Complete reference to core java interview questions basic to advanced level : SE to Team Lead level Part - 9

Collections

Q1.  What are limitations of object Arrays?

The main limitations of Object arrays are

• These are fixed in size ie once we created an array object there is no chance of increasing or decreasing size based on our requirement. Hence  If we don’t know size in advance , arrays are not recommended to use
• Arrays can hold only homogeneous elements. 
• There is no underlying data structure for arrays and hence no readymade method support for arrays. Hence for every requirement programmer has to code explicitly

       To over come  these problems collections are recommended to use

Q2. What are differences between arrays and collections?

Arrays	Collections
1.  Arrays r fixed in size and hence once we created an array we are not allowed to increase or decrease the size based on our requirement.	1. Collections are growable in nature and hence based on our requirement we can increase or decrease the size.
2.  Memory point of view arrays are not recommended to use	2. Memory point of view collections are recommended to use.
3. Performance point of view arrays are recommended to use	3. Performance point of view collections are not recommended to use.
4.  Arrays can hold only homogeneous elements	4. Collections can hold both homogeneous and heterogeneous elements.
5. Arrays can hold both primitives as well as objects	5. Collections can hold only objects.
6. For any requirement, there is no ready method support compulsory programmer has to code explicitly.	6. For every requirement ready made method support is available. Being a programmer we have to know how to use those methods and we are not responsible to implement those.

Q3. what are differences between arrays and ArrayList?
         
   Refer  the answer of  Q2

Q4. What are differences between arrays and Vector?
    
        Refer the answer of Q2

Q5. What is Collection API ?

It defines set of classes and interfaces which can be used for representing a group of objects as single entity

Q6.  What is Collection framework?

 It defines set of classes and inter faces which can be used for representing a group of objects as single entity

Q7.  What  is difference between Collections and Collection?

Collection is an interface which can be used for representing a group of individual objects as single entity  and it acts as root interface of collection frame  work.

Collections is an utility class to define several utility methods for Collection implemented class objects.

Q8.  Explain about Collection interface?

• This interface can be used to represent a group of objects as a single entity.
• It acts as root interface for entire collection framework.
• It defines the most commonly used methods which can be applicable for any collection implemented class object

Q9. Explain about List interface?

List interface is a child interface of Collection interface. This can be used to represent group of individual objects in as a single entity where

• Duplicates are allowed
• Insertion order is preserved

Q10.  Explain about Set interface?

Set is a child interface of Collection interface. it can be used to represent a group of individual objects as a single entity where

• Duplicate objects are not allowed.
• Insertion order is not preserved

11.  Explain about SortedSet interface?

it is child interface of Set interface. it can be used to represent a group of individual objects in to a single entity where

• All the objects are arranged in some sorting order (Can be natural sorting order or customizede).
• Duplicates are not allowed.

Q12.  Explain about NavigableSet ?

It is child interface of SortedSet and provides several utility methods for navigation purposes

• It doesn’t allows duplicates                                
• Insertion order is preserved
• It is introduced in 1.6 version

Q13. Explain about Queue interface?

If we want to represent a group of individual objects prior to processing, then we should go for Queue interface. It is child interface of Collection interface.

It has introduced in 1.5 version.

Q14. Explain about Map interface?

Remember it is not a child Interface of Collection Interface and hence Map and Collection Interfaces doesn’t have any relationship.

• It can be used for representing a group of Objects as key, value pairs.
• Both keys and values should be objects
• Keys can t be duplicated but values can be duplicated.
• it has  introduced in 1.2 version

Q15. Explain about SortedMap ?

• If we want to represent a group of objects as key value pairs where all the entries are arranged according some sorting order of keys then we should go for SortedMap.
• It is child interface of Map.
• It has  introduced in 1.2 version

Q16. Explain about NavigableMap?

• It is child interface of SortedMap and defines several method for navigation purpose
• It is introduced in 1.6 version

Q17.  Explain about ArrayList class?

 ArrayList is a Collection which can be used to represent a group of objects as a single entity.

• it is a implemented class for  List interface
• Introduced in 1.2 version
• The underlying data structure is resizable or growable array.
• Insertion order is preserved
• Duplicates are allowed
• Heterogeneous objects are allowed
• null insertion is possible
• This  class implements RandomAccess , Serializable , Cloneable interfaces
• Best choice  for retrieval purpose and worst if our frequent operation is insertion or deletion in the middle

Q18. What is RandomAccess Interface?

• If a collection class implements RandomAccess interface then we can access any of its element with the same speed.
• RandomAccess interface is marker interface and it dosent contains any methods.
• ArrayList and vector classes implements this interface.

