Monthly Archives: November 2011

Some Oracle and Java interview questions

(Updated 08-31-2013) For those brave hearts venture out the silicon valley/seattle, here is the must-do homework (LeetCode). Chinese version (here and here).

SQL: Inner join and out join

Function, (stored) procedure and package

What is a trigger in Oracle

“…Oracle lets you define procedures called triggers that run implicitly when an INSERT, UPDATE, or DELETE statement is issued against the associated table or, in some cases, against a view, or when database system actions occur. These procedures can be written in PL/SQL or Java and stored in the database, or they can be written as C callouts.

Triggers are similar to stored procedures. A trigger stored in the database can include SQL and PL/SQL or Java statements to run as a unit and can invoke stored procedures. However, procedures and triggers differ in the way that they are invoked. A procedure is explicitly run by a user, application, or trigger. Triggers are implicitly fired by Oracle when a triggering event occurs, no matter which user is connected or which application is being used…”

A shorter version at orafaq forum

Longer version at

7. Compare and contrast TRUNCATE and DELETE for a table.
Both the truncate and delete command have the desired outcome of getting rid of all the rows in a table. The difference between the two is that the truncate command is a DDL operation and just moves the high water mark and produces a now rollback. The delete command, on the other hand, is a DML operation, which will produce a rollback and thus take longer to complete.

8. Give the reasoning behind using an index.
Faster access to data blocks in a table.

11. Give some examples of the types of database contraints you may find in Oracle and indicate their purpose.
A Primary or Unique Key can be used to enforce uniqueness on one or more columns.
A Referential Integrity Contraint can be used to enforce a Foreign Key relationship between two tables.
A Not Null constraint – to ensure a value is entered in a column
A Value Constraint – to check a column value against a specific set of values.

Data Definition Language (DDL) statements are used to define the database structure or schema. Some examples:
CREATE – to create objects in the database
ALTER – alters the structure of the database
DROP – delete objects from the database
TRUNCATE – remove all records from a table, including all spaces allocated for the records are removed
COMMENT – add comments to the data dictionary
RENAME – rename an object

Data Manipulation Language (DML) statements are used for managing data within schema objects. Some examples:
SELECT – retrieve data from the a database
INSERT – insert data into a table
UPDATE – updates existing data within a table
DELETE – deletes all records from a table, the space for the records remain
MERGE – UPSERT operation (insert or update)
CALL – call a PL/SQL or Java subprogram
EXPLAIN PLAN – explain access path to data
LOCK TABLE – control concurrency

1. Describe the difference between a procedure, function and anonymous pl/sql block. Candidate should mention use of DECLARE statement, a function must return a value while a procedure doesn?t have to.

PL/SQL developer vs. Toad

SQL Server Reporting Services (Microsoft); SQL Server Reporting Services (wiki)

Oracle 10g reports (PDF); Oracle Reports Tutorial

LIMS (wiki)

(Updated 08-31-2013) OOPS Interview Questions

Top 20 Core Java Interview questions answers asked in Investment Bank

(Original) 30 Core Java questions (sys-con)

J2EE interview questions (

more j2ee interview questions

Java: hashMap and hashTable

Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.

Difference between Get and Post method on web (link one)

Lazy initialization, dependency injection

3 Things in OO:
data encapsulation, polymorphism, dynamic dispatch (when a method is invoked on an object, the object itself determines what code gets executed by looking up the method at run time in a table associated with the object. This feature distinguishes an object from an abstract data type (or module), which has a fixed (static) implementation of the operations for all instances. It is a programming methodology that gives modular component development while at the same time being very efficient.)

How to use static method? (Include class name)


Throwable interface (exception)

What is reflection

Interface vs. abstract class
non-static method
final: immutable, e.g. String
finalized method


Continue reading

JSF Richfaces best practice: drop down and data scroller

drop down box
Clear a list for selectionOneMenu every time, got burned by this in a “page number” drop down for Richfaces data scroller. And some other places.

Use validator
keep in mind it will overwrite the required = “true”, if the user does not select a valid value from drop down, it will give error message during Save method.

Assign 0 to “–select–” ?
Yes or no. Add 0 means 0 is a selectable option, so if one wants to enforce user to select something using required=”true”, it will pass this check. Thus comes in the validator, in which one can say “0” is not valid.

Page number for data scroller
1) t:saveState if we can not have session scope for dataBean;
2) the drop down tip above;
3) When “adding or deleting rows” and it cause the total number of pages changes, make sure set the correct page number in the “action” (save) methods.