Interview questions on Java Collections API
1. What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
2. What is the List interface?
The List interface provides support for ordered collections of objects.
3. What is the Vector class?
The Vector class provides the capability to implement a growable array of objects.
4. What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.
5. Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.
6. What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars
7. What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
8. What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
9. What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.
10. What is the highest-level event class of the event-delegation model?
The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.
11. What is the Collection interface?
The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates.
12. What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
13. What is the typical use of Hashtable?
Whenever a program wants to store a key value pair, one can use Hashtable.
14. I am trying to store an object using a key in a Hashtable. And some other object already exists in that location, then what will happen? The existing object will be overwritten? Or the new object will be stored elsewhere?
The existing object will be overwritten and thus it will be lost.
15. What is the difference between the size and capacity of a Vector?
The size is the number of elements actually stored in the vector, while capacity is the maximum number of elements it can store at a given instance of time.
16. Can a vector contain heterogeneous objects?
Yes a Vector can contain heterogeneous objects. Because a Vector stores everything in terms of Object.
17. Can an ArrayList contain heterogeneous objects?
Yes a ArrayList can contain heterogeneous objects. Because an ArrayList stores everything in terms of Object.
18. What is an enumeration?
An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It is a construct which collection classes return when you request a collection of all the objects stored in the collection. It allows sequential access to all the elements stored in the collection.
19. Considering the basic properties of Vector and ArrayList, where will you use Vector and where will you use ArrayList?
The basic difference between a Vector and an ArrayList is that, vector is synchronized while ArrayList is not. Thus whenever there is a possibility of multiple threads accessing the same instance, one should use Vector. While if not multiple threads are going to access the same instance then use ArrayList. Non synchronized data structure will give better performance than the synchronized one.
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Wednesday, April 9, 2008
Thursday, April 3, 2008
DataBase interview questions - Part 1
Collection of questions on DB and SQL
1. What is SQL?
SQL stands for 'Structured Query Language'.
2. What is SELECT statement?
The SELECT statement lets you select a set of values from a table in a database. The values selected from the database table would depend on the various conditions that are specified in the SQL query.
3. How can you compare a part of the name rather than the entire name?
SELECT * FROM people WHERE empname LIKE '%ab%'
Would return a recordset with records consisting empname the sequence 'ab' in empname .
4. What is the INSERT statement?
The INSERT statement lets you insert information into a database.
5. How do you delete a record from a database?
Use the DELETE statement to remove records or any particular column values from a database.
6.How could I get distinct entries from a table?
The SELECT statement in conjunction with DISTINCT lets you select a set of distinct values from a table in a database. The values selected from the database table would of course depend on the various conditions that are specified in the SQL query. Example
SELECT DISTINCT empname FROM emptable
7.How to get the results of a Query sorted in any order?
A:You can sort the results and return the sorted results to your program by using ORDER BY keyword thus saving you the pain of carrying out the sorting yourself. The ORDER BY keyword is used for sorting.
SELECT empname, age, city FROM emptable ORDER BY empname
8. How can I find the total number of records in a table?
You could use the COUNT keyword , example
SELECT COUNT(*) FROM emp WHERE age>40
9. What is GROUP BY?
A:The GROUP BY keywords have been added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called. Without the GROUP BY functionality, finding the sum for each individual group of column values was not possible.
10.What is the difference among "dropping a table", "truncating a table" and "deleting all records" from a table.
Dropping : (Table structure + Data are deleted), Invalidates the dependent objects ,Drops the indexes
Truncating: (Data alone deleted), Performs an automatic commit, Faster than delete
Delete : (Data alone deleted), Doesn’t perform automatic commit
11. What are the Large object types suported by Oracle?
Blob and Clob.
12.Difference between a "where" clause and a "having" clause.
Having clause is used only with group functions whereas Where is not used with.
13. What's the difference between a primary key and a unique key?
Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.
14. What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?
Cursors allow row-by-row prcessing of the resultsets.
Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information.
Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one rowundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Furthere, there are restrictions on the SELECT statements that can be used with some types of cursors.
Most of the times, set based operations can be used instead of cursors.
15. What are triggers? How to invoke a trigger on demand?
Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table.
Triggers can't be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined.
Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster.
16. What is a join and explain different types of joins.
Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table.
Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.
17. What is a self join?
Self join is just like any other join, except that two instances of the same table will be joined in the query.
1. What is SQL?
SQL stands for 'Structured Query Language'.
2. What is SELECT statement?
The SELECT statement lets you select a set of values from a table in a database. The values selected from the database table would depend on the various conditions that are specified in the SQL query.
3. How can you compare a part of the name rather than the entire name?
SELECT * FROM people WHERE empname LIKE '%ab%'
Would return a recordset with records consisting empname the sequence 'ab' in empname .
4. What is the INSERT statement?
The INSERT statement lets you insert information into a database.
5. How do you delete a record from a database?
Use the DELETE statement to remove records or any particular column values from a database.
6.How could I get distinct entries from a table?
The SELECT statement in conjunction with DISTINCT lets you select a set of distinct values from a table in a database. The values selected from the database table would of course depend on the various conditions that are specified in the SQL query. Example
SELECT DISTINCT empname FROM emptable
7.How to get the results of a Query sorted in any order?
A:You can sort the results and return the sorted results to your program by using ORDER BY keyword thus saving you the pain of carrying out the sorting yourself. The ORDER BY keyword is used for sorting.
SELECT empname, age, city FROM emptable ORDER BY empname
8. How can I find the total number of records in a table?
You could use the COUNT keyword , example
SELECT COUNT(*) FROM emp WHERE age>40
9. What is GROUP BY?
A:The GROUP BY keywords have been added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called. Without the GROUP BY functionality, finding the sum for each individual group of column values was not possible.
10.What is the difference among "dropping a table", "truncating a table" and "deleting all records" from a table.
Dropping : (Table structure + Data are deleted), Invalidates the dependent objects ,Drops the indexes
Truncating: (Data alone deleted), Performs an automatic commit, Faster than delete
Delete : (Data alone deleted), Doesn’t perform automatic commit
11. What are the Large object types suported by Oracle?
Blob and Clob.
12.Difference between a "where" clause and a "having" clause.
Having clause is used only with group functions whereas Where is not used with.
13. What's the difference between a primary key and a unique key?
Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.
14. What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?
Cursors allow row-by-row prcessing of the resultsets.
Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information.
Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one rowundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Furthere, there are restrictions on the SELECT statements that can be used with some types of cursors.
Most of the times, set based operations can be used instead of cursors.
15. What are triggers? How to invoke a trigger on demand?
Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table.
Triggers can't be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined.
Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster.
16. What is a join and explain different types of joins.
Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table.
Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.
17. What is a self join?
Self join is just like any other join, except that two instances of the same table will be joined in the query.
Wednesday, April 2, 2008
JDBC interview questions - Part 1
Here is a compilation of some frequently asked JDBC questions in job interviews
1. What is JDBC?
Ans : JDBC stands for Java Database Connectivity. It is an API which provides easy connection to a wide range of databases. To connect to a database we need to load the appropriate driver and then request for a connection object.
The Class.forName(….) will load the driver and register it with the DriverManager
2. What is a Transaction?
Ans: A transaction is a set of operations that should be completed as a unit. If one operation fails then all the other operations fail as well.
3. What are the steps in the JDBC connection?
Ans : While making a JDBC connection we go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
4. What is the difference between statements and prepared statements?
Ans : Prepared statements offer better performance, as they are pre-compiled. Prepared statements reuse the same execution plan for different arguments rather than creating a new execution plan every time. Prepared statements use bind arguments, which are sent to the database engine. This allows mapping different
requests with same prepared statement but different arguments to execute the same execution plan.
