Java Basics

Question 1

What is Java?

Answer 1

Java is a C-like, multiparadigmed, compiled programming language used for back-end development projects.

Question 2

What are the advantages of Java? (7 are listed here)

Answer 2

It: 1) is a mature language, 2) is maintained by Oracle, 3) is portable, 4) is platform independent, 5) supports multithreading, 6) is compiled, 7) has automatic garbage collection

Question 3

What is the JDK?

Answer 3

Java Development Kit. Provides an environment to develop and execute a Java program. Contains the compiler, JRE, and JVM.

Question 4

What is the JRE?

Answer 4

The Java Runtime Environment. It is all that’s needed to run an application. Contains the core library and JVM.

Question 5

JVM

Answer 5

Java Virtual Machine. Takes the byte code and executes it. Different operating systems can have different implementations of the JVM.

Question 6

What is a class?

Answer 6

A class is the blueprint of an object. It defines the states (fields), and behaviors (methods) that its instances may have.

Question 7

What is an object?

Answer 7

An object is an instance of the class. It is the representation of a real-world entity and has its own implementations of states and behaviors that are defined in its class.

Question 8

What is the root class from which every class extends.

Answer 8

The object class. Every other class extends the object class and inherits methods from it.

Question 9

What are some methods from the object class?

Answer 9

Equals(), getClass(), hashCode(), toString()

Question 10

What are the 8 primitive types in Java?

Answer 10

There is: byte, short, int, long, float, double, boolean, and char.

Question 11

Where are Strings stored?

Answer 11

Strings are stored in the heap. Strings declared literally are specifically stored in the string pool.

Question 12

Explain stack vs heap?

Answer 12

Stack is short term storage used for methods currently running while the heap is long term storage used throughout the application.

Question 13

What is the stack?

Answer 13

Short term storage that holds primitive and reference variables. It follows a LIFO (last in First out) pattern.

Question 14

What is the heap?

Answer 14

It is long term memory storage. Holds objects and is managed by the garbage collector

Question 15

What are annotations?

Answer 15

Annotations start with the ‘@’ and are used to provide supplemental information about a program. They can change the way a program is treated by the compiler. They help to associate metadata to the program elements.

Question 16

Why are strings immutable in Java?

Answer 16

Strings in Java are immutable, meaning they cannot be changed at a later time. This is because when a string is created, it is stored as a new object in the string pool, or it points to a string object of the same value. This helps the program save memory.

Question 17

What is the difference between String, StringBuilder, and StringBuffer?

Answer 17

String is immutable. StringBuilder and StringBuffer are mutable alternatives with methods that allow for manipulation.

Question 18

Difference between StringBuilder and StringBuffer?

Answer 18

StringBuilder is not thread safe, but faster. StringBuffer is thread safe, but slower.

Question 19

What are different variable scopes in Java?

Answer 19

There is class scope, instance scope, method scope, and block scope. Each one defines different “lifetimes” or the variables declared in them.

Question 20

What are the access modifiers in Java?

Answer 20

Public, default, protected, and private.

Question 21

Public classes are accessible where?

Answer 21

Throughout the program

Question 22

Default classes are accessible where?

Answer 22

Throughout the same package.

Question 23

Protected classes are accessible where?

Answer 23

Throughout the same package and subclasses.

Question 24

Private classes are accessible where?

Answer 24

Only within the same class.

Question 25

What are non access modifiers in Java?

Answer 25

Non-access modifiers introduce additional functionalities and include static, final, and abstract.

Question 26

What makes a class immutable?

Answer 26

A class is immutable when it is declared as final and private. A class is also immutable if no setter functions are declared within it. Objects within an immutable class are also immutable themselves.

Question 27

What data types are supported in switch statements?

Answer 27

Byte, short, char, int, and string.

Question 28

How to pass multiple values with a single parameter into a method?

Answer 28

By using varargs, also known as variable arguments. Varargs are shown as three dots at the end of a parameters sequence, and is used to indicate that the method may have a variable number of parameters.

Question 29

What methods are available in the object class?

Answer 29

There are 11 methods. Some of which are clone(), equals(), toString(), finalize(), hashCode, getClass().

Question 30

What is the difference between == and .equals?

Answer 30

== compares the memory address while .equals() compares the value.

Question 31

What is an enhanced for loop?

Answer 31

An enhanced for loop is also known as a for-each loop. And is used to iterate through elements of arrays and collections.

Question 32

What are the 3 usages of the super keyword?

Answer 32

1) refer to immediate parent class instance. 2) Invoke immediate parent class method. 3) Act as a constructor of the immediate parent class.

Question 33

What is the first line of any constructor?

Answer 33

The first line of a constructor must either be a call on another constructor in the same class (using this.), or a call on the superclass constructor (using super).

Question 34

How would you perform constructor chaining?

Answer 34

By calling a constructor within another constructor.

Question 35

What happens if you don’t define a constructor for a class? Can you still instantiate it?

Answer 35

You are given a default constructor and yes you can instantiate it.