Interview Wiki Java v1/2

Question 1

(Beginner) What is the MVC pattern?

Answer 1

The Model-View-Controller pattern.

Application logic is separated into three layers:

–a model, which knows about the data
–a view, which knows how to display things
–a controller, which implements application logic and knows how to map the raw data into the elements to be displayed.

Question 2

(Beginner) Are you familiar with any other design patterns other than MVC?

Answer 2

Common answers: Singleton, Factory, Observer, Facade

Question 3

(Beginner) What is the command line utility used to compile java source code into byte code?

Answer 3

javac

Question 4

(Beginner) How do you handle exceptions in Java?

Answer 4

try, catch, finally

Question 5

(Beginner) What is the difference between Final, finally and finalize()?

Answer 5

Final - declares a constant who’s value can’t be changed, or a method which can’t be overridden

finally
- is always called before leaving a try catch block.
- Used usually to close connections.
- Typically used for freeing resources.

finalize() - Called during garbage collection

Question 6

(Beginner) What is the difference between == operator and equals () method?

Answer 6

== compares objects (memory locations — are they the same instance)

.equals compares the object content based on how the class defines equality for instances of the class.

Question 7

(Intermediate) What is AJAX?

Answer 7

A) AJAX = Asynchronous Javascript and XML

A) AJAX allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes… allows parts of a web page to be updated, without reloading the whole page.

Question 8

(Intermediate) What is the difference between overriding and overloading methods?

Answer 8

A) Overwriting (Overriding) means having two methods with the same arguments, but different implementations. For example, a parent class has a method doWork() that does work for the what is present in the parent class. A child class extends the parent class, and needs to do work differently. The child class would override the doWork() method from the parent class.

A) Overloading means that you have multiple methods with the same name but different method signatures. For example, in the String class, there are 4 different indexOf methods: indexOf(int ch), indexOf(int ch, int fromIndex), indexOf(String str), indexOf(String str, int fromIndex)

Question 9

(Intermediate) What does protected mean?

Answer 9

protected - changes the scope of the class or method to only be accessible to other classes or sub classes in the same package

Question 10

(Advanced) What is the difference between SOAP and REST Web Services? (http://keydifferences.com/difference-between-soap-and-rest-web-services.html)

Answer 10

• SOAP is a protocol that is based on the HTTP protocol.
• REST is an architecture that uses the different HTTP methods to act on a resource.

Question 11

(Advanced) Which HTTP methods are supported by RESTful web services ? (http://www.restapitutorial.com/lessons/httpmethods.html)

Answer 11

GET, POST, PUT, PATCH and DELETE

Question 12

(Advanced) What do http methods do? What are they used for? (http://www.restapitutorial.com/lessons/httpmethods.html)

Answer 12

GET: The HTTP GET method is used to read (or retrieve) a representation of a resource. In the “happy” (or non-error) path, GET returns a representation in XML or JSON
POST: The POST verb is most-often utilized to create new resources. In particular, it’s used to create subordinate resources. That is, subordinate to some other (e.g. parent) resource. In other words, when creating a new resource, POST to the parent and the service takes care of associating the new resource with the parent, assigning an ID (new resource URI), etc.
PUT: PUT is most-often utilized for update capabilities, PUT-ing to a known resource URI with the request body containing the newly-updated representation of the original resource
PATCH: PATCH is used for modify capabilities. The PATCH request only needs to contain the changes to the resource, not the complete resource.
DELETE: DELETE is pretty easy to understand. It is used to delete a resource identified by a URI.

Question 13

(Advanced) What are the differences between a LinkedList and an ArrayList? When might you prefer a LinkedList over an ArrayList?

Answer 13

There are many differences.

—ArrayList provides efficient random access, but has expensive inserts/deletes in the middle of the list and semi-expensive size growth.

—LinkedList provides cheap insertions at any point in the list, but only supports sequential iteration starting from the ends, and also uses more memory.

Question 14

(Advanced) What are autoboxing and unboxing?

