## Pages

### CodeEval Balanced Smileys

This problem is part of the family of the ones asking to check if parentheses in a sentence are balanced. See for instance the simpler matching brackets one.

In this version, happy ':)' and sad ':(' smileys could be part of the the sentence, so that we have to think if a parentheses has to be considered in the balancing or skipped, being the representation of the mouth of the smiley.

I found it as CodeEval #84 problem, but it is a repost, originally coming from Facebook Hacker Cup 2013 Hackathon.

I came to a solution inferior to the one published by the Hacker Cup editors, so I just threw mine in the dustbin and I spent some time pondering on the other one, coming out with a slightly different variation. [By the way, I would suggest you to do the same. When you see a piece of code you don't fully understand, play around with it, adapt it to your style, make it yours.]

The base problem is pretty simple. We loop on each character in the string, any time we see an open bracket, we push it in a stack (or we emulate a push in a stack, using just a counter). When we see a close bracket, we pop from the stack the matching open bracket. In the end, the stack should be empty. If we try to pop when the stack is empty, the sentence is unbalanced.

Having to care to the smileys, we need a second (virtual) stack, where we push also the dubious open brackets, the one that could have a double interpretation.

Looking at the code should clarify the sense of this solution:
```min_open = 0  # 1
max_open = 0  # 2
for i in range(len(msg)):  # 3
if msg[i] == '(':  #4
max_open += 1
if i == 0 or msg[i-1] != ':':  # 5
min_open += 1
elif msg[i] == ')':  # 6
if i == 0:
return False
min_open = min_open - 1 if min_open else 0
if msg[i-1] != ':':  # 7
if max_open == 0:
return False
max_open -= 1
return True if min_open == 0 else False  # 8
```
1. Emulate the first stack. It keep track of the "clean" open brackets only.
2. The second "dirty" stack, containing also the brackets that could be seen as mouth for a sad smiley.
3. Loop an all the character in the sentence.
4. Open bracket, push it to the "dirty" stack (i.e. increase the max_open counter), and ...
5. ... only if it couldn't be seen as the mouth of a smiley, push it to the "clean" stack (i.e. increase the min_open counter)
6. Close bracket, if we are at the beginning, the sentence is unbalanced. Otherwise, pop the matching open bracket from the "clean" stack. If it was already empty, the sentence could be unbalanced. Not sure, however, because it could be a happy smiley. So ...
7. If the bracket could be seen as the mouth of a happy smiley, the "dirty" stack empty means the sentence is unbalanced, otherwise, pop a bracket from it.
8. Completed the loop, the sentence is balanced only if the "clean" stack is empty.

The full python script is on GitHub.

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### Calling a Stored Function from JDBC

The code we should write to call a stored procedure or function via JDBC is slightly different from the one we write to perform a SQL SELECT. A CallableStatement is used instead of a Statement plus a ResultSet to set the parameter, execute the call, and extract the result.

To check it, I have firstly created a stored function in my Oracle 12 instance, on the HR schema. It takes a SQL CHAR in input, and returns the same string with 'suffix' appended:
```create or replace FUNCTION foo (val CHAR)
RETURN CHAR AS
BEGIN
RETURN val || 'suffix';
END;
```
I ensured it works as I expected running this query:
```SELECT foo('hello') FROM DUAL;
```
The I have written a couple of tests to see how to get the same result in Java. Not just one, because we can use either the JDBC escape and the PL/SQL block syntax to achieve the same result, being the first
`{? = call foo(?)`
and the latter
`begin ? := foo(?); end;`
The core of both tests is in these few lines:
```cs = conn.prepareCall(call);  // 1
cs.registerOutParameter(1, Types.CHAR);  // 2
cs.setString(2, "aa");
cs.execute();
String result = cs.getString(1);  // 3
```
1. Assuming conn is a good java.sql connection to the HR user on my Oracle database, and call is either the JDBC escape or the PL/SQL block string showed above, the prepareCall() should return a good CallableStatement.
2. The callable statement has to know the type of the output parameter is a character string. Then we set the other parameter, with the string that we want to pass as input.
3. After executing the callable statement, we get the first (and only) result as a string.