Q19. Explain about LinkedList class?

LinkedList is a Collection implemented class which can be used for representing a group of objects as a single entity.

• LinkedList is the implemetation class for List interface
• Introduced in 1.2 version
• Underlying data Structure is   DoubleLinkedList
• Allows duplicates
• Insertion order is preserved
• Allows heterogeneous objects
• null insertion is possible
• LinkedList class implements Seriallizable and Cloneable interface but not RandomAccess interface
• Best choice  if frequent operation is insertion or deletion an objects in middle  but worst choice if frequent operation is retrieval.

Q20. Explain about Vector class?

     Vector is a legacy collection class which can be used to represent a group of objects.

• Introduced in 1.0 version. it is legacy class
• The underlying data structure is resizable or growable array.
• Insertion order is preserved
• Duplicates are allowed
• Heterogeneous objects are allowed
• It is a implemented class for  List interface
• null insertion is possible
•  Vector class implements RandomAccess ,Serializable,Cloneable interfaces
• Best Choice if frequent operation is retrieval and worst choice if frequent operation is insertion or deletion in the middle.
• All methods present in Vector class are synchronized hence Vector class object is thread safe.

Q21. What is difference between ArrayList and Vector?
   

ArrayList	Vector
1. No method is synchronized in the ArrayList class	1. All methods in Vector are synchronized.
2. ArrayList object is not thread safe.	2.  Vector is thread safe.
3. Relatively performance is high	3. Relatively performance is low
4. Introduced in 1.2 version and it is non legacy	4. Introduced in 1.0 version and it is legacy

Q22. How we can get synchronized version of ArrayList?

Collections class contains synchronizedList() method for this

                Public static List synchronizedList(List l)
               
                EX
                ArrayList l= new  ArrayList();
                List l2=Collections.synchronizedList(l);

  Similarly we can get synchronized versions of Set and Map objects by the following methods.

Public static List synchronizedSet(Set s)
Public static List synchronizedMap(Map m)

Q23. What is difference between size and capacity of a Collection Object?

size means number  of objects present  where as capacity means no of objects it can accommodate.

Q24. What is difference between ArrayList and Linked List?

ArrayList	LinkedList
1. The underlying data structure is resizable or growable array.	1. The underlying data structure is Double Linked List.
2.  This is Best choice if frequent operation is retrieval and worst choice if frequent operation is insertion or deletion in the middle.	2.  This is Best choice  if frequent operation is insertion or deletion in the middle and worst choice if frequent operation is retrieval .
3. This class implements Serializable , Cloneable and RandomAccess interfaces.	3. This class implements Serializable , Cloneable but not  RandomAccess interface.

Q25. What are legacy classes and interfaces present  in Collections framework ?

• Enumeration ---Interface
• Dictonary ------Abstract class
• Hashtable -----Concrete class
• Properties -----Concrete class
• Vector -----Concrete class
• Stack  -----Concrete class

Q26. what is difference Enumeration and Iterator?

Enumeration	Iterator
1. It is legacy interface and introduced in 1.0 version	1 It is non-legacy and introduced in 1.2 version
2Applicable only for legacy classes and it is not universal cursor	2Applicable for any Collection implemented class object.
3While iterating the elements we are not allowed to remove the objects just we can perform only read operation	3While iterating we can perform removal also in addition to read operation.
4By using elements() method we can get Enumeration object	4.   By using iterator() method we can get Iterator        object

Q27. What are limitations of Enumeration?

• While iterating the elements we are not allowed to perform removal operation
• It is applicable only for legacy classes and it is not a universal cursor.
• It can retrieve the elements only in forward direction

Q28. What is difference between enum and Enumeration?

An enum can be used to define a group of named constants .It has  introduced in 1.5 version

Ex
Class Beer{
                KO,KF,RC,FO
}

Enumeration is cursor to retrieve Objects one by one from Collection objects.

Q29. What is difference between Iterator and ListIterator?

• ListIterator is the child interface of the Iterator
• Iterator is the single direction cursor where as ListIterator is bidirectional cursor.
• While iterating the elements by Iterator we can perform only read and remove operations. But by using ListIterator we can perform read,removal, replace and addition of new objects also.
• Iterator is applicable for every Collecton implemented class object but ListIterator  is applicable only for List implemented class objects.
• Iterator can be get by using iterator() of Collection interface where as ListIterator can be get by using listIterator() method of List interface
• both are introduced in 1.2 version

Q30. What is relation between ListIterator and Iterator?
   