Answer 14

A) Boxing is the process of converting a primitive type (such as an int) to an object type (such as Integer), and auto-unboxing is the reverse process. Prior to Java 5, this had to be done with manual casts, but Java 5 and higher will automatically perform the conversions where necessary. One common scenario where this is useful is when working with generic collections - only object types can be used with generics, so without autoboxing, storing e.g., primitive ints in an ArrayList would require a significant amount of manual casting between int and Integer. (More details)

Question 15

(Advanced) What will unboxing do with null values?

Answer 15

A NullPointerException will be thrown because you can’t have null invoke a method.

Ex: To unbox an Integer object to int, Java calls the “Integer Object”.intValue() method.

Question 16

(Advanced) What is Cross Site Scripting (XSS) and how would you protect against it?

Answer 16

It is a Security Vulnerability that allows the user to inject javascript into a webpage and can get the page to execute that script.

Hackers will use this to steal a users session cookies and compromise their account.

To protect against it you can use:
1. URL Encoding and Use Reg Expressions to Filter out non Alpha Numeric Characters.
2. black list or white list to validate inputs.

Question 17

(Advanced) If you could change one thing about Java what would it be?

Answer 17

Come up with answer - no answer was provided

Question 18

(Java EE Questions) What is a deployment descriptor?

Answer 18

Typically it is a file that holds configuration information about how an application should be deployed in a JavaEE environment. For instance, in a web application. the web.xml file defines what servlets receive which URL mappings, what the index files are, what the error conditions are, and what roles are required for certain HTTP methods.

Question 19

(Java EE Questions) What is an EJB?

Answer 19

An EJB is an Enterprise Java Bean. It is a Java class that is defined to interact directly with the JavaEE environment.

Question 20

(Java EE Questions) What benefits do we get from using EJBs?

Answer 20

We can put JavaEE extensions in an EJB for things like transactions, validations, restart/retry logic.

Question 21

(Java EE Questions) What is a Web container?

Answer 21

A web container is a Java application that serves web applications that are packaged in the form of WARs (Web Application Resource).

Technically, these are different from web application servers

Question 22

(Java EE Questions) What is a Web Application Server?

Answer 22

Implementation of a JavaEE server

Question 23

(Java EE Questions) When to use web containers vs. web application servers?

Answer 23

—Web container used if you do not need a full stack JavaEE server.
—Most web applications probably only ever need a web container.
—If you need to do stuff with EJB’s, you will either need to add frameworks to the web container or just run a web application server.

Question 24

(java v2 Core) Discuss some of the features of Java. Comparedd to other languages.

4 things

Answer 24

generics, autoboxing, annotations, varArgs

Generics - provides compile-time (static) type safety for collections and eliminates the need for most typecasts (type conversion).
AutoBoxing - Automatic conversions between primitive types (such as int) and primitive wrapper classes (such as Integer
Annotations - allows language constructs such as classes and methods to be tagged with additional data, which can then be processed by metadata-aware utilities.
Varargs - The last parameter of a method can now be declared using a type name followed by three dots (e.g. void drawtext(String… lines)). In the calling code any number of parameters of that type can be used and they are then placed in an array to be passed to the method, or alternatively the calling code can pass an array of that type.

Question 25

(java v2 Core) What is an interface? Why would you use one?

Answer 25

1) An Interface defines what has to be implemented;
2) Insulation from implementation, development can occur separately

_ _ ___

• Provides a standard implementation of some behaviors. You subclass and provide implementations for other methods.
• Commonly used in frameworks to provide structure and consistent implementation.

Question 26

(java v2 Core) What is an abstract class? How is it different from an interface? Why would you use one?

Answer 26

Abstract class
—cannot be instantiated but allows implementation of methods

Implementation
—Provides a standard implementation of some behaviors. You subclass and provide implementations for other methods.
—Commonly used in frameworks to provide structure and consistent implementation.

Question 27

(java v2 Core) Can you explain static variables vs. instance variables?

Answer 27

—Static variables are “shared” among instances. They exist within the class.

—The instance variables as named are specific to each instance and are not shared across instances.

