Week 6

Question 1

ORM Framework features, advantages, and disadvantages

Answer 1

• Object Relational Mapping is a tool that uses objects to connect the OOP language and database systems
• Benefits - ORM maps an object to the table, can hide the details of SQL queries from OO logic, provides methods for automatic versioning and timestamping, provides caching support for better performance, best suited for large projects, injected transaction management, configurable logging, faster development of applications
• some ORM tools are Hibernate, JPA, Active JPA, iBATIS, IBM Pure Query, etc

Question 2

What methods are available in the Object class (4)?

Answer 2

.clone
.hashcode
.equals
.toString

Question 3

How would you clone an object?

Answer 3

First, tag the class with the Cloneable marker interface.

Next, invoke clone().

The clone method is declared in java.lang.Object and does a shallow copy.

Question 4

What is an enhanced for loop?

Answer 4

Enhanced for loops allow easier traversal of Collections (actually any arrays or Iterables)

Syntax - for (object x : collection){. . .}

Question 5

What is try-with-resources?

What interface must the resource implement to use this feature?

Answer 5

• a Java 7 feature
• allows for automatically closing resources in a try/catch block using try(resource) {…} syntax
• Must implement the AutoCloseable interface

Question 6

How would you make numbers in your code more readable?

Answer 6

Use the underscore ( _ ) for numeric literals; must be placed between numbers (EX - 45,000 = 45_000)

Question 7

What are covariant return types?

Answer 7

A method is allowed to return objects that are child classes of the return type.

Also, when overriding a method, the return type of the new method can be a child class of the original return type

Question 8

What are 3 usages of the super() keyword?

Answer 8

1. to refer to immediate parent class instance variable.
2. super() is used to invoke immediate parent class constructor (also can pass params)
3. to invoke immediate parent class method.

Question 9

Can you overload / override a main method? static? private? default? protected?

Answer 9

Main method - overload, cannot override b/c is static method.

Static method - overload, cannot override b/c belongs to class (not inherited).

Private method - overload, cannot override b/c doesn’t get inherited.

Default method - both.

Protected method - both (override if inherited).

Question 10

What is the difference between FileReader and BufferedReader?

Answer 10

FileReader is just a Reader which reads a file, so it reads characters and uses the platform-default encoding.

BufferedReader reads text from a character-input stream, buffering characters so as to provide for the efficient reading of characters, arrays, and lines (e.g. can read one line at a time).

You can wrap a BufferedReader around a FileReader

Question 11

How would you pass multiple values with a single parameter into a method?

Answer 11

Use varargs

Question 12

What is static block?

Answer 12

• used for static initialization

- executed only once - upon creation of the first object of a class or access to static methods of a class

Question 13

What are static imports?

Answer 13

a static method or variable from a class - syntax: import static

Question 14

How would you clone an object?

Answer 14

use the .clone() method inherited from object class

Question 15

What makes a class immutable (6)?

Answer 15

1. Declare the class as final so it can’t be extended.
2. Make all fields private so that direct access is not allowed.
3. Don’t provide setter methods for variables.
4. Make all mutable fields final so that it’s value can be assigned only once.
5. Initialize all the fields via a constructor performing deep copy.
6. Perform cloning of objects in the getter methods to return a copy rather than returning the actual object reference.

Question 16

If 2 objects are equal, do they have the same hashcode? If not equal?

Answer 16

If two objects have the same hashcode then they are NOT necessarily equal.

But if objects are equal, then they MUST have same hashcode.

Question 17

What data types are supported in switch statements (6)?List all non-access modifiers?

Answer 17

1. byte
2. char
3. enums
4. int
5. short
6. string

Question 18

List all non-access modifiers (9)?

Answer 18

1. abstract
2. default
3. final
4. native
5. static
6. strictfp
7. synchronized
8. transient
9. volatile

Question 19

Which collections cannot hold null values (4)?

Answer 19

HashTable
TreeSet
ArrayDeque
PriorityQueue

Question 20

What is the .forEach() merthod in Java 8?

Answer 20

The forEach() method accepts what is called a functional interface as its parameter (specifically a Consumer), which the lambda expression ( -> ) then implements at runtime.

Question 21

If 2 interfaces have default methods and you implement both, what happens?

Answer 21

The code will NOT compile unless you override the method.

However, the code WILL compile if one interface is implemented further up in the class hierarchy than the other - in this case, the closest method implementation in the hierarchy will be called.

Question 22

If 2 interfaces have same variable names and you implement both, what happens?

