Quiz 3

Question 1

JDBC

Answer 1

Java Data Base Connectivity. An API that consists of libraries of classes used to interact with a relational database from a java application

Question 2

JDBC API allows you to access databases from just about any database vendor (T/F)

Answer 2

True

Question 3

JDBC API lets you create a Connection object which allows you to log into the database (T/F)

Answer 3

True

Question 4

JDBC API lets you create a Statement object to send queries/update statements to the database source (T/F)

Answer 4

True

Question 5

JDBC API lets you create a PreparedStatement object that can be cached and executed only once (T/F)

Answer 5

False. It can be executed multiple times.

Question 6

JDBC API lets you use a CallableStatement for executing stored procedures on the database (T/F)

Answer 6

True

Question 7

JDBC API lets you process the results returned from the DB in a Collection object (T/F)

Answer 7

False. It is returned in a ResultSet object.

Question 8

JDBC API lets you close the connection to the DB (T/F)

Answer 8

True

Question 9

JDBC API looks after all of the translation work needed for your java app and the database to talk to each other (T/F)

Answer 9

True

Question 10

What is needed to run a JDBC API?

Answer 10

A JDBC compatible database driver which must be loaded by your java application code in order to be used. Applications can load and use more than one driver depending on the back-end DB.

Question 11

What are the steps to access a database via a Java app?

Answer 11

1. Load JDBC driver
2. Create Connection object and pass in DB URL, user and pass as args
3. Create Statement/Prepared Statement object
4. Execute the query/stored procedure
5. Process the results
6. Close ResultSet (if used)
7. Close Statement object
8. Close the Connection object

Question 12

Types of DB Drivers

Answer 12

1. JDBC to ODBC bridge (inefficient, unreliable)
2. Native-API partly Java
3. JDBC to Network protocol pure Java
4. Native protocol pure Java (most efficient)

Question 13

Model View Controller (MVC)

Answer 13

Design pattern that helps you break code into functional segments with clear boundaries as to what each segment does (compound design pattern)

Question 14

MVC architecture

Answer 14

Design GUI using three separate code entities:

1. Model - maintain the data to be displayed in GUI
2. View - displayes data contained in model
3. Controller - handles user inputs and updates model

Question 15

If you have a view registered as a listener for the model then it can automatically update itself when the event fires (T/F)

Answer 15

True

Question 16

Multiple views can be serviced by the same model (T/F)

Answer 16

True

Question 17

TableModel

Answer 17

Interface that specifies 9 methods to manipulate data from a ResultSet:

1. addTableModelListener
2. getColumnClass
3. getColumnCount
4. getColumnName
5. getRowCount
6. getValueAt
7. isCellEditable
8. removeTableModelListener
9. setValueAt

Question 18

Process

Answer 18

• Can be thought as the entire program
• Every process has its own address space
• Process based multitasking means 2+ programs can be run concurrently
• Process based multitasking is controlled by the OS

Question 19

Thread

Answer 19

• A small part of a process
• All threads in a process share the address space
• Thread based multitasking means a process can run 2+ threads concurrently
• Thread based multitasking is under the control of the JVM

Question 20

Option 1 for creating threads in Java

Answer 20

Have the class being run as a thread implement the Runnable interface. Runnable represents a job that can execute concurrently with other jobs. Coder must only implement the run() method.

Question 21

Option 2 for creating threads in Java

Answer 21

Extend the class from the Thread class, which also implements the Runnable interface, and override the Thread class run() method with your own code.

Question 22

run() is automatically invoked by the JVM but should still be called explicitly (T/F)

Answer 22

False. Coder should not call run() explicitly, otherwise the code will run inside the current thread, no new thread will be used.

Question 23

What does the start() method do?

Answer 23

Signals the JVM that a thread is ready and waiting to run, so it joins the queue of threads waiting to run.

Question 24

Stop(), suspend() and resume() are unstable and should not be used (T/F)

Answer 24

True

Question 25

What does the yield() method do?

Answer 25

Causes the thread to be polite and give up its remaining time to allow another thread to run.

Question 26

What does the sleep() method do?

Answer 26

Safely and temporarily stop a thread (used instead of suspend() ). Needs to be put inside a try-catch.

Question 27

What are the constant thread priorities?

Answer 27

``` MIN = 1 NORM = 5 MAX = 10 ```

Question 28

Round-Robin Scheduling

Answer 28

If all runnable threads have equal priority then each gets an equal time on the CPU.

Question 29

What is thread contention / thread starvation?

Answer 29

When a lower priority thread may never get to run if there is a higher priority thread or if an equal priority thread never yields.

Question 30

To avoid contention coders must ensure higher priority threads invoke sleep() / yield() periodically (T/F)

Answer 30

True

Question 31

What is the Lost Update Scenario? (Concurrent Processing)

Answer 31

When two users try to modify and access the same data and one was unable to complete the process in one time slice.

Question 32

What was done to prevent Lost Update?

Answer 32

Transaction rules were created. When a transaction is started, the row gets locked to prevent other users from accessing it.

Question 33

Synchronized Data Structure

Answer 33

Puts locks in place when a thread accesses an element of the vector and blocks other threads from accessing it until the first thread completes its transaction.

Question 34

Access to values in an arrayList structure that is synchronized is faster and safe to use anytime (T/F)

Answer 34

False. They are only safe to use in single thread applications.

Question 35

Race Condition

Answer 35

If thread access is not synchronized either by the coder or some feature of the data structure which can cause data corruption. Occurs when several threads try to access the same data during their time slice.

Question 36

Thread-Safe

Answer 36

A class or data structure that will not permit race conditions to occur.

Question 37

Coders must prevent more than one thread at a time from entering the critical region of the program where variable / data structure is accessed in order to prevent race conditions (T/F)

Answer 37

True

Question 38

Most methods with a return value will not be corrupted by multi thread access (T/F)

Answer 38

False. Basically any method can be corrupted.

Question 39

Lock Interface

Answer 39

Defines lock(), unlock(), and newCondition() and allows creation of locks with specified fairness policies.

Question 40

True Fairness Policy

Answer 40

The longest waiting thread will be the first to obtain the lock and do business.

Question 41

True fairness policy may result in slower performance but prevents thread starvation where a thread is unable to obtain a lock and cannot carry out its task (T/F)

Answer 41

True

Question 42

Atomic classes can perform their operations atomically, meaning the whole operation is completed as a unit and cannot be interrupted (T/F)

Answer 42

True

Question 43

What are the Atomic classes?

Answer 43

AtomicInteger, AtomicBoolean, AtomicLong, AtomicReference

Question 44

AtomicInteger Class

Answer 44

Contains several methods which treat read-calculate-write as one atomic task, such that all steps are completed without interruptions (thread-safe).

Question 45

Why not synchronize every method?

Answer 45

1. Time cost (runs slowly) | 2. Defeats the purpose of multi-threading (every thread has to wait for the method to be available)

Question 46

Volatile

Answer 46

Applied to a variable. Does not do any locking. Informs the JVM that the variable will be accessed concurrently by multiple threads

Question 47

For variables changed using R-U-W steps, volatile will prevent possible lost updates (T/F)

Answer 47

False. It will not prevent lost updates and should not be relied upon to do so.

Question 48

Thread States

Answer 48

1. New 2. Runnable 3. Running 4. Blocked 5. Terminated/Dead

Question 49

Calling yield() does not automatically result in a thread giving up its time (T/F)

Answer 49

True

Question 50

wait() is a method of the Thread class (T/F)

Answer 50

False. It is from the Object class.

Question 51

How to stop a thread properly?

Answer 51

Call interrupt() rather than stop()