Cybage Java interview questions and answers.

Java Interview Questions and Answers covered in this post:

    What is the difference between Composition and Aggregation?

	Composition	Aggregation
1.	Tightly couple relationship.	Loosely coupled relationship.
2

 

    Can we overload or override the main method?

-   We can overload the main() method but we can’t override the main() method. The main () method can’t override because the static method can’t be overridden. The static method belongs to the class, not the object.

package simplifiedjava.crackedInterview;  public class Parent {        public static void main(String[] args) {             // TODO Auto-generated method stub       }        public static void main(String[] args, int num) {             // TODO Auto-generated method stub       } }  package simplifiedjava.crackedInterview;  public class Child extends Parent{        public static void main(String[] args) {             // TODO Auto-generated method stub       } }

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    What is the difference between String s = "abc" and String s = new String("abc")?

-  String s=”abc”: On this line String object will not create on heap memory. ‘abc’ string will be directly stored on the string constant pool.

-    String s = new String(“abc”): new String(“abc”): On this line String object will be created on heap memory and ‘abc’ will be stored in string constant pool.

    What is SerialVersionUID? Who provides this SerialVersionUID? How will you create it?

-     SerialVersionUID is a unique identifier of the class.

-   SerialVersionUID represent the version of the class. This comes into the picture while serialization and deserialization.

-    JVM uses it to compare the versions of the class ensuring that the same class was used during Serialization is loaded during Deserialization.

-   the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization.

-    You can refer the below image to generate the SerialVerionUID. 

    What is the difference between Annonymous Inner class and Lambda Expression?

	Annonymous Inner class	Lambda Expression
1.	It is a class without a name.	It is a function without a name.
2.	An anonymous inner class can extend an abstract class or concrete class.	Lambda expression cannot extend an abstract class or concrete class.
3.	An anonymous inner class can implement an interface that has multiple abstract methods as well as a functional interface.	Lambda expression can implement only a functional interface.
4.	Inside the inner class, we can declare an instance variable.	Inside lambda expression, we can declare only local variables.
5.	An anonymous inner class can be instantiated.	Lambda expression cannot be instantiated.
6.	An anonymous interface is the best choice if we want to handle multiple methods.	Lambda expression is the best choice if we want to handle the interface with a single abstract method.
7.	Inside the Anonymous inner class, ‘this’ always refers current anonymous inner class object but not the outer class object.	Inside lambda expression, this always refers current outer class object.
8.	For an anonymous inner class at the time of compilation, a separated .class file will be generated.	For the lambda expression, separate .class file won’t be generated.

    Explain the internal working of HashMap?

-   Hashmap internally stores mapping in the form of Map.Entry object which contains key and value.

-   Hashmap works on hashing principle.

-  We use the put() method to insert elements in hashmap and get() method to retrieve elements from the hashmap object.

-    Internal working of inserting an element in hashmap.

1.       When we use the put() method to insert key and value objects into a hashmap, internally hashmap uses hashing technique to identify the bucket reference.

2.       Once the bucket is identified object is stored in Map.Entry object in the form of key and value.

3.       It may happen multiple objects can have the same hashcode so multiple objects will be stored in the same bucket.

4.       If one bucket has multiple objects then those objects are stored in the form of a LinkedList.

5.       We must override hashcode() and equals() method.

6.       Hashcode() method calculates the hashcode as an integer value and the equals() method is used to identify the key if the key is matching then it will replace the existing element otherwise that element will be stored next to the current element.

-            Internal working of retrieving element in hashmap.

1.       When you want to retrieve an object from hashmap then you can call the get() method and pass the key object.

2.       Hashmap generates the hashcode for that key and finds the appropriate bucket.

3.       If there is only one element available in that bucket then simply that element will be returned.

4.       If we want to retrieve an object from this linked list, we need an extra check to search for the correct element, this is done by the equals() method.

-          Please refer to the below example.

package simplifiedjava.crackedInterview;  import java.util.HashMap;  public class HashMapDemo {        public static void main(String[] args) {                        HashMap<Integer, String> hMap = new HashMap<Integer, String>();             hMap.put(1, "One");             hMap.put(10, "Ten");             hMap.put(100, "Hundred");             hMap.put(1000, "Thousand");                        System.out.println(hMap);                       System.out.println(hMap.get(100));       } } Output: {1=One, 100=Hundred, 1000=Thousand, 10=Ten} Hundred

 

Graphical Representation of Hashmap.

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

    What is the difference between HashSet and LinkedHashSet?

	HashSet	LinkedHashSet
1.	HashSet implements using a HashTable.	LinkedHashSet implements using LinkedList + HashSet.
2.	Insertion order never preserved.	LinkedHashSet insertion order preserved.
3.	Unique elements without insertion order.	Unique elements with insertion order.
4.	Introduced in Java 1.2	Introduced in Java 1.4

    What is the difference between Traditional Collection and Concurrent Collection?

	Traditional Collection	Concurrent Collection
1.	All traditional collections are not thread-safe.	All Concurrent collections are thread-safe.
2.	Thread-safe traditional collections performance is low as compared to Concurrent collections.	Concurrent Collections performance is high as compared to the traditional collection.
3.	While one thread is executing the collection object no other thread is allowed to access the same collection object.	In Concurrent collection, while one thread is executing a collection object another thread is allowed to access the same collection object.
4.	ConcurrentModificationException can be thrown when multiple threads execute the same resources.	ConcurrentModificationException cannot occur when multiple threads executing the same resources.
5.	Traditional Thread-safe classes are follows.             1.       Vector             2.       Stack                                                             3.       Hashtable             4.       Properties	Concurrent collections are as follows.             1.       ConcurrentHashMap             2.       CopyOnWriteArrayList             3.       CopyOnWriteArraySet

    How would you implement Multiple Inheritance in java?

-          One way you can implement multiple interfaces.

-          The second one is, one interface extends multiple interfaces and class implements the first interface. 

• 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

Upcoming questions in next post:

1. What are the access modifiers and its scope?

2. Can you explain public static void main(String[] args) Method.

3. What is ternary operator in java.

4. Can you specify the scenario where you can implement String, StringBuffer and StringBuilder.

5. How to achieve Thread safety in java.

6. What is deadlock and How to avoid a situation of dead lock.

7. What is NotSerializableException. When it can be thrown. 

8. What is HashSet? Can you explain internal working of Hashset?

9. What is rehashing?

10.What will happen if I declared multiple abstract methods in a functional interface.

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