Answer 22

The code will compile unless you make a reference to the variable (this is an ambiguous reference).

You must explicitly define the variable by using the interface name: int a = INTERFACENAME.a;

Question 23

Why does HashTable not take null key?

Answer 23

The HashTable hashes the keys given as input, and the null value cannot be hashed

Question 24

What new syntax for creating variables was introduced with Java 10?

Answer 24

The var keyword, with type inference

Question 25

Is there an interactive REPL (read, evaluate, print, loop) tool for Java like there is for languages like Python?

Answer 25

Yes, the jshell tool introduced in Java 9

Question 26

What are collection factory methods?

Answer 26

They allow you to directly populate collections

Question 27

Can you catch more than one exception in a single catch block?

Answer 27

Yes, use the | operator

Question 28

What are the 7 logging levels?

Answer 28

Least to most restrictive:

1. ALL => all levels
2. DEBUG => designates fine-grained informational events that are most useful to debug an application
3. INFO => informational messages that highlight the progress of the application at the coarse grained level
4. WARN => designates potentially harmful situations
5. ERROR => designates error events that might still allow the application to continue running
6. FATAL => severe error events that presumably lead the application to abort
7. OFF => highest possible level, intended to turn off logging

Question 29

Why is it important to use a logger instead of writing to standard output?

Answer 29

You can set logging thresholds, as well as keeping track of application events in a log file

There is no advantage to using a logger

Loggers can reduce design complexity

Loggers are easier to configure

Question 30

What file formats can you use for log4j configuration (4)?

Answer 30

1. XML
2. JSON
3. YML
4. Properties

Question 31

What is JPA?

Answer 31

• Java Persistence API
• standard API for accessing, persisting, and managing data between Java objects/classes and relational databases
• defined in the javax.persistence package
• uses Java Persistence Query Language to person database operations
• uses EntityManager interface to create, read, and delete operations for instances of mapped entity classes

Question 32

What is Hibernate?

Answer 32

• an ORM tool for Java programming language
• open-source persistent framework
• a flexible and powerful ORM solution to map Java classes to database tables
• an implementation of JPA, so it follows the common standards provided by the JPA
• defined in the org.hibernate package
• uses Hibernate Query Language (HQL)
• Hibernate’s Session interface is an extension of the JPA EntityManager interface

Question 33

What are the benefits of using Hibernate over JDBC (4)?

Answer 33

• used to overcome JDBC’s drawbacks

1. transparent persistence - reduces the lines of connection code
2. database independent - can be used to connect with any database, and changing the SQL sent to a database is as simple as changing the database dialect in the config file
3. provides an abstraction - many common tasks are implemented for us internally (like establishing a connection or performing CRUD)
4. supports dual-level caching mechanism - retains the objects in the cache to reduce repeated hits to the database; highly scalable and optimizes the app’s performance

Question 34

Tell me about some of the JPA annotations you have worked with (15 listed)? What do they do? How do you specify multiplicity relationships with JPA annotations?

Answer 34

@Column - maps the field to the column table (attributes include name, length, nullable, and unique)
@Entity - used to mark a class as a mapped/persistence class. MUST have a no-args constructor with package visibility
@Table - used to specify the table details that used to persist the entity in the database
@GeneratedValue - used to instruct the database to generate a value for the field automatically
@Id - used to mark the field as a primary key column
@JoinColumn - specifies a column for joining an entity association or element collection
@JoinTable - used in the mapping of associations. It is specified on the owning side of an association
@NamedQuery - is a way to use any query by some meaningful name (like an alias)
@PersistenceContext - handles a set of entities which hold data to be persisted in some persistence store
@Transient - tells the hibernate, not to add this particular column

Specify multiple relationships with JPA using @ForeignKey, @OneToOne, @OneToMany, @ManyToOne, @ManyToMany

Question 35

What are the interfaces of Hibernate (5)?

Answer 35

ALL are defined in the org.hibernate package

1. Session - session objects are created using the SessionFactory and are used to perform CRUD operations.
2. SessionFactory - used to create the Session Object. The SessionFactory object is used by all threads of an application.
3. Query - obtained by calling Session.createQuery(); can be used to expose extra functionality beyond that provided by Session.iterate() and Session.find()
4. Transaction - associated with a Session and usually instantiated by a call to Session.beginTransaction(); used whenever we perform any operation and based upon that operation there is some change in database
5. Criteria - a simplified API for retrieving entities by composing Criterion objects
   
   • restrictions - class in hibernate that provides several methods that can be used as conditions (known as Criterion). These conditions are added to a criteria object with the add() method
   • projections - used in order to query only a subset of the attributes of an entity or group of entities you’re querying with Criteria

Question 36

What is multi-threading?

