Core Java Questions

Question 1

Explain each keyword in public static void main(String args[])?

Answer 1

a. This is the method that is called by the JVM when starting a program
b. Public - is an access modifier that says that this method is accessible by any class
c. static - says that this method can be accessed without instantiating a class
d. void - the method returns no value
e. String args[] - contains command line arguments that can be passed while running the program

Question 2

What is an object and what is a class

Answer 2

a. Class - is the template or blueprint through which we can create objects in our programs
b. Object - is an instance of a class that represents an entity that the program is trying to model

Question 3

What is static in Java

Answer 3

a. Keyword that is used for memory management
b. Can be used with classes, variables, methods and block
c. Static members belong to the class instead of a specific instance
d. Static means one per class

Question 4

What is a constructor?

Answer 4

a. A special method that is used to instantiate an object
b. It has the same name as the class it is declared in
c. It has no explicit return type
d. It can be overloaded.

Question 5

Types of constructors

Answer 5

a. Default Constructor - when it has no params and provides default values to the properties of the object
b. Parametized Constructor - used to provide different values to the properties of an object when instantiated

Question 6

What is this keyword

Answer 6

a. can be used to refer to the current class instance variable
b. can be used to invoke current class method
c. this() can be used to invoke current class constructor
d. can be passed as an argument in the method call
e. can be passed as argument in the constructor
d. can be used to return the current class instance from the method.

Question 7

What is meant by the local variable and the instance variable

Answer 7

a. Local variable - is a variable that is declared inside a method
b. instance variable - is declared inside a class but outside a method

Question 8

What is inheritance

Answer 8

a. Process by which a class gains the properties and methods of another class.

Question 9

What is encapsulation

Answer 9

a. Process of hiding implementation details from other classes.

Question 10

What is polymorphism

Answer 10

a. The ability to perform an action in several different ways
b. There are two type - Static(compile) time polymorphism and dynamic(runtime) polymorphism

Question 11

What is method overloading

Answer 11

a. When a class has more than one method with the same name but different signature(different parameters)

Question 12

What is method overriding

Answer 12

a. When a method of a superclass is inherited by a subclass and the implementation is changed.

Question 13

What are the different access modifiers

Answer 13

a. Access modifiers - are keywords used to set the visibility of classes, constructors and data members
b. Default - visible only within the package
c. Private - visible within the class only
d. Protected - visible within the package and subclasses
e. Public - visible to all classes within or outside the package

Question 14

What is an Interface

Answer 14

a. An interface is used to achieve abstraction and loose coupling
b. In Java, a class cannot extend more than one class but with interfaces, a class can implement as many interfaces as possible.
c. interfaces have methods that are abstract by default but have no implementation
d. They can also have variables which can be public, static and final by default.

Question 15

What is an Abstract Class?

Answer 15

a. A class that is declared using the keyword abstract
b. It can have methods that don’t have a body (abstract) as well as concrete methods(those with a body)
c. An abstract class cannot be instantiated but it must be extended to use the abstract and non abstract methods

Question 16

What is the difference between Abstract class and Interface?

Answer 16

a. Abstract class can extend only one class or abstract class while Interfaces can extend a number of interfaces
b. An abstract class can have both abstract and concrete methods while an interface can only have abstract methods
c. An abstract keyword is required for an abstract method in an abstract class but in an interface the abstract keyword is optional as the method by default is abstract
d. An abstract class can have protected and public abstract methods while an interface can have only public abstract methods
e. An abstract class can have static, final or static final variables with any access modifies but interfaces can only have public static final as the access modifier

Question 17

What is the final keyword ?

Answer 17

a. Final variables cannot be changed when initialized
b. Final method cannot be overridden
c. Final class cannot be subclassed

Question 18

What is a thread

Answer 18

a. The flow of execution of a program
b. They are used to perform tasks concurrently and asynchronously
c. Every java program has at least one thread called the main thread created by the JVM
d. The user can define their own thread by extending the Thread class or implementing the Runnable interface.

Question 19

What is the difference between String, StringBuffer and StringBuilder?

Answer 19

a. String is a sequence of characters, an immutable object
b. StringBuffer and StringBuilder are classes used to manipulate strings
c. StringBuilder is not thread safe but is faster when compared to StringBuffer
d. StringBuffer is thread safe and synchronized but is slower than StringBuilder.

Question 20

What is the difference between Array and ArrayList?

Answer 20

a. An array is a fixed length of items while ArrayList is dynamic in length
b. Arrays support primitive data types and objects while Arraylist can only store objects
c. Array performs faster than ArrayList due to being strongly typed

Question 21

What is the Collection Framework?

Answer 21

a. Is a combination of classes and interfaces that are used to store and manipulate data that is in the form of objects

Question 22

What is the difference between List and Set in Java

Answer 22

a. List is an interface in java used to create an ordered collection of objects that allows duplicates and null values.
b. List implementations are - ArrayList, LinkedList
c. Set is an interface used to create an unordered collection that doesn’t allow duplicate elements and can only have one null value.
d. Set implantations are - HashSet, TreeSet

Question 23

What is the difference between Set and Map

Answer 23

a. Set allows for an ordered collection of items while Map contains key value pairs
b. Set contains unique values where Map can contain unique keys with duplicate values.

Question 24

What is the difference between HashMap and HashTable

Answer 24

a. HashMap is non synchronized while HashTable is synchronized
b. HashTable is thread safe and can be shared between multiple threads
c. HashMap doesn’t guarantee the order of the map will remain constant over time.

Question 25

What is an Exception and the different types of Exceptions

Answer 25

a. Is an unwanted event that interrupts the normal flow of a program
b. There are two types - Checked and Unchecked
c. Checked - are checked by the compiler at the time of compilation eg SQLException, IOException, ClassNotFoundException
d. Unchecked exceptions are those not checked during compile time by the compiler
e. Eg ArithmeticException, NullPointerException. ArrayIndexOutOfBounds

Question 26

What are the exception handling keywords

Answer 26

a. try - used to
b. catch
c. finally
d. throw - used to throw an exception
e. throws - delcare an exception