5. Which are te types of JDBC drivers ?
Ans: a. JDBC-ODBC Bridge Driver (Type-1 driver)
b. Native API Partly Java Driver (Type-2 driver)
c. Net protocol pure Java Driver (Type-3 driver)
d. Native protocol Pure Java Driver (Type-4 driver)
6.What are the types of resultsets in JDBC3.0? How you can retrieve information of resultset?
Ans: ScrollableResultSet and ResultSet. We can retrieve information of resultset by using java.sql.ResultSetMetaData interface. You can get the instance by calling the method getMetaData() on ResulSet object.
7. How can we store java objects in database ?
Ans: We can store java objects by using setObject(), setBlob() and setClob() methods in PreparedStatement.
8. What do you mean by isolation level?
Ans: Isolation means that the business logic can proceed without consideration for the other activities of the system.
9. List some exceptions thrown by JDBC
A. BatchUpdateException , DataTruncation , SQLException , SQLWarning
10. In which interface the methods commit() & rollback() are defined ?
Ans: java.sql.Connection interface
11. How to binary data is stored in the database ?
Ans: The binary data such as images are stored as binary streams
- getBinaryStream(), setBinaryStream()). It will not be visible in database, it is stored in form of bytes, to make it visible we have to use any one frontend tool.
12. How to check null value in JDBC?
A. By using the method wasNull() in ResultSet ,it returns boolean value. Returns whether the last column read had a value of SQL NULL. Note that you must first call one of the getXXX methods on a column to try to read its value and then call the method wasNull to see if the value read was SQL NULL.
1. What is JDBC?
Ans : JDBC stands for Java Database Connectivity. It is an API which provides easy connection to a wide range of databases. To connect to a database we need to load the appropriate driver and then request for a connection object.
The Class.forName(….) will load the driver and register it with the DriverManager
2. What is a Transaction?
Ans: A transaction is a set of operations that should be completed as a unit. If one operation fails then all the other operations fail as well.
3. What are the steps in the JDBC connection?
Ans : While making a JDBC connection we go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
4. What is the difference between statements and prepared statements?
Ans : Prepared statements offer better performance, as they are pre-compiled. Prepared statements reuse the same execution plan for different arguments rather than creating a new execution plan every time. Prepared statements use bind arguments, which are sent to the database engine. This allows mapping different
requests with same prepared statement but different arguments to execute the same execution plan.
5. Which are te types of JDBC drivers ?
Ans: a. JDBC-ODBC Bridge Driver (Type-1 driver)
b. Native API Partly Java Driver (Type-2 driver)
c. Net protocol pure Java Driver (Type-3 driver)
d. Native protocol Pure Java Driver (Type-4 driver)
6.What are the types of resultsets in JDBC3.0? How you can retrieve information of resultset?
Ans: ScrollableResultSet and ResultSet. We can retrieve information of resultset by using java.sql.ResultSetMetaData interface. You can get the instance by calling the method getMetaData() on ResulSet object.
7. How can we store java objects in database ?
Ans: We can store java objects by using setObject(), setBlob() and setClob() methods in PreparedStatement.
8. What do you mean by isolation level?
Ans: Isolation means that the business logic can proceed without consideration for the other activities of the system.
9. List some exceptions thrown by JDBC
A. BatchUpdateException , DataTruncation , SQLException , SQLWarning
10. In which interface the methods commit() & rollback() are defined ?
Ans: java.sql.Connection interface
11. How to binary data is stored in the database ?
Ans: The binary data such as images are stored as binary streams
- getBinaryStream(), setBinaryStream()). It will not be visible in database, it is stored in form of bytes, to make it visible we have to use any one frontend tool.
12. How to check null value in JDBC?
A. By using the method wasNull() in ResultSet ,it returns boolean value. Returns whether the last column read had a value of SQL NULL. Note that you must first call one of the getXXX methods on a column to try to read its value and then call the method wasNull to see if the value read was SQL NULL.
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