty, 30=Thirty, 20=Twenty}

     

        What is TreeMap? Can you explain how the objects will be stored in TreeMap?

    -          TreeMap is a data structure where elements are stored in some natural sorting order.

    -          Underlying data structure is the RED-BLACK tree.

    -          Insertion order never maintains. Maintain some natural sorting order.

    -          Duplicates keys are not allowed but duplicate values are allowed.

    -          If we are depending on natural sorting order then keys should be homogeneous and comparable otherwise it will through ClassCastException.

    -          If we are defining our comparator then keys need not be homogeneous or comparable.

    -          We can insert homogeneous as well as Heterogeneous (Different types of Object) objects in the Treemap.

    -          Null is strictly now allowed after java 7.

    -          Till java 6, For non-empty Treemap if we are trying to insert an entry with the null key then we will get NullPointerException. For empty tree map at first entry with null key is allowed but after inserting new key you will get NullPointerException.

    -          Please refer to the below example.

    package simplifiedjava.crackedInterview; 

    import java.util.TreeMap; 

    public class TreeMapDemo { 

          public static void main(String[] args) {           

                TreeMap<Integer, String> tMap = new TreeMap<Integer, String>();

                tMap.put(100, "Hundred");

                tMap.put(1, "One");

                tMap.put(1000, "Thousand");

                tMap.put(10, "Ten");

                System.out.println(tMap);    

          }

    }

    Output: {1=One, 10=Ten, 100=Hundred, 1000=Thousand}

     

        Is it possible to store elements in decending order in TreeMap?

    -          Yes, it is possible to store elements in treeMap in descending order.

    -          There are a couple of ways we can store elements in descending order which is as follows.

    1.       You can provide a Comparator object in the TreeMap constructor.

    2.       You can use Collections.reverse() method to it. 

     

    package simplifiedjava.crackedInterview; 

    import java.util.Collections;

    import java.util.Comparator;

    import java.util.TreeMap; 

    public class TreeMapDemo { 

          public static void main(String[] args) {           

                TreeMap<Integer, String> tMap = new TreeMap<Integer, String>(new ReverseOrderTreeElements());

                tMap.put(100, "Hundred");

                tMap.put(1, "One");

                tMap.put(1000, "Thousand");

                tMap.put(10, "Ten");

                System.out.println(tMap);                      

          }

    } 

    class ReverseOrderTreeElements implements Comparator<Integer>{ 

          @Override

          public int compare(Integer o1, Integer o2) {

                return o2.compareTo(o1);

          }    

    }

    Output: {1000=Thousand, 100=Hundred, 10=Ten, 1=One} 

    package simplifiedjava.crackedInterview; 

    import java.util.Collections;

    import java.util.Comparator;

    import java.util.TreeMap; 

    public class TreeMapDemo { 

          public static void main(String[] args) {           

                TreeMap<Integer, String> tMap = new TreeMap<Integer, String>(Collections.reverseOrder());

                tMap.put(100, "Hundred");

                tMap.put(1, "One");

                tMap.put(1000, "Thousand");

                tMap.put(10, "Ten");

                System.out.println(tMap);                      

          }

    }

    Output: {1000=Thousand, 100=Hundred, 10=Ten, 1=One}

     

        What is Hashtable? What are the characteristics of Hashtable?

    -          Hashtable is a thread-safe version of HashMap.

    -          Hashtable also maintain the elements in the form of Key/value pair.

    -          Underlying data structure of Hashtable is Hashtable.

    -          Insertion order never maintains.

    -          Insertion depends on the hashcode of the key.

    -          Duplicate keys are not allowed but duplicate values are allowed.

    -          Heterogeneous (Different types of Object) objects are allowed.

    -          Null is not allowed for both keys as well as value. Otherwise, it will through NullPointerException.

    -          Every method defined inside HashTable is synchronized.

    -          Hashtable is Clonable because Hashtable implements a Clonable interface.

    -          Hashtable is Serializable because Hashtable implements a Serializable interface.

    -          Hashtable is the worst choice if want to perform search or sort operation.

    -          Hashtable has not implemented RandomAccess.

    -          Please refer to the below example.

    package simplifiedjava.crackedInterview; 

    import java.util.Hashtable; 

    public class HashTableDemo { 

          public static void main(String[] args) {

              Hashtable<Integer, String> tMap = new Hashtable<Integer, String>();

                tMap.put(100, "Hundred");

                tMap.put(1, "One");

                tMap.put(1000, "Thousand");

                tMap.put(10, "Ten");

                System.out.println(tMap);          

          }

    }

    Output: {10=Ten, 1000=Thousand, 1=One, 100=Hundred}



    • Java interview questions and answers all MNC - Click here
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    • 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
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    • 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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