// static var ex
public class Car {
private String name;
private String engine;

public static int numberOfCars;

public Car(String name, String engine) {
    this.name = name;
    this.engine = engine;
    numberOfCars++;
}

// getters and setters } @Test public void whenNumberOfCarObjectsInitialized_thenStaticCounterIncreases() {
new Car("Jaguar", "V8");
new Car("Bugatti", "W16");
 
assertEquals(2, Car.numberOfCars); }

–instance var example
public class Studentsrecords
{
/* declaration of instance variables. */
public String name; //public instance
String division; //default instance
private int age; //private instance
#######

Question 28

(java v2 Core) What does the term ‘final’ mean in Java, as applied to a class, a method, or a variable?

Answer 28

A) A class declared as final can not be subclassed. All methods in a final class are implicitly final. Declaring a class as final has no affect on the references to the objects of that class when it is used as a variable.
B) A ‘final’ method cannot be overridden
C) Declaring a variable as final means it cannot be modified.

Question 29

(java v2 Core) Does the final keyword have different effects between a primitive and an object?

Answer 29

Yes

For primitive) variables the value cannot be changed (ie int x=10; assigns x a value of 10 forever.

For an object variable), it simply means you cannot change the variable to point to another object, but you can still modify the values inside the object!

Question 30

(java v2 Core) Do you have experience with Java Collections classes?

Answer 30

HashMap, Hashtable, ArrayList, Map, List
If yes,

Question 31

(java v2 Core) What is the main difference in an ArrayList and a List?

Answer 31

a List is an interface….

An ArrayList is an implementation of the List interface.

Question 32

(java v2 Core) Is Java pass-by-reference or pass-by-value? What is the difference?

Answer 32

Java is pass-by-value but it passes the reference which makes it behave like pass-by-reference.

Question 33

(java v2 Core) What is encapsulation?

Answer 33

Hides the implementation behind code (an interface).

Two features: instance variables are protected, getter and setter methods provide access to instance variables.

Encapsulation is done to hide “sensitive” data from users. To achieve this, you must:
declare class variables/attributes as private
provide public get and set methods to access and update the value of a private variable.
public class Person {
private String name; // private = restricted access

// Getter
public String getName() {
return name;
}

// Setter
public void setName(String newName) {
this.name = newName;
}
}
public class Main {
public static void main(String[] args) {
Person myObj = new Person();
myObj.setName(“John”); // Set the value of the name variable to “John”
System.out.println(myObj.getName());
}
}

// Outputs “John”

Question 34

(java v2 Core) Can you call one constructor from another if a class has multiple constructors? How?

Answer 34

Yes, using “this” keyword.

A constructor in Java is a special method that is used to initialize objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes:

// Create a Main class
public class Main {
int x; // Create a class attribute

// Create a class constructor for the Main class
public Main(int y) { // you don’t need parameters, but you can write them if you want them.
x = 5; // Set the initial value for the class attribute x
z = y;
}

public static void main(String[] args) {
Main myObj = new Main(); // Create an object of class Main (This will call the constructor)
System.out.println(myObj.x); // Print the value of x
}
}

// Outputs 5

Question 35

(JEE Questions) Have you worked with web services? SOAP or REST?
[If yes for SOAP or REST] What are they, explain SOAP or REST?

Answer 35

A)
* Web services is a self contained, self modular application that is designed to support inter operable machine-to-machine interaction over a network.
* The interface is described by a WSDL and the communication is done with SOAP messages.
* SOAP is a protocol specification for exchanging structured information in the implementation of Web Services.
* JAX-RPC/JAX-WS
* RESTful Service (Representational State Transfer) – uses HTTP methods (put/get/delete/post) along with the URI, instead of SOAP/XML payload to define client requests.
* For example: http://example.com/resources/item17
o Get: Retrieve a representation of item 17.
o Delete: Delete item 17
o Put: Replace item 17 with the passed in representation or create it (if it doesn’t exist).

https://spring.io/guides/tutorials/rest

Question 36

(JEE Questions) Compare and contrast SOAP and REST

Answer 36

A) SOAP has a Web Services Description Language (WSDL)
B) SOAP is a very structured, formalized process for inter service communication. All XML.
—Where REST is guidelines and does not have to be XML.

Question 37

(JEE Questions) In a REST URI or URL, what is the difference between a query parameter and a path parameter?