      ListIterator is child interface of Iterator

Q31. Explain about HashSet class?

• The underlying data structure is Hashtable
• null values are accepted
• duplicates are not allowed
• insertion order is based on hashcode of the object hence insertion order is not preserved
• best  suitable if frequent operation is  search operations
• HashSet  class implements Serializable and Cloneable
• it is implementation class for Set interface
• heterogeneous objects are allowed
• it is introduced in 1.2 version

Q32. If we are trying to insert duplicate values in Set what will happen?

 If we are trying to insert duplicate objects to the HashSet  , we wont get any compile time or run time errors just the add(Object o) returns false and it doesn’t add that object.

Q33. What is LinkedHashSet?
It is the child class of HashSet. The main difference between HashSet and LinkedHashSet is:

In the case of HashSet insertion order is not preserved , but in the case of LinkedHashSet insertion will be preserved.

Q34. Differences  between HashSet and LinkedHashSet?

HashSet	LinkedHashSet
1The Underlying datastructure is Hashtable	1The underlying datastructure is combination of LinkedList and Hashtable
2Insertion Order is not preserved	2     Insertion order is preserved.
3Introduced in 1.2 version	3     Introduced in 1.4 version

Q35. What are major enhancements in 1.4 version of collection frame work?

LinkedHashSet
 LinkedHashMap
IdentityHashMap
                                                                      

Q36. Explain about TreeSet?
     
 It is Collection object which can be used to represent a group of objects according to some sorting order.

• The underlying datastructure is Balanced tree
• Duplicates are not allowed
• All objects are stored according to some sorting order hence insertion order is not preserved
• Heterogeneous objects are not allowed violation leads to ClassCastException
• For an Empty TreeSet as firs element null value can be inserted but after inserting that first value if we are trying to insert any other objects then we will get NullPointerException
• For an non empty TreeSet if we are trying to  inser null value at run time u will get NullPointerException

Q37. What are differences between List and Set interfaces?

List	Set
1Insertion Order is preserved	1Insertion Order is not preserved
2Duplicate Objects are allowed	2     Duplicate Objects are not allowed
3The implemented classes are ArrayList,LinkedList , Vector and Stack classes	3   The implemented classes are HashSet,            LinkedHashSet and Tree

Q38. What is Comparable interface?

• This interface can be used for defining natural sorting order of the objects.
• It is present in java.lang package
• It contains a method public int compareTo(Object obj1)

Q39. What is Comparator interface?

• This interface can be used for implementing customized sorting order.
• It is present in java.util package
• It contains two methods
• public int compare(Object ,Object)
• public boolean equals(Object)

Q40. What are differences between Comparable and Comparator?

Comparable	Comparator
1This can be used for natural sorting order	1This can be used for implementing customized sorting
2This interface present in java.lang package	2     This is present in java.util package
3Contains only one method: public int compareTo(Object obj1)	3     It contains two methods. public int compare(Object ,Object) public Boolean equals(Object)
4    It is marker interface	4  It is not a marker interface.

Q41. What is difference between HashSet and TreeSet?

HashSet	TreeSet
1The underlying data structure is Hashtable	1The underlying data structure is balanced tree
2Heterogeneous objects are allowed	2       Heterogeneous objects are not allowed  bydefalut
3Insertion order is not preserved and it is based on hashcode of the objects	3   Insertion order is not preserved and all the objects are inserted according to some sorting order.
4null insertion is possible	4   As the first element only null insertion is   possible and in all other cases we will get NullPointerException

Q42. What is Entry interface?

It is inner interface of Map.
In the Map each key value pair is considered as Entry object.

interface Map{
                //more code here
                interface Entry{                
                                Object  getKey()
                                Object  getValue()
                                Object  setValue(Object new) }     }         

Q43. Explain about HashMap?
It is a Map Object which can be used used to represent a group of objects as key-value pairs.

• The underlying data structure is Hashtable
• Duplicaes keys are not allowed duplicate values are allowed
• Insertion order is not preserved because insertion is based on hashcode of keys.
• Heterogeneous objects are allowed for  both keys and values
• null key is allowed  only once
• null values  are allowed multiple times
• Introduced in 1.2 version

Q44. Explain about LinkedHashMap?

It is child class of HashMap. It is exactly same as HashMap except the following difference.

In the case of HashMap the insertion order is not preserved but in the case of LinkedHashMap insertion order is preserved. Introduced in 1.4 version

Q45. Differences between HashMap and LinkedHashMap ?

