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Core Java Interview Questions Page3

     

Q 1. How can I get the full path of Explorer.exe ?
Ans :
Generally Java sandbox does not allow to obtain this reference so that system can be access from an applet loaded via the Internet.  

 
Q 2. : How do I limit the scope of a file chooser?

Ans :
Generally FileFilter is used to limit file selection. You could write your own custom filter that only takes files in a specified directory at a particular instance. This does not interrupt users from browsing the file system but ensures that no other files is selected.

 
Q 3. Is it possible to convert a string to an abstract path ?

Ans :
The class java.io.File class contains several methods which create abstract file paths, that can be created on the file system or can be used to test whether the actual file paths exist. Constructor File(String) receives an argument having a complete abstract path name. The constructor File(File, String) takes File and String where argument File contains parent directory reference and the String is a path name under that directory. 

  
Q 4. What's the difference between a data store and a database ?

Ans :
Generally the term data store is used for a storage system such as relational database that may be used to store and retrieve information. For example, the term data store is used in Enterprise Java Beans (EJB) specification allowing application server vendors to implement storage according to their choice. Mostly data stores are implemented in the form of a relational database, but it can take other forms also.

 
Q 5. Why not statically import a database driver class ?

Ans :
The standard import statement makes a class available to the class that imports it so that its public variables and method can be used.  Importing and using specific JDBC driver implementations is done in the same way, provided they are available on the application's classpath. Java SQL package and JDBC scheme are designed that deal with database interfaces in a generic way that is not bound to use any single driver implementation. Multiple drivers can be loaded in a single application.
 
Loading a specific driver and invoking its connect(String, Properties) method directly may be possible, using the static Class.forName(String) method and getting connections through the DriverManager's getConnection() methods is more suitable. This makes it possible to configure application's database driver using external configuration or properties, instead of hard coding the class references. It only requires declaration of Java interface types for the driver, connection, statements, rather than vendor-specific classes.

  
Q 6. How can I insert multiple records at a time ?

Ans :
Use of multiple inserts to a database with a single SQL statement does not make sense from a database management point of view. INSERT statement creates records and these records should be distinct from each other otherwise they will be redundant and inefficient use of storage. An UPDATE statement can update multiple records by using a single statement and it is a more appropriate way of managing the user data. Inserting multiple records by using a PreparedStatement is a better way of adding multiple different records to the database. To add multiple records to the database PreparedStatement uses iterator.

 
Q 7. How can I insert a new row between two others ?

Ans :
Relational database does not required to insert records in a particular sequence. Inserting a record to the database the application usually inserts the records at the end of the table. If listing of data in a particular sequence is important for you application then you should generally use a data field to determine the sort order. Lets take an example, Suppose you want to sort the data by a by a customer's last name and first name by using an SQL ORDER BY statement, then you would use the query:

SELECT * FROM customer ORDER BY last_name, first_name;

Q 8. How can I select records between two dates ?
Ans :
In order to select records by date, a database table must have a DATE or TIMESTAMP field for using in the WHERE condition of the query statement. E.g. For a table having product order you must have a date_ordered field of DATE data type. Secondly, since the SQL query input is submitted in the form of a string therefore the two dates must be type cast to DATE data types.

  
Q 9. How do I get the keys from a Map ?

Ans :
The method keySet() is used to get the keys in a map rather than the values, which returns a java.util.Set object containing all the keys. Then get an Iterator from the key set, or process the keys by using other methods of the Set interface.

 
Q 10. How do I insert slashes in a date ?

Ans :
Java uses java.text.SimpleDateFormat in order to format Date types. This class receives a string having time pattern and applies it to the given date like a template. For instance use the pattern "dd/MM/YYYY" in order to create a date with slashes.

 
Q 11. How can I get the difference between two dates in days ?

Ans :
In order to compare dates by day, deduct the millisecond time of the earlier date from the later one and divide the result by the number of milliseconds in a day. This method checks whether the two dates are same and then applies after(Date) method to check the deduction is the right way round. In order to compare in a better way by hour or minute, checking of any possible timezone offset is also important.

  
Q 12. Why are wait(), notify() and notifyall() methods defined in the Object class ?

Ans :
All of the methods like wait(), notify() and notifyall() are defined in the Object class. These are the necessary methods for all the objects to undergo multithreading. All of these are part of every object created in java  language programming. These methods are discussed in detailed on the Java Software Development Kit JavaDoc page for the Object class.

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Posted on: April 18, 2011

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