The actual code, that you could find on GitHub, class GettingStartedTest methods callFoo(), testFunctionCallJdbcEscape(), and testFunctionCallPlSqlBlock(), is a bit more verbose because I have to provide all the standard boilerplate, initializing, testing, cleaning up.

Reference: Oracle Database JDBC Developer's Guide 12c Release 2 (12.2) 2.8 Stored Procedure Calls in JDBC Programs

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### Selecting on Oracle via JDBC

Once we decided how to establish a connection to the database, performing a SELECT on it is pretty simple. Here I'm going to use OracleDataSource as connection provider, using pure JDBC doesn't add any complexity.

The only problem in this Java code is that is slightly boring. So boring that there are a few wrapper aiming at keeping the advantages of JDBC easing its usage. See for instance the Spring JdbcTemplate.

Anyway, firstly we open a connection, then a statement, on which we execute a query (a select, in this case) that return a resultset. We normally do something on the resultset, and then we can close what we have opened, in reversed order, resultset, statement, connection. The code is made even more boring by the need of try-catching the SQLExceptions that could be thrown. Last straw in this boredom parade, is that before closing the JDBC objects we have to ensure they have been created, checking them for not being null.

So, our typical plain JDBC code could be seen in three steps:

Initialization

```// ...
Connection conn = null;
Statement stmt = null;
ResultSet rs = null;
try {
conn = ods.getConnection();  // 1
stmt = conn.createStatement();
rs = stmt.executeQuery("SELECT first_name FROM employees");  // 2

// ...
```
1. I'm using the OracleDataSourceConnector I described in the previous post. Nothing fancy, anyway, I simply get a connection to a Oracle database for the well known hr example user.
2. Having extracted a statement from the connection, I execute a query, that is plain SQL SELECT. The resulting table, here consisting of just one column, is returned in a ResultSet.

Operation

Let's do something with the fetched data. Here I copy the first names to a list of string, then I sort them and I ensure the first and last are what I expect. Usually this part of the job has more sense.
```List<String> results = new ArrayList<String>();
while (rs.next()) {
}

assertEquals(107, results.size());
Collections.sort(results);
assertEquals("Winston", results.get(106));
```
The employees table on HR should have 107 rows, first names should range from Adam to Winston, however, your result may vary.

Cleanup

Finally (actually, the code is normally performed in the finally block following the try one where the initialization and operation happened - meaning that we would always perform this step, whatever happens there), being done with the data, we can close result set, statement, and connection.
```assertNotNull(rs);
rs.close();

assertNotNull(stmt);
stmt.close();

assertNotNull(conn);
conn.close();
```
Notice that we have to strictly follow the inverted order seen in initialization.
Notice also that each close() method could throw and exception so, again, these calls have to be in a try-catch block.

See full code on GitHub, class GettingStartedTest, method testSelect().

Reference: Oracle Database JDBC Developer's Guide 12c Release 2 (12.2) 2.4 Sample: Connecting, Querying, and Processing the Results

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### Connecting to Oracle via JDBC

There are a couple of ways to connect to a recent Oracle Database, going through the standard DriverManager class or the specific OracleDataSource one. Let's see both of them. And, when we are there, let's add also some consideration on connecting to MySql via DriverManager.

It's a bit of an overkill, but I have written a little abstract class that is going to be the root of a hierarchy of classes for providing access to database through JDBC:
```public abstract class Connector {
public abstract Connection getConnection() throws SQLException;

public String getDatabaseVersion(Connection conn) throws SQLException {
}
}
```
The point of it is that each different concrete class has its own way to get a database connection but all of them would use it in the same way. So the getConnection() method is abstract where the actual method performing JDBC operation, like getDatabaseVersion(), would act in the same way.

Using OracleDataSource

The OracleDataSourceConnector extends Connector, and its raison d'être is keeping an instance of a OracleDataSource object as its private data member. Initialized in the constructor, is used by the the getConnection() method to return a database connection.
```public class OracleDataSourceConnector extends Connector {
private OracleDataSource ods;

public OracleDataSourceConnector(String url, String user, String password) throws SQLException {
ods = new OracleDataSource();
ods.setURL(url);
ods.setUser(user);
}

@Override
public Connection getConnection() throws SQLException {
return ods.getConnection();
}
}
```
The good thing about OracleDataSource is that it has its own pool of connections that is managed implicitly. On the flip side, it is not standard JDBC. That means extra work if we want to adapt our code to use a different database.

