Q: Can u explain the collections hierarchy?
A:
Interface Implementation Historical
Set HashSet TreeSet
Vector
List ArrayList LinkedList
Stack
Hashtable
Map HashMap TreeMap
Properties
Q: What is the List Interface?
List represents an ordered collection of objects, In a List user can specify the position into which an
object must be inserted, and Users can access elements by their index and can also search for objects in
the List.
Q: What are the different implementations of List interface?
A: ArrayList, LinkedList and Vector.
Q: What is the difference between ArrayList and Vector?
A: The underlying implementation of ArrayList and vector is same, the only difference is that all the
methods in Vector is synchronized and hence vector is thread Safe. Moreover, In an Vector we can
specify the amount by which the size of an Vector should increase when it becomes full.
Q: How do you decide when to use ArrayList and When to use LinkedList?
A: In an ArrayList direct access to any element based on its index is very fast ,where as in linked list
the access to elements by index is comparatively slow. In array list Insertion and deletion are very slow
where as in LinkedList it is very fast. So I would use ArrayList where I need fast retrieval by index and
I would use LinkedList when I need to do many insertion and deletion.
Q: Why is insersion and deletion in ArrayList slow compared to LinkedList?
A: ArrayList internally uses and array to store the elements, when that array gets filled by inserting
elements a new array of roughly 1.5 times the size of the original array is created and all the data of old
array is copied to new array. During deletion, all elements present in the array after the deleted
elements have to be moved one step back to fill the space created by deletion. In linked list data is
stored in nodes that have reference to the previous node and the next node so adding element is simple
as creating the node an updating the next pointer on the last node and the previous pointer on the new
node. Deletion in linked list is fast because it involves only updating the next pointer in the node before
the deleted node and updating the previous pointer in the node after the deleted node.
Q: What is an Iterator?
A: Iterator is an Interface whose implementations are returned by the iterator() method in the collection
classes, Iterators are used for iterating through the elements in a collection, It also allows us to remove
elements during the iteration.
Q: How do you traverse through a collection using its Iterator?
A: Iterators have methods hasNext() and next(). Using hasNext() we can find whether the iterator has
more elements and by calling next() we can get the next element.
Q: How do you remove elements during Iteration?
A: Iterator also has a method remove() when remove is called, the current element in the iteration is
deleted.
Q: Why are Iterators returned by ArrayList called Fail Fast?
A: Because, if list is structurally modified at any time after the iterator is created, in any way except
through the iterator's own remove or add methods, the iterator will throw a
ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails
quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in
the future.
Q: What is a Set?
A: A set is a collection of Objects in which duplicates are not allowed, it models the mathematical set.
Q: What are the main Implementations of the Set interface?
A: HashSet, TreeSet, LinkedHashSet and EnumSet?
Q: What is an EnumSet?
A: An EnumSet is a specialized set for use with enum types, all of the elements in the EnumSet type
that is specified, explicitly or implicitly, when the set is created.
Q: What is a HashSet?
A: A HashSet is an Implementation of the SetInterface which internally uses a Hash Table like
datastructure(actually elements are stored as keys in a HashMap) structure. It does not guarantee that
the insersion order will be maintained and order may not remain constant with time.
Q: What is a TreeSet?
A: TreeSet is a Set implementation that keeps the elements in sorted order. The elements are sorted
according to the natural order of elements or by the comparator provided at creation time.
Q: Are HashSet and TreeSet thread safe?
A: Both are not thread safe.
Q: Explain Map Interface?
A: A Map is a Collection of Key Value Pairs, Each Key is mapped to one value, Keys cannot be
duplicated. We can get the value from the map by providing the key.
Q: What is Dictionary Class?
A: Dictionary Class is obsolete now it was similar to Map but it was an Abstract Class.
Q: What Are the Different Collection Views That Maps Provide?
A: Maps Provide Three Collection Views
1) Key Set - allow a map's contents to be viewed as a set of keys.
2) Values Collection - allow a map's contents to be viewed as a set of values.
3) Entry Set - allow a map's contents to be viewed as a set of key-value mappings.
Q: What are the main implementations of Map?
A: HashMap, HashTable, TreeMap and EnumMap.