Answer 36

Handling multiple threads / paths of execution in your program

Question 37

In what ways can you create a thread?

Answer 37

By extending the thread class or by implementing the runnable interface

you must call threads with .start() method to start it as a new thread of execution

Question 38

Lifecycle of a thread?

Answer 38

When created, in NEW state.

When .start() is called, it goes to RUNNABLE state.

When .run() is called, goes to RUNNING state.

If .sleep() or .wait() is called, will go to WAITING.

If dependent on another thread to release a lock, it will go to BLOCKED state.

When finished executing, will be in DEAD state and cannot be restarted.

Question 39

What is deadlock?

Answer 39

Deadlock occurs when one thread is waiting on a resource held by another thread, which is waiting on a resource the first has. In this scenario, execution of both threads is blocked and the program is stuck indefinitely.

Deadlock occurs when a thread ends unexpectedly and disrupts execution of the program

Deadlock occurs when two threads call each other and get stuck in an infinite loop

Deadlock occurs when the program terminates before a thread can finish executing

Question 40

What is a thread?

Answer 40

A special method that executes only once

A class which allows for more efficient and faster processing

A representation of a path of execution, allowing for concurrent processing

A separate program which can be run only after the end of the current program

Question 41

What is the synchronized keyword used for?

Answer 41

addressing the problem of deadlock by preventing access to the resource by more than one thread at a given time

synchronizing the execution of many separate threads so they finish execution at the same time

preventing access to the resource from outside the class

enabling the class to be serialized

Question 42

Threads can be created by..?

Answer 42

1. passing a Thread into the constructor of Runnable: new Runnable(Thread)
2. extending the Thread class and overriding the .start() method
3. extending the Runnable class and overriding the .run() method

Question 43

Tell me how you set up hibernate? What files are you editing, what goes in them, etc.

Answer 43

• configuration files would go into an XML file or properties file:
  1. XML file (named hibernate.cfg.xml) - contains the database and session related configuration. Database configuration includes the JDBC connection URL, Database user credentials, driver class, and hibernate dialect.
  2. properties file (named hibernate.properties) - also used to configure Hibernate
• if both are present, then hibernate gives priority to a hibernate.cfg.xml file
• the difference between a hibernate.cfg.xml and hibernate.properties file is that in an XML file we can directly map classes using the element, but there is no way to configure this in a properties file

Question 44

Explain Hibernate configuration: SQL dialect?

Answer 44

Dialect allows Hibernate to generate SQL optimized for a particular relational database.

Hibernate generates queries for the specific database based on the Dialect class.

A hibernate dialect gives information to the framework of how to convert hibernate queries(HQL) into native SQL queries

Question 45

Explain Hibernate configuration: hbm2ddl?

Answer 45

automatically validates and exports DDL (data definition language) to the schema when the sessionFactory is created

Question 46

Explain Hibernate configuration: show_sql?

Answer 46

If this property value is true then it enables the logging of all hibernate-generated SQL statements to the console.

Question 47

explain eager vs lazy loading?

Answer 47

Eager loading - the process whereby a query for one type of entity also loads related entities as part of the query. Eager loading is achieved by the use of the Include method.

Lazy loading - delaying the loading of related data, until you specifically request for it.

While lazy loading delays the initialization of a resource, eager loading initializes or loads a resource as soon as the code is executed. Eager loading also involves pre-loading related entities referenced by a resource.

Question 48

Under what circumstances would your program throw a LazyInitializationException?

Answer 48

when Hibernate needs to initialize a lazily fetched association to another entity without an active session context

Question 49

What is HQL? What makes it different from SQL?

Answer 49

• Hibernate Query Language - object-oriented query language of the Hibernate framework.
• It supports OOP concepts like polymorphism, inheritance, and abstraction.
• It is database-independent and easy to learn.
• different from SQL because HQL queries against persistent objects instead of tables/columns

Question 50

What is the Criteria API?

Answer 50

• provides an object oriented approach for querying a database and getting results; a programmatic and typesafe way to fetch data from the relational database

Question 51

What is caching?

What is the difference between L1 and L2 cache?

Answer 51

Hibernate performs caching to optimize the performance of an application. It’s used to reduce the number of database hits by storing data locally in a cache.

