Interview Prep

Question 1

What is polymorphism?

Answer 1

Using the same method to do different things eg when extending a class. For example, the Animal class might contain the method makeSound() and the Pig class which extends the Animal class might use the makeSound() method to print “oink” by overriding this method

Question 2

What are the main features of Java?

Answer 2

portable, robust, secure, platform-independent, OOP language, supports multithread

Question 3

What is an Array?

Answer 3

All elements have the same data type. Arrays are a fixed same and this size cannot be changed once declared. Accessing an invalid index will cause an exception

Question 4

Where are Arrays created?

Answer 4

On the heap memory, not on the stack

Question 5

What is the syntax to create an Array?

Answer 5

dataType[] arrayVariableName = new dataType[arraySize];

Question 6

Is it possible to create an Array without giving it a size?

Answer 6

No - this will cause a compile-time error

Question 7

What is a runtime error?

Answer 7

A runtime error occurs when a program does not contain any syntax errors but features a command the computer can not perform, such as dividing by 0

Question 8

What are the basic principles of OOP?

Answer 8

Abstraction, Encapsulation, Inheritance, Polymorphism

Question 9

What is abstraction in Java?

Answer 9

Abstraction means hiding the detailed information from the user. Abstraction can be achieved by interfaces and abstract classes.

Question 10

What is inheritance in Java?

Answer 10

Inheritance allows a Java class or interface to inherit properties and behaviour from another class or interface. Inheritance can be gained by implementing interfaces or extending classes.

Question 11

What is an object in Java

Answer 11

An object is an instance of a class. A class is a blueprint for an object, containing its basic properties and behaviour. The new keyword is used to instantiate a class and make an object.
For example, Animal.java has a class named Animal. Animal animal = new Animal() is an object or instance of this class

Question 12

What is method overloading?

Answer 12

When a method has the same name but a different number of arguments

Question 13

Does Java support multiple inheritance?

Answer 13

No, but multiple inheritance can be gained by using interfaces

Question 14

What is the difference between a method and a constructor?

Answer 14

A constructor is a method within a class with the same name as the class and no return type.

Question 15

Can the main method be overloaded?

Answer 15

Yes

Question 16

What is the difference between an error and an exception?

Answer 16

Error is generated by the environment at runtime. The exception is generated by the application intentionally or mistakenly. An exception can be checked or unchecked, but the error is only unchecked.

Question 17

What is the difference between static and non-static methods?

Answer 17

Static methods belong to a class but non-static methods belong to an object. You don’t need to instantiate a class to access the static methods, but you have to instantiate the class to get access to non-static methods.

Question 18

What are the different ways to create threads in Java?

Answer 18

There are two ways:

1. Implement Runnable interface
2. By extending Thread class

Question 19

What is synchronisation in Java?

Answer 19

Synchronization is a technique to control access of a method with multiple threads at the same time. If we declare a method synchronized, then only one thread can use this method at a time. This is used for Thread safety.

Question 20

What is the final keyword used for?

Answer 20

We cannot change the value of a final variable. We are not able to inherit a final class and cannot override a final method.

Question 21

What is meant by garbage collection in Java?

Answer 21

Cleaning an object from memory which is not used is called garbage collection. It’s an important feature of Java and does not need to be done manually because Java does this automatically.

Question 22

What are the types of joins in SQL?

Answer 22

Cross join
Inner join
Left outer join
Right outer join
Full outer join

Question 23

What is an inner join in SQL?

Answer 23

An inner join retrieves data where the records from the first table are equal to the records from the second table, based on a set condition.

Question 24

What is a left outer join in SQL

Answer 24

When using this type of join, you will retrieve all the rows from the first (or left) table and the matching rows from the second (or right) table. If there is no matching row in the second table, you will get a NULL value.

Question 25

What is a full outer join in SQL?

Answer 25

Using this join will result in retrieving all the rows from the first table and all the rows from the second table. If there are non-matching rows, both from the first and second tables, they will be shown as a NULL value.

Question 26

What is a cross join in SQL?

Answer 26

This type of join means that every row from one table is joined to every row from another table. It multiplies rows. If you’re working with a huge amount of data, this is not advisable. This type of join is also called the Cartesian product.