Q: What is EnumMap?
A: A specialized Map implementation for use with enum type keys. All of the keys in an enum map
must come from a single enum type that is specified, explicitly or implicitly, when the map is created.
Q:What is a HashMap?
A: HashMap is an implementation of HashMap that is based on the hash table data structure.
Q: What is the HashTable Class then?
A: HashTable is also like HashMap, It is implementation of map that is based on the hash table data
structure but the difference is that HashTable is synchronized and does not allow null keys or values.
While HashMap is not synchronized and allows null as key and value.
Q:What is a TreeMap?
A: TreeMap actually implements the SortedMap interface which extends the Map interface. In a
TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's
class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data
structure.
Q: What is a KeySet View?
A: KeySet is a set returned by the keySet() method of the Map Interface, It is a set that contains all the
keys present in the Map.
Q: What is a Values Collection View?
A: Values Collection View is a collection returned by the valueSet() method of the Map Interface, It
contains all the objects present as values in the map.
Q: What is a EntrySet View?
A: Entry Set view is a set that is returned by the entrySet() method in the map and contains Objects of
type Map.Entry each of which has both Key and Value.
29 MySQL interview questions
By admin | September 5, 2006
1. How do you start and stop MySQL on Windows? - net start MySQL, net stop MySQL
2. How do you start MySQL on Linux? - /etc/init.d/mysql start
3. Explain the difference between mysql and mysqli interfaces in PHP? - mysqli is the object-
oriented version of mysql library functions.
4. What’s the default port for MySQL Server? - 3306
5. What does tee command do in MySQL? - tee followed by a filename turns on MySQL
logging to a specified file. It can be stopped by command notee.
6. Can you save your connection settings to a conf file? - Yes, and name it ~/.my.conf. You
might want to change the permissions on the file to 600, so that it’s not readable by others.
7. How do you change a password for an existing user via mysqladmin? - mysqladmin -u root
-p password "newpassword"
8. Use mysqldump to create a copy of the database? - mysqldump -h mysqlhost -u username -p
mydatabasename > dbdump.sql
9. Have you ever used MySQL Administrator and MySQL Query Browser? Describe the
tasks you accomplished with these tools.
10.What are some good ideas regarding user security in MySQL? - There is no user without a
password. There is no user without a user name. There is no user whose Host column contains
% (which here indicates that the user can log in from anywhere in the network or the Internet).
There are as few users as possible (in the ideal case only root) who have unrestricted access.
11.Explain the difference between MyISAM Static and MyISAM Dynamic. - In MyISAM
static all the fields have fixed width. The Dynamic MyISAM table would include fields such as
TEXT, BLOB, etc. to accommodate the data types with various lengths. MyISAM Static would
be easier to restore in case of corruption, since even though you might lose some data, you
know exactly where to look for the beginning of the next record.
12.What does myisamchk do? - It compressed the MyISAM tables, which reduces their disk
usage.
13.Explain advantages of InnoDB over MyISAM? - Row-level locking, transactions, foreign
key constraints and crash recovery.
14.Explain advantages of MyISAM over InnoDB? - Much more conservative approach to disk
space management - each MyISAM table is stored in a separate file, which could be compressed
then with myisamchk if needed. With InnoDB the tables are stored in tablespace, and not much
further optimization is possible. All data except for TEXT and BLOB can occupy 8,000 bytes at
most. No full text indexing is available for InnoDB. TRhe COUNT(*)s execute slower than in
MyISAM due to tablespace complexity.
15.What are HEAP tables in MySQL? - HEAP tables are in-memory. They are usually used for
high-speed temporary storage. No TEXT or BLOB fields are allowed within HEAP tables. You
can only use the comparison operators = and <=>. HEAP tables do not support
AUTO_INCREMENT. Indexes must be NOT NULL.
16.How do you control the max size of a HEAP table? - MySQL config variable
max_heap_table_size.
17.What are CSV tables? - Those are the special tables, data for which is saved into comma-
separated values files. They cannot be indexed.
18.Explain federated tables. - Introduced in MySQL 5.0, federated tables allow access to the
tables located on other databases on other servers.