L1 - checks for cached data; enabled by default and can’t be disabled; a mandatory cache through which all requests must pass; associated with the Session object and each object caches data independently so there is NO sharing of cached data across sessions and cached data is deleted when session closes; only useful for repeated queries in the same session

L2 - responsible for caching objects and sharing data across sessions; disabled by default but we can enable it through configuration (adding properties to the hibernate configuration file); assciated with the SessionFactory object and is shared among all sessions created using the same session factory;

Question 52

How do you enable second-level/L2 caching?

Answer 52

• we can enable it through configuration by adding the following properties to the hibernate configuration file:

true
org.hibernate.cache.ehcache.EhCacheRegionFactory

• to make an entity eligible for L@ caching we annotate it @Cache(usage = CacheConcurrencyStrategy.( ))

Question 53

What is JPQL?

Answer 53

• Java Persistence Query Language - object-oriented query language used to perform database operations.
• uses the EntityManager interface to create, read, and delete operations for instances of mapped entity classes

Question 54

Explain Named Queries

Answer 54

• an SQL expression with a predefined unchangeable query string; can be defined in hibernate mapping file or inan entity class
• annotations used in entity class:
  @NamedQueries - used to define the multiple HQL expressions.
  @NamedQuery - used to define the single HQL expression. has 2 attributes:
  - name - used to specify a name by which a session object can locate the query.
  - query - used to specify the HQL statments.
  @NamedNativeQueries - used to define the multiple native SQL expressions.
  @NamedNativeQuery - used to define the single native SQL expression.

Question 55

What are the 3 object states in Hibernate?

Answer 55

An object of a persistent class (a class mapped to a relational database table) can be in one of three different states. These states are defined in relation to a persistence context (Session object).

1. transient state - When an object is created using the new operator and not yet associated with a Hibernate Session, then the object state is transient; doesn’t represent a row; garbage collected if the app does not hold a reference
2. persistent state - The object state is persistent when it is associated with the hibernate session; represents a row in the database and has an identifier value; transient can become persistent by associating them with a Session using save() /Persist() /or saveOrUpdate() methods;when data comes from database using get() or load() method, it’s persistent
3. detached state - When a persistent object has its session closed, then it becomes detached; any changes made to detached objects will not be saved automatically to the db; if a detached instance is reattached with a new Session, it becomes persistent again; the Session class’ close(), evict(Object), and clear() methods are used to move a persistent object to the detached state; the Session class’ update(Object) and merge(Object) methods can be used to reattach detached objects to a session.

Question 56

Can you write native SQL with Hibernate? Is this a good idea?

Answer 56

Yes. In hibernate, we can directly write SQL statements using Native SQL, but using HQL or JPQL is recommended

Question 57

How would you configure Hibernate to print to the console all SQL statements run?

Answer 57

Add ‘show_sql’ property to the Hibernate configuration file

Question 58

What is automatic dirty checking?

Answer 58

Hibernate provides a feature called Automatic Dirty checking where changes to a persistent object are automatically saved to the database when the session is flushed or the transaction is committed. So the code does not need to invoke an explicit save or update.

Question 59

What is Transactional Write Behind?

Answer 59

• when Hibernate tries to defer the Persistence Context (first-level cache) flushing up until the last possible moment

Question 60

What are the 7 transaction propagation strategies?

Answer 60

1. Mandatory - if there is an active transaction, then it will be used. If there isn’t an active transaction, then Spring throws an exception
2. Nested - Spring checks if a transaction exists, and if so, it marks a save point
3. Never - Spring throws an exception if there’s an active transaction
4. Not supported - If a current transaction exists, first Spring suspends it, and then the business logic is executed without a transaction
5. Required - the default propagation. Spring checks if there is an active transaction, and if nothing exists, it creates a new one
6. Requires new - Spring suspends the current transaction if it exists, and then creates a new one
7. Supports - Spring first checks if an active transaction exists. If a transaction exists, then the existing transaction will be used. If there isn’t a transaction, it is executed non-transactional

Question 61

What is the difference between Dynamic Insert and Dynamic Update?

Answer 61

When the dynamic-insert property is set to true, Hibernate does not include null values for properties (for properties that aren’t set by the application) during an INSERT operation.

With the dynamic-update property set to true, Hibernate does not include unmodified properties in the UPDATE operation.

Question 62

What is a proxy?

Answer 62

• Hibernate generates proxies to provide lazy loading for to-one associations, and you can use them to improve the performance of some of your write operations; generates the proxy class as a subclass of your entity class