What is covariant return type?
A covariant return type lets you override a superclass method with a return type that subtypes the superclass method's return type. So we can use covariant return types to minimize upcasting and downcasting.
class Parent {
Parent foo () {
System.out.println ("Parent foo() called");
return this;
}
}

class Child extends Parent {
Child foo () {
System.out.println ("Child foo() called");
return this;
}
}

class Covariant {
public static void main(String[] args) {
Child c = new Child();
Child c2 = c.foo(); // c2 is Child
Parent c3 = c.foo(); // c3 points to Child
}
}

--What an I/O filter?
An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
--What modifiers can be used with a local inner class?
A local inner class may be final or abstract.
--What is autoboxing ?
Automatic conversion between reference and primitive types.

--How can I investigate the physical structure of a database?
The JDBC view of a database internal structure can be seen in the image below.

* Several database objects (tables, views, procedures etc.) are contained within a Schema.
* Several schema (user namespaces) are contained within a catalog.
* Several catalogs (database partitions; databases) are contained within a DB server (such as Oracle, MS SQL

The DatabaseMetaData interface has methods for discovering all the Catalogs, Schemas, Tables and Stored Procedures in the database server. The methods are pretty intuitive, returning a ResultSet with a single String column; use them as indicated in the code below:

public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we
// use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]",
"[login]", "[passwd]");

// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();

// Get all Catalogs
System.out.println("\nCatalogs are called '" + dbmd.getCatalogTerm()
+ "' in this RDBMS.");
processResultSet(dbmd.getCatalogTerm(), dbmd.getCatalogs());

// Get all Schemas
System.out.println("\nSchemas are called '" + dbmd.getSchemaTerm()
+ "' in this RDBMS.");
processResultSet(dbmd.getSchemaTerm(), dbmd.getSchemas());

// Get all Table-like types
System.out.println("\nAll table types supported in this RDBMS:");
processResultSet("Table type", dbmd.getTableTypes());

// Close the Connection
conn.close();
}
public static void processResultSet(String preamble, ResultSet rs)
throws SQLException
{
// Printout table data
while(rs.next())
{
// Printout
System.out.println(preamble + ": " + rs.getString(1));
}

// Close database resources
rs.close();
}
---How do I find all database stored procedures in a database?
Use the getProcedures method of interface java.sql.DatabaseMetaData to probe the database for stored procedures. The exact usage is described in the code below.

public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]", "[login]", "[passwd]");

// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();

// Get all procedures.
System.out.println("Procedures are called '" + dbmd.getProcedureTerm() +"' in the DBMS.");
ResultSet rs = dbmd.getProcedures(null, null, "%");

// Printout table data
while(rs.next())
{
// Get procedure metadata
String dbProcedureCatalog = rs.getString(1);
String dbProcedureSchema = rs.getString(2);
String dbProcedureName = rs.getString(3);
String dbProcedureRemarks = rs.getString(7);
short dbProcedureType = rs.getShort(8);

// Make result readable for humans
String procReturn = (dbProcedureType == DatabaseMetaData.procedureNoResult ? "No Result" : "Result");

// Printout
System.out.println("Procedure: " + dbProcedureName + ", returns: " + procReturn);
System.out.println(" [Catalog | Schema]: [" + dbProcedureCatalog + " | " + dbProcedureSchema + "]");
System.out.println(" Comments: " + dbProcedureRemarks);
}

// Close database resources
rs.close();
conn.close();
}

How can I investigate the parameters to send into and receive from a database stored procedure?
Use the method getProcedureColumns in interface DatabaseMetaData to probe a stored procedure for metadata. The exact usage is described in the code below.