19.What is SERIAL data type in MySQL? - BIGINT NOT NULL PRIMARY KEY
AUTO_INCREMENT
20.What happens when the column is set to AUTO INCREMENT and you reach the
maximum value for that table? - It stops incrementing. It does not overflow to 0 to prevent
data losses, but further inserts are going to produce an error, since the key has been used
already.
21.Explain the difference between BOOL, TINYINT and BIT. - Prior to MySQL 5.0.3: those
are all synonyms. After MySQL 5.0.3: BIT data type can store 8 bytes of data and should be
used for binary data.
22.Explain the difference between FLOAT, DOUBLE and REAL. - FLOATs store floating
point numbers with 8 place accuracy and take up 4 bytes. DOUBLEs store floating point
numbers with 16 place accuracy and take up 8 bytes. REAL is a synonym of FLOAT for now.
23.If you specify the data type as DECIMAL (5,2), what’s the range of values that can go in
this table? - 999.99 to -99.99. Note that with the negative number the minus sign is considered
one of the digits.
24.What happens if a table has one column defined as TIMESTAMP? - That field gets the
current timestamp whenever the row gets altered.
25.But what if you really want to store the timestamp data, such as the publication date of the
article? - Create two columns of type TIMESTAMP and use the second one for your real data.
26.Explain data type TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE
CURRENT_TIMESTAMP - The column exhibits the same behavior as a single timestamp
column in a table with no other timestamp columns.
27.What does TIMESTAMP ON UPDATE CURRENT_TIMESTAMP data type do? - On
initialization places a zero in that column, on future updates puts the current value of the
timestamp in.
28.Explain TIMESTAMP DEFAULT ‘2006:09:02 17:38:44′ ON UPDATE
CURRENT_TIMESTAMP. - A default value is used on initialization, a current timestamp is
inserted on update of the row.
29.If I created a column with data type VARCHAR(3), what would I expect to see in MySQL
table? - CHAR(3), since MySQL automatically adjusted the data type.
31 more MySQL questions
By admin | September 30, 2006
1. What is DDL, DML and DCL? - If you look at the large variety of SQL commands, they can
be divided into three large subgroups. Data Definition Language deals with database schemas
and descriptions of how the data should reside in the database, therefore language statements
like CREATE TABLE or ALTER TABLE belong to DDL. DML deals with data manipulation,
and therefore includes most common SQL statements such SELECT, INSERT, etc. Data Control
Language includes commands such as GRANT, and mostly concerns with rights, permissions
and other controls of the database system.
2. How do you get the number of rows affected by query? - SELECT COUNT (user_id) FROM
users would only return the number of user_id’s.
3. If the value in the column is repeatable, how do you find out the unique values? - Use
DISTINCT in the query, such as SELECT DISTINCT user_firstname FROM users; You can
also ask for a number of distinct values by saying SELECT COUNT (DISTINCT
user_firstname) FROM users;
4. How do you return the a hundred books starting from 25th? - SELECT book_title FROM
books LIMIT 25, 100. The first number in LIMIT is the offset, the second is the number.
5. You wrote a search engine that should retrieve 10 results at a time, but at the same time
you’d like to know how many rows there’re total. How do you display that to the user? -
SELECT SQL_CALC_FOUND_ROWS page_title FROM web_pages LIMIT 1,10; SELECT
FOUND_ROWS(); The second query (not that COUNT() is never used) will tell you how many
results there’re total, so you can display a phrase "Found 13,450,600 results, displaying 1-10".
Note that FOUND_ROWS does not pay attention to the LIMITs you specified and always
returns the total number of rows affected by query.
6. How would you write a query to select all teams that won either 2, 4, 6 or 8 games? -
SELECT team_name FROM teams WHERE team_won IN (2, 4, 6, 8)
7. How would you select all the users, whose phone number is null? - SELECT user_name
FROM users WHERE ISNULL(user_phonenumber);
8. What does this query mean: SELECT user_name, user_isp FROM users LEFT JOIN isps
USING (user_id) - It’s equivalent to saying SELECT user_name, user_isp FROM users LEFT
JOIN isps WHERE users.user_id=isps.user_id
9. How do you find out which auto increment was assigned on the last insert? - SELECT
LAST_INSERT_ID() will return the last value assigned by the auto_increment function. Note
that you don’t have to specify the table name.
