OCP - 4
NIO.2, JDBC
NIO.2, JDBC
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Flashcards
What are NIO Views for?
A view is a group of realte related attributes for a particular file system.
They increase performance by reading all metadata in one operation rather than accessing the fs multiple times.
How can you read all Metadata of a Path?
BasicFileAttributes data = Files.readAttributes(path, BasicFileAttributes.class)
How can you write Metadata of a Path?
BasicFileAttributeView data = Files.getFileAttributeView(path, BasicFileAttributeView.class)
How can you travese a directory using NIO2?
Using the Files.walk(...) method which returns a Stream<Path>
What are the arguments of the Files.fild(...) method?
FIles.filnd(Path path, int maxDepth, BiPredicate<Path,BasicFileAttributes> predicate)
What does Files.list(path) return?
A stream of Paths of all the child paths of the given one
What is the difference between Files.readAllLines and Files.lines?
- Files.readAllLines returns a List of Strings
- Files.readLines returns a Stream of Strings
- The latter is usually better as it does not require the entire file to be stored in memory
Assuming the current directory is /seals/harp/food, what is the result of executing the following code?
final Path path = Paths.get(".").normalize(); // h1
int count = 0;
for(int i=0; i<path.getNameCount(); ++i) {
count++;
}
System.out.println(count);
What are the 4 Key Concepts of JDBC?
- Driver: Knows how to get a connection to the database
- Connection: Knows how to communicate with the database
- Statement: Konws how to run the SQL
- ResultSet: Knows what was returned by a SELECT Query
What is the format of a JDBC URL?
protocol:Product/Vendor Name:DatabaseSpecific Connection Details
eg. jdbc:postgres://localhost:5432/zoo
How can you connect to a database using JDBC?
DriverManager.getConnection("jdbc:derby:zoo")
DriverManager.getConnection("jdbc:derby:zoo", "username", "password")
How can jdbc determine which driver to use?
A driver does advertise itself via a file called java.sql.Driver in the META-INF/services directory. Otherwise, the Class.forName("fqn.of.driver") has to be used.
How can you obtain a JDBC statement?
connection.createStatement()
connection.createStatement(ResultSet.Type_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY)
What is the difference between the different ResultSet Types?
- TYPE_FORWARD_ONLY: Usually sufficient, only next() is availbale.The view is static (new updates are not visible)
- TYPE_SCROLL_INSENSITIVE Can scroll in any directoy,.The view is static (new updates are not visible)
- TYPE_SCROLL_SENSITIVE: Can scroll in any directoy, the view is not static: new updates are visible. Not supported by most dirvers
What is the difference of the different result set concurrency modes?
- CONCUR_READ_ONLY
- CONCUR_UPDATABLE - not universally supported.
What is the difference between statment.execute() , statment.executeUpdate() and statement.executeQuery()?
- statment.executeUpdate() - intended for DELETE, INSERT, UPDATE: Returns the number of affected rows
- statement.executeQuery() - intended for SELECT: Returns a Result Set
- statment.execute(): Generic: Returns true for a SELECT, otherwise false
What is the result of this code:
int result = stmt.executeUpdate("select * from animal");
An Exception is thrown: A resut was returned when none was expected
How does a Result set work?
- It is a Cursor, meaning it points to a row (or a virtual row above/below the table)
- navigate using next() and - if supported - previous(), absolte(), relative(), ...
- multiple get___ Methods return the value of the column index/name in the current row
What has to be considered when working with column indices in Result Sets?
The columns are counted starting with 1 rather than 0
What happens if you don't call and check rs.next() before calling rs.get___(..)
The get___(..) method might throw an SQLException
How can you convert a java.sql.Date, java.sql.Time and java.sql.TimeStamp Object into a LocalDate/LocalTime, LocalDateTime?
Using the convenient Time.toLocalTime(), TimeStamp.toLocalDateTime(), Date.toLocalDate methods
Why do the return type of first()/last() and beforeFirst()/afterLast() differ?
The beforeFirst and afterLast do always work, making a return type redundant.
Where does the cursor point to when calling rs.absolute(0)
Same as rs.beforeFirst()
Where does the cursor point to when calling rs.absolute(-1)
To the last entyr in the table
What implicit rules corresponding to a JDBC connection, Statement and ResultSet exist?
- Closing a Connection also closes the Statement and ResultSet
- Closing a Statement also closes the ResultSet
- JDBC automatically clsoes a ResultSet when you run another SQL statement from the same Statement
What is the difference between a paht.toRealPaht() and paht.toAbsolutePath()?
toAbsolutePaht does not operate on the fs ("theory") while toRealPath will resolve symbolic links . toRealPaht will throw an exceptiom if the file does not exist