I have written a tester to check the functionality, OracleDataSourceConnectorTest. See it on GitHub for full reference. There are only a few things that I want to stress here.
```public class OracleDataSourceConnectorTest {
private static final String URL = "jdbc:oracle:thin:@localhost:1521/orclpdb";  // 1
private static final String USER = "hr";
private static final String PASSWORD = "hr";

private static OracleDataSourceConnector ods;
private static Connection conn;

@BeforeClass
public static void setUp() {  // 2
try {
ods = new OracleDataSourceConnector(URL, USER, PASSWORD);
conn = ods.getConnection();
} catch (SQLException e) {
fail(e.getMessage());
}
}

// ...
}
```
1. I'm using the thin Oracle JDBC driver, the other choice is the OCI one. See the Oracle documentation if you wonder which one to use. Short answer is, usually thin is the one you want to peek. My database is local, on the standard port, and the service is named orclpdb. Your setup may vary.
2. I setup the connector and a connection through this static method called only once before the tests in the class are called. The sense is that I don't want to repeat expensive operations without a reason. So, when nothing is against it, I would reuse connector and connection in more tests.

And here is a test that requires its own connector, because I want to perform a disruptive negative test:
```@Test
OracleDataSourceConnector connector = null;
try {
connector = new OracleDataSourceConnector(URL, "Unknown", PASSWORD);  // 1
} catch (SQLException e) {
fail(e.getMessage());
}

try {
connector.getConnection();  // 2
fail("No connection expected for unknown user");
} catch (SQLException e) {
String expectedState = "72000";
assertEquals(expectedState, e.getSQLState());

int expectedCode = 1017;
assertEquals(expectedCode, e.getErrorCode());
}
}
```
1. I pass a bas user name to the connector. No failure is expected here, since no connection is actually done.
2. I expect the failure to happen here. Oracle should react with a ORA-01017 error code, that is included in the SQL state 72000, SQL execute phase errors.

The other test ensures that I can actually get to the database:
```@Test
public void testGetDatabaseNameVersion() {
try {
String expected = "Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production";
String actual = ods.getDatabaseVersion(conn);
assertEquals(expected, actual);
} catch (SQLException e) {
fail(e.getMessage());
}
}
```
Using DriverManager

If I am not interested in the connection pooling service offered by OracleDataSource, and I prefer to keep as generic as I can, I could use these other solution. In the old days, it required to perform an explicit registration of the JDBC driver:
```Class.forName(klass)
```
Where klass is the actual class name, like "oracle.jdbc.driver.OracleDriver" or "com.mysql.jdbc.Driver".

Now this is done implicitly by the DriverManager, that leads to a very slim connector:
```public class PlainConnector extends Connector {
private String url;
private String user;

public PlainConnector(String url, String user, String password) {
this.url = url;
this.user = user;
}

@Override
public Connection getConnection() throws SQLException {
}
}
```
The constructor just store a copy of the data for connecting to the database, the getConnection() uses them going through DriverManager.

I have written another test case to see that actually the plain connector works as the OracleDataSource one. An then another one to see what I have to change to access with the same class a different database, here MySql.

This test case is accessing a MySql database that I have installed locally and on which I have added a user named "hr", to keep it close to the Oracle one. You can see how minimal are the required changes.

I had to change the URL:
```private static final String URL = "jdbc:mysql://localhost:3306/hr?useSSL=false";
```
Notice the parameter useSLL set to false, to remove the MySql warning "Establishing SSL connection without server's identity verification is not recommended." In this test we can live without id verification.

In case of bad user, I expect now 28000 as SQL state and 1045, access denied, as error code.

And, the database version now should be something like "5.7.19-log"

Ah, right. Remember to put in your application build path the jar for oracle JDBC (currently named ojdbc8.jar) and for MySql (mysql-connector-java-5.1.43-bin.jar) and to have the databases up and running, if you want your tests to succeed.

Reference: Oracle Database JDBC Developer's Guide 12c Release 2

Full Java code on GitHub.

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