HashMap	LinkedHashMap
1.The underlying data structure is Hashtable	1.The underlying data structure is a combination of Hashtable and linkedlist
2.Insertion order is not preserved and it is based on hashcode of keys	2    Insertion order is preserved
3.Introduced in 1.2 version	3   Introduced in 1.4 version.

Q46. Differences between HashMap and Hashtable?

HashMap	Hashtable
1.The underlying data structure is Hashtable	1.The underlying data structure of Hashtable
2.No method is synchronized and hence HashMap object is not thread safe	2 .All methods are synchronized and hence it is   thread safe
3.Performance is high	3.   Performance is low
4.null insertion is possible for both keys and values	4.   null insertion is not possible for both key and value violation leads to NullPointerException
5.Introduced in 1.2 version and it is non legacy	5.   Introduced in 1.0 version and it is legacy

Q47. What is IdentityHashMap?

It is exactly same as HashMap except the following difference.

     In the HashMap JVM uses equals() method to identify duplicate keys  but in the  case of IdentityHashMap JVM uses == operator for this.

Q48. What is difference between HashMap and IdentityHashMap?
     
     Refer Q47 for the answer.

Q49. What is WeakHashMap?
      
   It is exactly same as HashMap except the following difference.

 In case of HashMap an Object is not eligible for garbage collection if it is associated with HashMap even though it dosent have any external references.  ie HashMap dominates garbage collector.

But in case of WeakHashMap , if an Object is not having any external references then it is always eligible for garabage collectoion even though it is associated with weakHashMap.  ie  garbage collector dominates WeakHashMap

Q50. What is difference between HashMap and WeakHashMap?
  Refer Q49 for the answer.

Q51. What is TreeMap?

 TreeMap can be used to store a group of objects as key-value pairs where all the entries are arranged according to some sorting order of keys.

• The underlying data structure is RED-BLACK Tree
• Duplicates keys are not allowed but values can be duplicated.
• Insertion order is not preserved because insertion is based on some sorting order
• If we are depending on Natural sorting order then keys should be homogeneous(violation leads to ClassCastException)  but values need not homogeneous
• In case of customized sorting order we can insert  heterogeneous keys and values
• For empty TreeMap as first entry with null values are allowed but after inserting that entry if we are trying to insert any other entry we will get NullPointerException
• For non empty TreeMap if we are trying to insert null keys we will get NullPointerException
• There are no restrictions for null values.

Q52. What is Hashtable
     
     Hashtable is a legacy Map and can be used to store objects as key value pairs.

• The underlying data sturucture is Hashtabe
• Duplicates keys are not allowed but duplicate values are allowed
• null insertion is not possible for both keys and values
• all methods are synchronized
• insertion order is not preserved because it is  based on hashcode  of keys
• heterogeneous Objects are allowed for both keys and values
• introduced in 1.0 version it is legacy class

Q53. What is PriorityQueue?

It represents a data structure to hold group of individual objects prior to processing based on some priority .it can be natural sorting order and it can be customized sorting order described by Comparator.
It is the implementation class of Queue interface.

• Insertion order is not preserved because here insertion is done based on some sorting order
• Duplicates are not allowed
• null insertion is not possible even as first element also
•  If we are depending on natural sorting order Objects should be homogeneous  violation leads to ClassCastException
•  If we are depending on customized  sorting order Objects can be heterogeneous also.

Q54. What is Arrays class?

• It is utility class for arrays.
• It defines several utility methods for arrays like sorting an array or searching an element in array
• present in java.util package

Q55. We are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList? 
ArrayList
Q56. Why ArrayList is faster than Vector?  

All methods present in the Vector are synchronized  and hence  any method can be executed by only one thread at a time. It slows down the execution.

But in ArrayList,  no method is synchronized and hence multiple thread are allowed execute simultaneously which speed up the execution.

What is the difference between private, protected, and public?

These keywords are for allowing privileges to components such as java methods and variables.
Public: accessible to all classes
Private: accessible only to the class to which they belong
Protected: accessible to the class to which they belong and any subclasses.
Access specifiers are keywords that determines the type of access to the member of a class. These are:
* Public
* Protected
* Private
* Defaults

2. What's the difference between an interface and an abstract class? Also discuss the similarities. (Very Important)

Abstract class is a class which contain one or more abstract methods, which has to be implemented by sub classes. Interface is a Java Object containing method declaration and doesn't contain implementation. The classes which have implementing the Interfaces must provide the method definition for all the methods
Abstract class is a Class prefix with a abstract keyword followed by Class definition. Interface is a Interface which starts with interface keyword.
Abstract class contains one or more abstract methods. where as Interface contains all abstract methods and final declarations
Abstract classes are useful in a situation that Some general methods should be implemented and specialization behavior should be implemented by child classes. Interfaces are useful in a situation that all properties should be implemented.