Answer 37

• Query parameters are the name values pairs passed with the http request. (Everything after the question mark)
• Path parameter is part of the URI pattern and have the {} in path

Query Param:
@GetMapping(“/api/foos”)
@ResponseBody
public String getFoos(@RequestParam String id) { // id is the query parameter
return “ID: “ + id;
}
http://localhost:8080/spring-mvc-basics/api/foos?id=abc
—-
path param:
@GetMapping(“/api/employees/{id}”)
@ResponseBody
public String getEmployeesById(@PathVariable String id) {
return “ID: “ + id;
}
id is the path parameter, 111 in the example below.
http://localhost:8080/api/employees/111

Question 38

(JEE Questions) What are some of the ways that you would secure a rest endpoint?

Answer 38

A) HTTP basic auth
B) SPENGO, certificate authentication
C) HTTP as your protocol, Encryption

Question 39

(JEE Questions) Do you have experience with JEE web applications?
[If yes] What is a servlet?

Answer 39

• Servlet is a special Java class intended to handle web requests.
• Receives an Http Request and returns and Http Response
• It implements methods such as ‘doPost’ and ‘doGet’
• Instances of servlets are created and managed by the application server

Question 40

(Database/Persistence) What databases have you used?

Answer 40

personal answer: mssql, db2, snowflake, netezza, oracle

Question 41

(Database/Persistence) Why use stored procedures?

Answer 41

—Solid answer: “stored procedures provides several advantages including
- better performance,
- higher productivity,
- ease of use,
- increased scalability.”

—Good answer: https://docs.oracle.com/cd/F49540_01/DOC/java.815/a64686/01_intr3.htm

Question 42

(Database/Persistence) Explain the pros and cons of using indexes.

Answer 42

Pros: Speed up query access
Cons: Slows down inserts and possible updates

Question 43

(Database/Persistence) Inner join vs outer join

Answer 43

Inner join returns records that match in both tables
Outer join returns all the rows from one table and the matching ones from a second table.

Question 44

(Database/Persistence) What are some good practices for error handling?

Answer 44

—Checked vs. Unchecked Exceptions
—logging errors
—passing context across tiers
—translating system errors into friendly UI messages
—ensuring exceptions never escape to the client
—etc

Exception – checked
In general, checked exceptions represent errors outside the control of the program. For example, the constructor of FileInputStream throws FileNotFoundException if the input file does not exist. Java verifies checked exceptions at compile-time. Therefore, we should use the throws keyword to declare a checked exception:

private static void checkedExceptionWithThrows() throws FileNotFoundException {
File file = new File(“not_existing_file.txt”);
FileInputStream stream = new FileInputStream(file);
}

Exception - unchecked
If a program throws an unchecked exception, it reflects some error inside the program logic. For example, if we divide a number by 0, Java will throw ArithmeticException:

private static void divideByZero() {
int numerator = 1;
int denominator = 0;
int result = numerator / denominator;
}

Question 45

(Design) What design patterns are you familiar with?
[Ask this question so onsite can go more into this]

Answer 45

Façade, Command, Singleton, Factory, Adapter, Bridge, Proxy, Template

The most popular approach (for Singletons) is to implement a Singleton by creating a regular class and making sure it has:

a private constructor
a static field containing its only instance
a static factory method for obtaining the instance

We’ll also add an info property for later use only. So our implementation will look like this:

public final class ClassSingleton {

private static ClassSingleton INSTANCE;
private String info = "Initial info class";

private ClassSingleton() {        
}

public static ClassSingleton getInstance() {
    if(INSTANCE == null) {
        INSTANCE = new ClassSingleton();
    }
    
    return INSTANCE;
}

// getters and setters }

Question 46

(Design) What are some things you would consider if you wanted to make your code reusable?

Answer 46

Possibilities:

1) focus on encapsulation,
2) package in a jar,
3) javadoc,
4) test harness,
5) make clear distinction between public methods (contract with the consumer) and private (implementation-specific). documentation,

6) extra cost to design/develop/test
7) expose as a web service, ensure dependencies are well-known and isolated from the consumer,

Question 47

(Design) What techniques do you use to grow your skills and technical knowledge? What technologies are you excited about? Where should companies be focusing?