10.What does –i-am-a-dummy flag to do when starting MySQL? - Makes the MySQL engine
refuse UPDATE and DELETE commands where the WHERE clause is not present.
11.On executing the DELETE statement I keep getting the error about foreign key constraint
failing. What do I do? - What it means is that so of the data that you’re trying to delete is still
alive in another table. Like if you have a table for universities and a table for students, which
contains the ID of the university they go to, running a delete on a university table will fail if the
students table still contains people enrolled at that university. Proper way to do it would be to
delete the offending data first, and then delete the university in question. Quick way would
involve running SET foreign_key_checks=0 before the DELETE command, and setting the
parameter back to 1 after the DELETE is done. If your foreign key was formulated with ON
DELETE CASCADE, the data in dependent tables will be removed automatically.
12.When would you use ORDER BY in DELETE statement? - When you’re not deleting by
row ID. Such as in DELETE FROM techinterviews_com_questions ORDER BY timestamp
LIMIT 1. This will delete the most recently posted question in the table
techinterviews_com_questions.
13.How can you see all indexes defined for a table? - SHOW INDEX FROM
techinterviews_questions;
14.How would you change a column from VARCHAR(10) to VARCHAR(50)? - ALTER
TABLE techinterviews_questions CHANGE techinterviews_content
techinterviews_CONTENT VARCHAR(50).
15.How would you delete a column? - ALTER TABLE techinterviews_answers DROP
answer_user_id.
16.How would you change a table to InnoDB? - ALTER TABLE techinterviews_questions
ENGINE innodb;
17.When you create a table, and then run SHOW CREATE TABLE on it, you occasionally
get different results than what you typed in. What does MySQL modify in your newly
created tables? -
1. VARCHARs with length less than 4 become CHARs
2. CHARs with length more than 3 become VARCHARs.
3. NOT NULL gets added to the columns declared as PRIMARY KEYs
4. Default values such as NULL are specified for each column
18.How do I find out all databases starting with ‘tech’ to which I have access to? - SHOW
DATABASES LIKE ‘tech%’;
19.How do you concatenate strings in MySQL? - CONCAT (string1, string2, string3)
20.How do you get a portion of a string? - SELECT SUBSTR(title, 1, 10) from
techinterviews_questions;
21.What’s the difference between CHAR_LENGTH and LENGTH? - The first is, naturally,
the character count. The second is byte count. For the Latin characters the numbers are the
same, but they’re not the same for Unicode and other encodings.
22.How do you convert a string to UTF-8? - SELECT (techinterviews_question USING utf8);
23.What do % and _ mean inside LIKE statement? - % corresponds to 0 or more characters, _
is exactly one character.
24.What does + mean in REGEXP? - At least one character. Appendix G. Regular Expressions
from MySQL manual is worth perusing before the interview.
25.How do you get the month from a timestamp? - SELECT
MONTH(techinterviews_timestamp) from techinterviews_questions;
26.How do you offload the time/date handling to MySQL? - SELECT
DATE_FORMAT(techinterviews_timestamp, ‘%Y-%m-%d’) from techinterviews_questions; A
similar TIME_FORMAT function deals with time.
27.How do you add three minutes to a date? - ADDDATE(techinterviews_publication_date,
INTERVAL 3 MINUTE)
28.What’s the difference between Unix timestamps and MySQL timestamps? - Internally Unix
timestamps are stored as 32-bit integers, while MySQL timestamps are stored in a similar
manner, but represented in readable YYYY-MM-DD HH:MM:SS format.
29.How do you convert between Unix timestamps and MySQL timestamps? -
UNIX_TIMESTAMP converts from MySQL timestamp to Unix timestamp,
FROM_UNIXTIME converts from Unix timestamp to MySQL timestamp.
30.What are ENUMs used for in MySQL? - You can limit the possible values that go into the
table. CREATE TABLE months (month ENUM ‘January’, ‘February’, ‘March’,…); INSERT
months VALUES (’April’);
31.How are ENUMs and SETs represented internally? - As unique integers representing the
powers of two, due to storage optimizations.