NOTE! This method can only discover parameter values. For databases where a returning ResultSet is created simply by executing a SELECT statement within a stored procedure (thus not sending the return ResultSet to the java application via a declared parameter), the real return value of the stored procedure cannot be detected. This is a weakness for the JDBC metadata mining which is especially present when handling Transact-SQL databases such as those produced by SyBase and Microsoft.

public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we
// use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver")
;
// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]",
"[login]", "[passwd]");

// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();

// Get all column definitions for procedure "getFoodsEaten" in
// schema "testlogin" and catalog "dbo".
System.out.println("Procedures are called '" + dbmd.getProcedureTerm() +"' in the DBMS.");
ResultSet rs = dbmd.getProcedureColumns("test", "dbo", "getFoodsEaten", "%");

// Printout table data
while(rs.next())
{
// Get procedure metadata
String dbProcedureCatalog = rs.getString(1);
String dbProcedureSchema = rs.getString(2);
String dbProcedureName = rs.getString(3);
String dbColumnName = rs.getString(4);
short dbColumnReturn = rs.getShort(5);
String dbColumnReturnTypeName = rs.getString(7);
int dbColumnPrecision = rs.getInt(8);
int dbColumnByteLength = rs.getInt(9);
short dbColumnScale = rs.getShort(10);
short dbColumnRadix = rs.getShort(11);
String dbColumnRemarks = rs.getString(13);


// Interpret the return type (readable for humans)
String procReturn = null;

switch(dbColumnReturn)
{
case DatabaseMetaData.procedureColumnIn:
procReturn = "In";
break;
case DatabaseMetaData.procedureColumnOut:
procReturn = "Out";
break;
case DatabaseMetaData.procedureColumnInOut:
procReturn = "In/Out";
break;
case DatabaseMetaData.procedureColumnReturn:
procReturn = "return value";
break;
case DatabaseMetaData.procedureColumnResult:
procReturn = "return ResultSet";
default:
procReturn = "Unknown";
}

// Printout
System.out.println("Procedure: " + dbProcedureCatalog + "." + dbProcedureSchema
+ "." + dbProcedureName);
System.out.println(" ColumnName [ColumnType(ColumnPrecision)]: " + dbColumnName
+ " [" + dbColumnReturnTypeName + "(" + dbColumnPrecision + ")]");
System.out.println(" ColumnReturns: " + procReturn + "(" + dbColumnReturnTypeName + ")");
System.out.println(" Radix: " + dbColumnRadix + ", Scale: " + dbColumnScale);
System.out.println(" Remarks: " + dbColumnRemarks);
}

// Close database resources
rs.close();
conn.close();
}

How do I check what table-like database objects (table, view, temporary table, alias) are present in a particular database?
Use java.sql.DatabaseMetaData to probe the database for metadata. Use the getTables method to retrieve information about all database objects (i.e. tables, views, system tables, temporary global or local tables or aliases). The exact usage is described in the code below.

NOTE! Certain JDBC drivers throw IllegalCursorStateExceptions when you try to access fields in the ResultSet in the wrong order (i.e. not consecutively). Thus, you should not change the order in which you retrieve the metadata from the ResultSet.

public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we
// use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]",
"[login]", "[passwd]");

// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();

// Get all dbObjects. Replace the last argument in the getTables
// method with objectCategories below to obtain only database
// tables. (Sending in null retrievs all dbObjects).
String[] objectCategories = {"TABLE"};
ResultSet rs = dbmd.getTables(null, null, "%", null);

// Printout table data
while(rs.next())
{
// Get dbObject metadata
String dbObjectCatalog = rs.getString(1);
String dbObjectSchema = rs.getString(2);
String dbObjectName = rs.getString(3);
String dbObjectType = rs.getString(4);

// Printout
System.out.println("" + dbObjectType + ": " + dbObjectName);
System.out.println(" Catalog: " + dbObjectCatalog);
System.out.println(" Schema: " + dbObjectSchema);
}

// Close database resources
rs.close();
conn.close();
}

--How can I connect from an applet to a database on the server?
There are two ways of connecting to a database on the server side.
1. The hard way. Untrusted applets cannot touch the hard disk of a computer. Thus, your applet cannot use native or other local files (such as JDBC database drivers) on your hard drive. The first alternative solution is to create a digitally signed applet which may use locally installed JDBC drivers, able to connect directly to the database on the server side.
2. The easy way. Untrusted applets may only open a network connection to the server from which they were downloaded. Thus, you must place a database listener (either the database itself, or a middleware server) on the server node from which the applet was downloaded. The applet would open a socket connection to the middleware server, located on the same computer node as the webserver from which the applet was downloaded. The middleware server is used as a mediator, connecting to and extract data from the database.
---Many connections from an Oracle8i pooled connection returns statement closed. I am using import oracle.jdbc.pool.* with thin driver. If I test with many simultaneous connections, I get an SQLException that the statement is closed.
ere is an example of concurrent operation of pooled connections from the OracleConnectionPoolDataSource. There is an executable for kicking off threads, a DataSource, and the workerThread.

The Executable Member
package package6;

/**
* package6.executableTester
*/
public class executableTester {
protected static myConnectionPoolDataSource dataSource = null;
static int i = 0;

/**
* Constructor
*/
public executableTester() throws java.sql.SQLException
{
}

/**
* main
* @param args
*/
public static void main(String[] args) {

try{
dataSource = new myConnectionPoolDataSource();
}
catch ( Exception ex ){
ex.printStackTrace();
}

while ( i++ < 10 ) {
try{
workerClass worker = new workerClass();
worker.setThreadNumber( i );
worker.setConnectionPoolDataSource( dataSource.getConnectionPoolDataSource() );
worker.start();
System.out.println( "Started Thread#"+i );
}
catch ( Exception ex ){
ex.printStackTrace();
}
}
}

}

The DataSource Member

package package6;
import oracle.jdbc.pool.*;

/**
* package6.myConnectionPoolDataSource.
*
*/
public class myConnectionPoolDataSource extends Object {
protected OracleConnectionPoolDataSource ocpds = null;

/**
* Constructor
*/
public myConnectionPoolDataSource() throws java.sql.SQLException {
// Create a OracleConnectionPoolDataSource instance
ocpds = new OracleConnectionPoolDataSource();

// Set connection parameters
ocpds.setURL("jdbc:oracle:oci8:@mydb");
ocpds.setUser("scott");
ocpds.setPassword("tiger");

}

public OracleConnectionPoolDataSource getConnectionPoolDataSource() {
return ocpds;
}

}

The Worker Thread Member

package package6;
import oracle.jdbc.pool.*;
import java.sql.*;
import javax.sql.*;

/**
* package6.workerClass .
*
*/
public class workerClass extends Thread {
protected OracleConnectionPoolDataSource ocpds = null;
protected PooledConnection pc = null;

protected Connection conn = null;

protected int threadNumber = 0;
/**
* Constructor
*/

public workerClass() {
}

public void doWork( ) throws SQLException {

// Create a pooled connection
pc = ocpds.getPooledConnection();

// Get a Logical connection
conn = pc.getConnection();

// Create a Statement
Statement stmt = conn.createStatement ();

// Select the ENAME column from the EMP table
ResultSet rset = stmt.executeQuery ("select ename from emp");

// Iterate through the result and print the employee names
while (rset.next ())
// System.out.println (rset.getString (1));
;

// Close the RseultSet
rset.close();
rset = null;

// Close the Statement
stmt.close();
stmt = null;

// Close the logical connection
conn.close();
conn = null;

// Close the pooled connection
pc.close();
pc = null;

System.out.println( "workerClass.thread#"+threadNumber+" completed..");

}

public void setThreadNumber( int assignment ){
threadNumber = assignment;
}

public void setConnectionPoolDataSource(OracleConnectionPoolDataSource x){
ocpds = x;
}

public void run() {
try{
doWork();
}
catch ( Exception ex ){
ex.printStackTrace();
}
}

}

The OutPut Produced

Started Thread#1
Started Thread#2
Started Thread#3
Started Thread#4
Started Thread#5
Started Thread#6
Started Thread#7
Started Thread#8
Started Thread#9
Started Thread#10
workerClass.thread# 1 completed..
workerClass.thread# 10 completed..
workerClass.thread# 3 completed..
workerClass.thread# 8 completed..
workerClass.thread# 2 completed..
workerClass.thread# 9 completed..
workerClass.thread# 5 completed..
workerClass.thread# 7 completed..
workerClass.thread# 6 completed..
workerClass.thread# 4 completed..

The oracle.jdbc.pool.OracleConnectionCacheImpl class is another subclass of the oracle.jdbc.pool.OracleDataSource which should also be looked over, that is what you really what to use. Here is a similar example that uses the oracle.jdbc.pool.OracleConnectionCacheImpl. The general construct is the same as the first example but note the differences in workerClass1 where some statements have been commented ( basically a clone of workerClass from previous example ).
The Executable Member

package package6;
import java.sql.*;
import javax.sql.*;
import oracle.jdbc.pool.*;

/**
* package6.executableTester2
*
*/
public class executableTester2 {
static int i = 0;
protected static myOracleConnectCache
connectionCache = null;

/**
* Constructor
*/
public executableTester2() throws SQLException
{
}

/**
* main
* @param args
*/
public static void main(String[] args) {
OracleConnectionPoolDataSource dataSource = null;

try{

dataSource = new OracleConnectionPoolDataSource() ;
connectionCache = new myOracleConnectCache( dataSource );

}
catch ( Exception ex ){
ex.printStackTrace();
}

while ( i++ < 10 ) {
try{
workerClass1 worker = new workerClass1();
worker.setThreadNumber( i );
worker.setConnection( connectionCache.getConnection() );
worker.start();
System.out.println( "Started Thread#"+i );
}
catch ( Exception ex ){
ex.printStackTrace();
}
}
}
protected void finalize(){
try{
connectionCache.close();
} catch ( SQLException x) {
x.printStackTrace();
}
this.finalize();
}

}

The ConnectCacheImpl Member

package package6;
import javax.sql.ConnectionPoolDataSource;
import oracle.jdbc.pool.*;
import oracle.jdbc.driver.*;
import java.sql.*;
import java.sql.SQLException;

/**
* package6.myOracleConnectCache
*
*/
public class myOracleConnectCache extends
OracleConnectionCacheImpl {

/**
* Constructor
*/
public myOracleConnectCache( ConnectionPoolDataSource x)
throws SQLException {
initialize();
}

public void initialize() throws SQLException {
setURL("jdbc:oracle:oci8:@myDB");
setUser("scott");
setPassword("tiger");
//
// prefab 2 connection and only grow to 4 , setting these
// to various values will demo the behavior
//clearly, if it is not
// obvious already
//
setMinLimit(2);
setMaxLimit(4);

}

}

The Worker Thread Member

package package6;
import oracle.jdbc.pool.*;
import java.sql.*;
import javax.sql.*;

/**
* package6.workerClass1
*
*/
public class workerClass1 extends Thread {
// protected OracleConnectionPoolDataSource
ocpds = null;
// protected PooledConnection pc = null;

protected Connection conn = null;

protected int threadNumber = 0;
/**
* Constructor
*/

public workerClass1() {
}

public void doWork( ) throws SQLException {

// Create a pooled connection
// pc = ocpds.getPooledConnection();

// Get a Logical connection
// conn = pc.getConnection();

// Create a Statement
Statement stmt = conn.createStatement ();

// Select the ENAME column from the EMP table
ResultSet rset = stmt.executeQuery
("select ename from EMP");

// Iterate through the result
// and print the employee names
while (rset.next ())
// System.out.println (rset.getString (1));
;

// Close the RseultSet
rset.close();
rset = null;

// Close the Statement
stmt.close();
stmt = null;

// Close the logical connection
conn.close();
conn = null;

// Close the pooled connection
// pc.close();
// pc = null;

System.out.println( "workerClass1.thread#
"+threadNumber+" completed..");

}

public void setThreadNumber( int assignment ){
threadNumber = assignment;
}

// public void setConnectionPoolDataSource
(OracleConnectionPoolDataSource x){
// ocpds = x;
// }

public void setConnection( Connection assignment ){
conn = assignment;
}

public void run() {
try{
doWork();
}
catch ( Exception ex ){
ex.printStackTrace();
}
}

}

The OutPut Produced

Started Thread#1
Started Thread#2
workerClass1.thread# 1 completed..
workerClass1.thread# 2 completed..
Started Thread#3
Started Thread#4
Started Thread#5
workerClass1.thread# 5 completed..
workerClass1.thread# 4 completed..
workerClass1.thread# 3 completed..
Started Thread#6
Started Thread#7
Started Thread#8
Started Thread#9
workerClass1.thread# 8 completed..
workerClass1.thread# 9 completed..
workerClass1.thread# 6 completed..
workerClass1.thread# 7 completed..
Started Thread#10
workerClass1.thread# 10 completed..