How to sort list in reverse order

To sort the elements of the List in the reverse natural order of the strings, get a reverse Comparator from the Collections class with reverseOrder(). Then, pass the reverse Comparator to the sort() method. 
List list = new ArrayList(); 
Comparator comp = Collections.reverseOrder();
Collections.sort(list, comp) 

What is difference between Arrays and ArrayList?

Arrays are created of fix size whereas ArrayList is of not fix size. It means that once array is declared as : 
int [] intArray= new int[6]; 
intArray[7]   // will give ArraysOutOfBoundException. 
Also the size of array cannot be incremented or decremented. But with arrayList the size is variable.
Once the array is created elements cannot be added or deleted from it. But with ArrayList the elements can be added and deleted at runtime. 
List list = new ArrayList();
list.add(1);
list.add(3);
list.remove(0) // will remove the element from the 1st location.
ArrayList is one dimensional but array can be multidimensional. 
            int[][][] intArray= new int[3][2][1];   // 3 dimensional array 

What is difference between HashMap and HashTable?

Both collections implements Map. Both collections store value as key-value pairs. The key differences between the two are
1. Access to the Hashtable is synchronized on the table while access to the HashMap isn't. You can add it, but it isn't there by default.
2. Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't. If you change the map while iterating, you'll know. • Fail-safe - “if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException”
3. HashMap permits null values and only one null key, while Hashtable doesn't allow key or value as null.

How can Arraylist be synchronized without using Vector?

 Arraylist can be synchronized using: 
• Collection.synchronizedList(List list) 
Other collections can be synchronized: 
• Collection.synchronizedMap(Map map) 
• Collection.synchronizedCollection(Collection c) 

What is difference between ArrayList and vector?

Synchronization - ArrayList is not thread-safe whereas Vector is thread-safe. In Vector class each method like add(), get(int i) is surrounded with a synchronized block and thus making Vector class thread-safe. 
Data growth - Internally, both the ArrayList and Vector hold onto their contents using an Array. When an element is inserted into an ArrayList or a Vector, the object will need to expand its internal array if it runs out of room. A Vector defaults to doubling the size of its array, while the ArrayList increases its array size by 50 percent. 

Oracle PL/SQL WHERE CURRENT OF & FOR UPDATE

WHERE CURRENT OF & FOR UPDATE

The WHERE CURRENT OF clause is used in some UPDATE and DELETE statements.

The WHERE CURRENT OF clause in an UPDATE or DELETE statement states that the most recent row fetched from the table should be updated or deleted. We must declare the cursor with the FOR UPDATE clause to use this feature.

Inside a cursor loop, WHERE CURRENT OF allows the current row to be directly updated.

When the session opens a cursor with the FOR UPDATE clause, all rows in the return set will hold row-level exclusive locks. Other sessions can only query the rows, but they cannot update, delete, or select with FOR UPDATE.

Oracle provides the FOR UPDATE clause in SQL syntax to allow the developer to lock a set of Oracle rows for the duration of a transaction.

The syntax of using the WHERE CURRENT OF clause in UPDATE and DELETE statements follows:

WHERE [CURRENT OF cursor_name | search_condition]

Oracle PL/SQL: SQL ROLLUP

SQL ROLLUP

You can use WITH ROLLUP to generate a summary row for each group.

Example 1 of ROLLUP

Suppose we have the following table - Sales:

EmpId	Yr	Sales
1	2005	12000.00
1	2006	18000.00
1	2007	25000.00
2	2005	15000.00
2	2006	6000.00
3	2006	20000.00
3	2007	24000.00

A simple Group by results in:

SELECT Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY Yr

Yr	Sales
2005	27000.00
2006	44000.00
2007	49000.00

A simple group by with ROLLUP results in:

SELECT Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY Yr WITH ROLLUP

Yr	Sales
2005	27000.00
2006	44000.00
2007	49000.00
NULL	120000.00

Another example of group by with ROLLUP:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY EmpId, Yr WITH ROLLUP

EmpId	Yr	Sales
1	2005	12000.00
1	2006	18000.00
1	2007	25000.00
1	NULL	55000.00
2	2005	15000.00
2	2006	6000.00
2	NULL	21000.00
3	2006	20000.00
3	2007	24000.00
3	NULL	44000.00
NULL	NULL	120000.00

Dataset table row count - OLEDB Data Source VB.Net

The DataSet contains copy of the data we requested through the SQL statement. The DataSet consists of DataTableCollection and their DataRelationCollection. The DataTableCollection contains zero or more DataTable objects. The data inside Table is in the form of Rows and Columns . The following VB.NET source code shows how to find the number of rows in a table that resides in the Dataset from OLEDB Data Source.

Imports System.Data.OleDb
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)

Handles Button1.Click
        Dim connetionString As String
        Dim connection As OleDbConnection
        Dim oledbAdapter As OleDbDataAdapter
        Dim ds As New DataSet
        Dim sql As String

        connetionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=Your mdb filename;"
        sql = "Your SQL Statement Here"

        connection = New OleDbConnection(connetionString)
        Try
            connection.Open()
            oledbAdapter = New OleDbDataAdapter(sql, connection)
            oledbAdapter.Fill(ds, "OLEDB Temp Table")
            oledbAdapter.Dispose()
            connection.Close()

            MsgBox("number of Row(s)   -   " & ds.Tables(0).Rows.Count)

        Catch ex As Exception
            MsgBox("Can not open connection ! ")
        End Try
    End Sub
End Class