Answer 47

No answer provided

Question 48

(Behavioral) When you have encountered a problem that you knew would impact your project, how did you handle the situation? When would you decide to escalate an issue? What could you have done differently?

Answer 48

No answer provided

Question 49

(Behavioral) What is a transient variable?

Answer 49

A variable that can be serialized but will not be.

Serialization is the conversion of the state of an object into a byte stream; deserialization does the opposite. After serialization, we can then save to a database or transfer over a network.

transient is a variables modifier used in serialization. At the time of serialization, if we don’t want to save value of a particular variable in a file, then we use transient keyword. When JVM comes across transient keyword, it ignores original value of the variable and save default value of that variable data type.

transient keyword plays an important role to meet security constraints. There are various real-life examples where we don’t want to save private data in file. Another use of transient keyword is not to serialize the variable whose value can be calculated/derived using other serialized objects or system such as age of a person, current date, etc.
Practically we serialized only those fields which represent a state of instance, after all serialization is all about to save state of an object to a file. It is good habit to use transient keyword with private confidential fields of a class during serialization.

// A sample class that uses transient keyword to
// skip their serialization.
class Test implements Serializable
{
// Making password transient for security
private transient String password;

// Making age transient as age is auto- 
// computable from DOB and current date. 
transient int age; 
  
// serialize other fields 
private String username, email; 
Date dob; 
  
// other code  }

Question 50

(Behavioral) Why make a class final?

(Behavioral) Why make a method final?

Answer 50

Class: Dependency on the implementations…
…provided in the class.

method: You might wish to make a method final if it has an implementation that should not be changed and it is critical to the consistent state of the object. (ie the method blue should always return the constant BLUE.)

Question 51

(Behavioral) What principal of OO design does this break?

Answer 51

Inheritance

Question 52

(Behavioral) When would you override the hashcode() method?

Answer 52

When an object’s uniqueness is defined differently

than the standard hashcode() method;

Question 53

(Behavioral) What are the access control modifiers in JAVA (e.g. public, etc.)?

Answer 53

public - objects outside of the class have access
private - only the owning class has access
default(package) - only classes in the package have access
protected - only the owning class and subclasses have access

Question 54

(Agile Engineering) What is TDD? Why would you want to use it?

Answer 54

no answer

Question 55

(Agile Engineering) What is Continuous Integration?

Answer 55

DevOps best practice, allowing developers to frequently merge code changes into a central repository where builds and tests then run.

Automated tools are used to assert the new code’s correctness before integration

Question 56

(Agile Engineering) Explain why you should focus on simple design vs future proofing/goldplating

Answer 56

Future-proofing software can be defined as the practice of designing, developing, and implementing software solutions in a way that allows them to adapt and remain effective in the face of future technological advancements, changing user needs, and evolving industry trends.

Gold plating in project management refers to the unnecessary addition of extra features, functionality or any type of enhancements to a project that are beyond its agreed-upon requirements

Question 57

(Agile Engineering) Which command would you use to list the previous commands you entered or get more information about those executed commands?

Answer 57

Use the “history” command

Question 58

(Agile Engineering) How would you parse out from the “history” command only those commands which included the text “scp”?

Answer 58

history | grep “scp”

Question 59

(Agile Engineering) How would you redirect the standard output to a file?

Answer 59

Use the “>” character. Example, history | grep “scp” > /tmp/history_withscp.txt

Question 60

(Agile Engineering) Will the “>” character append the standard output to the end of the file?

Answer 60

No, it overwrites the content of the file and creates the file if necessary

Question 61

(Agile Engineering) What is used to append the standard output to a file?

Answer 61

“»” is used

Question 62

(Agile Engineering) Which command is used to change the permissions of the file to read only for all?

Answer 62

chmod 444 /tmp/history_withscp.txt

Question 63

What is the difference between web app server and web container?

Answer 63

application server implements a full JavaEE stack and the container does not.

A web container is missing EJB’s, JMS, built in security, etc.

Question 64

When would you override the equals() method?

Answer 64

You can override the equals method in our class to check whether two objects have same data or not.