Differences are as follows:

* Interfaces provide a form of multiple inheritance. A class can extend only one other class.
* Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
* A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
* Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast.

Similarities:

* Neither Abstract classes or Interface can be instantiated.

How to define an Abstract class?
A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.
Example of Abstract class:

abstract class testAbstractClass {
    protected String myString;
    public String getMyString() {
    return myString;
}
public abstract string anyAbstractFunction();
}

How to define an Interface?
Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Example of Interface:

public interface sampleInterface {
    public void functionOne();
    public long CONSTANT_ONE = 1000;
}

3. Question: How you can force the garbage collection?

Garbage collection automatic process and can't be forced. You could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

Garbage collection is one of the most important feature of Java, Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program can't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.

4. What's the difference between constructors and normal methods?

Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times and it can return a value or can be void.

5. Can you call one constructor from another if a class has multiple constructors

Yes. Use this() to call a constructor from an other constructor.

6. Explain the usage of Java packages.

This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

7. Explain in your own words the "bottom line" benefits of the use of an interface.

The interface makes it possible for a method in one class to invoke methods on objects of other classes, without the requirement to know the true class of those objects, provided that those objects are all instantiated from classes that implement one or more specified interfaces. In other words, objects of classes that implement specified interfaces can be passed into methods of other objects as the generic type Object, and the methods of the other objects can invoke methods on the incoming objects by first casting them as the interface type.

8. What are some advantages and disadvantages of Java Sockets?

Some advantages of Java Sockets: 
Sockets are flexible and sufficient. Efficient socket based programming can be easily implemented for general communications. Sockets cause low network traffic. Unlike HTML forms and CGI scripts that generate and transfer whole web pages for each new request, Java applets can send only necessary updated information.

Some disadvantages of Java Sockets:
Security restrictions are sometimes overbearing because a Java applet running in a Web browser is only able to establish connections to the machine where it came from, and to nowhere else on the network   Despite all of the useful and helpful Java features, Socket based communications allows only to send packets of raw data between applications. Both the client-side and server-side have to provide mechanisms to make the data useful in any way.

9. Explain the usage of the keyword transient?

Transient keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

10. What's the difference between the methods sleep() and wait()

The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

11. What would you use to compare two String variables - the operator == or the method equals()?

I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.

12. Why would you use a synchronized block vs. synchronized method?

Synchronized blocks place locks for shorter periods than synchronized methods.

13. What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?

You do not need to specify any access level, and Java will use a default package access level.

14. Can an inner class declared inside of a method access local variables of this method?

It's possible if these variables are final.

15. What can go wrong if you replace && with & in the following code:
String a=null; if (a!=null && a.length()>10) {...}

A single ampersand here would lead to a NullPointerException.

16. What's the main difference between a Vector and an ArrayList?

Java Vector class is internally synchronized and ArrayList is not synchronized.

17. Describe the wrapper classes in Java.

Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

Following table lists the primitive types and the corresponding wrapper classes:
Primitive Wrapper
boolean  - java.lang.Boolean
byte - java.lang.Byte
char - java.lang.Character
double - java.lang.Double
float - java.lang.Float
int - java.lang.Integer
long - java.lang.Long
short - java.lang.Short
void - java.lang.Void

18. How could Java classes direct program messages to the system console, but error messages, say to a file?

The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:
Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);

19. How do you know if an explicit object casting is needed?

If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:
Object a; Customer b; b = (Customer) a;

20. When you assign a subclass to a variable having a supeclass type, the casting is performed automatically. Can you write a Java class that could be used both as an applet as well as an application?

Yes. Add a main() method to the applet.

21. If a class is located in a package, what do you need to change in the OS environment to be able to use it?

You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.javIn this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\>java com.xyz.hr.Employee

22. What's the difference between J2SDK 1.5 and J2SDK 5.0?

There's no difference, Sun Microsystems just re-branded this version.

23. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?

Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

24. Name the containers which uses Border Layout as their default layout?

Containers which uses Border Layout as their default are: window, Frame and Dialog classes.

25. You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use:
ArrayList or LinkedList?

ArrayList

26. When should the method invokeLater()be used?

This method is used to ensure that Swing components are updated through the event-dispatching thread.

27. How can a subclass call a method or a constructor defined in a superclass?

Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

28. What do you understand by Synchronization?

Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.
E.g. Synchronizing a function: