Intermediate

Question 1

What are arrays in java?

Answer 1

An array is an object containing a fixed number of values of the same type

Question 2

What are varargs?

Answer 2

• Varargs are used to pass an arbitrary number of arguments to a method but we can also pass an array directly as the argument
• Each method can only have one varargs parameter and the varargs parameter needs to be the last argument

Question 3

What is the disadvantage of varargs?

Answer 3

They can lead to Heap Pollution

Question 4

What does the static keyword mean?

Answer 4

The static keyword means that the particular member belongs to a type itself, rather than to an instance of that type

Question 5

What is important about static fields (class variables)?

Answer 5

A SINGLE copy of that field is created and shared among all instances of that class, this static variable is stored in the heap memory

Question 6

When to use static fields?

Answer 6

• When the value of the variable is independent of objects
• When the value is supposed to be shared across all objects

Question 7

What is important about static methods?

Answer 7

• Static methods belong to a class not an object and cannot be overwritten
• Static methods cannot use this or super keywords

Question 8

When to use static methods?

Answer 8

Static methods are used to access or manipulate static variables and other static methods that do not depend upon objects; static methods are widely used in utility and helper classes

Question 9

What is the benefit of static classes?

Answer 9

A static class allows to create a class within a class and provides a way of grouping elements that are only used in one place -> keeps the code more organized and readable

Question 10

In what two types can nested class architecture be divided?

Answer 10

1. Nested classes that are declared as static are called static nested classes
2. Nested classes that are non-static are called inner classes

Question 11

What is the difference between inner classes and static nested classes?

Answer 11

Inner classes have access to all members of the enclosing class, whereas static nested classes only have access to static members of the outer class

Question 12

Why to use static nested classes?

Answer 12

• Grouping classes that will be used only in one place increases encapsulation
• Code is closer to the place where it will be used -> Increases readability, and maintainability

Question 13

What are enums?

Answer 13

• The keyword enum denotes a special type of class that always extends the java.lang.Enum class
• Enum is short for enumerations
• Basic example for enum is days of the week
• Enum declaration looks similar to a class declaration
• Enum entries are typically written in uppercase
• Enums can have fields and a constructor

Question 14

What are implementation design patterns using enums?

Answer 14

• Singleton Pattern
• Strategy Pattern

Question 15

What does the final keyword mean?

Answer 15

• The final keyword allows to set limitations on extensibility -> classes marked as final cannot be extended
• The final keyword in a class declaration does not mean that the objects of this class are immutable, we just cannot extend it
• Methods marked as final cannot be overwritten
• Variables marked as final cannot be reassigned -> once a final variable is initialized, it cannot be changed
• A final reference variable cannot be reassigned but that does not mean that the object it refers to is immutable -> the properties of this object can be changed freely

Question 16

What is the aim of generics?

Answer 16

The aim of generics is to reduce bugs and add an extra layer of abstraction over types

Question 17

When are generics useful?

Answer 17

Generics are useful if you would run into a code duplication like e.g. an IntegerPrinter class or a StringPrinter class where all classes would hold the exact same code only the types of the variables would be different -> What we want is one class that is flexible for many different types

Question 18

What are generic wildcards?

Answer 18

Todo is to print a list containing any type of thing -> Use a ? as a type parameter in the angle brackets

Question 19

What is one restriction of generics in java?

Answer 19

One restriction is that the type parameter cannot be a primitive type, because generics are a compile-time feature, meaning the type parameter is erased and all generic types are implemented as type object

Question 20

How to read and write user input in java?

Answer 20

• Reading from system.in
• Writing to system.out
• Using the console class for input and output

Question 21

What are checked exceptions?

Answer 21

• Checked exceptions represent errors outside the control of the program
• They are verified at compile-time
• The throws keyword is used to declare a checked exception

Question 22

What are unchecked exceptions?

Answer 22

• Unchecked exceptions reflect some error inside the program logic
• They are not verified at compile-time

Question 23

What are common unchecked exceptions?

Answer 23

NullpointerException, ArrayIndexOutOfBoundsException and IllegalArgumentException

Question 24

How to remember when to use checked and unchecked exceptions?

Answer 24

If a client can reasonably be expected to recover from an exception, make it a checked exception. If a client cannot do anything to recover from the exception, make it an unchecked exception

Question 25

What are the main reasons for introducing custom exceptions?

Answer 25

• Business logic exceptions -> exceptions that are specific to the business logic and workflow. These help the application users or developers to understand what the exact problem is
• To catch and provide specific treatment to a subset of existing java exceptions

Question 26

How to create custom checked and unchecked exceptions?

Answer 26

• Custom checked exceptions have to extend the java.lang.Exception class
• Custom unchecked exceptions have to extend the java.lang.RuntimeException

Question 27

What is try with resources?

Answer 27

Try with resources allows us to declare resources to be used in a try block with the assurance that the resources will be closed after the execution of that block

Question 28

What is an effectively final variable?

Answer 28

Effectively final means the variable does not change after the first assignment

Question 29

When are the Comparator and Comparable interfaces useful?

Answer 29

When working with custom types, or trying to compare objects that are not directly comparable, we need to make use of a comparison strategy -> Make use of Comparator and Comparable interfaces

Question 30

What is Comparable?

Answer 30

Comparable is an interface defining a strategy of comparing an object with other objects of the same type. This is called the classes “natural ordering”

Question 31

What is Comparator?

Answer 31

The comparator interface defines a compare(arg1, arg2) method with two arguments that represent compared objects, and works similarly to the Comparable.compareTo() method

Question 32

When to use Comparator or Comparable?

Answer 32

• Comparable interface is a good for defining the default ordering, it is the main way of comparing objects
• Sometimes we cannot modify the source code of the class whose objects we want to sort, thus making the use of Comparable impossible -> Comparator allows us to avoid additional code to our domain classes

Question 33

What is a list?

Answer 33

A list represents an ordered sequence of values where some value may occur more than one time

Question 34

What is an arraylist?

Answer 34

An arraylist is one of the list implementations built atop an array, which is able to dynamically grow and shrink as you add and remove elements

Question 35

What is a linkedlist?

Answer 35

A linkedlist is a doubly-linked list implementation of the list and dequeue interfaces which implements all optional list operations and permits all optional list operations and permits all elements including null

Question 36

What is the difference between arraylist and linkedlist?

Answer 36

• Arraylist is faster in random access than linkedlist
• Insertion, addition and removal operations are faster in linkedlist
• linkedlist consumes more memory

Question 37

What are features of a hashset?

Answer 37

• It stores unique elements and permits null
• It is backed by a hashmap
• It does not maintain insertion order
• It is not thread-safe

Question 38

When is a hashset useful?

Answer 38

Useful whenever you want a collection of your elements without any duplicates and the particular order is not important

Question 39

What is a typical use case for hashset?

Answer 39

Removing duplicate items from a list

Question 40

What are other possible set implementations?

Answer 40

• TreeSet
• LinkedHashSet

Question 41

How does a hashset maintains uniqueness?

Answer 41

• When we put an object into a hashset, it uses the object’s hashcode value to determine if an element is not in the set already
• Each hashcode value corresponds to a certain bucket location which can contain various elements, for which the calculated hash value is the same. But two objects with the same hashcode might not be equal. So, objects within the same bucket will be compared using the equals() method

Question 42

What is the rule of thumb for the performance of a hashset?

Answer 42

• A high initial capacity is good for a large number of entries coupled with little to no iteration
• A low initial capacity is good for few entries with a lot of iteration

Question 43

What is a hashmap?

Answer 43

Hashmap is a map with key-value mapping, which means that every key is mapped to exactly one value that we can use the key to retrieve the corresponding value from a map

Question 44

What is the advantage of a hashmap?

Answer 44

The advantage of a hashmap is that the time complexity to insert and retrieve a value is O(1) on average

Question 45

What is an iterator and what are its pros and cons?

Answer 45

• An iterator is one of many ways we can traverse a collection
• Iterators made it possible to remove elements from a collection we are iterating over
• Iterators do not guarantee the iteration order

Question 46

What is an anonymous inner class?

Answer 46

• Is like a nested class without a name; it provides a class definition and instantiates it at the same time
• Implements functional interfaces and abstract classes without creating additional sub-classes

Question 47

What is a lambda expression?

Answer 47

• Implements functional interface
• Is essentially a method definition without a name
• Makes code more concise and readable
• Provides a way to pass a function as a method argument

Question 48

Anonymous class vs. lambda expression?

Answer 48

• Anonymous classes are used for interfaces and abstract classes
• Lambda expressions are only used for functional interfaces

Question 49

What are functional interfaces?

Answer 49

Functional interfaces are interfaces that have exactly one method

Question 50

How is the performance of lambda expressions compared to anonymous classes?

Answer 50

Lambda expression is better compared to the anonymous class in terms of performance because an anonymous class lead to an extra class file on the compilation, this takes additional time during class loading and verification during runtime