Differences between iBatis and Hibernate.

There are major differences between iBatis and Hibernate but both the solutions work well, given their specific domain. Personally I would suggest you should use iBATIS if:

You want to create your own SQL's and are willing to maintain them.

your environment is driven by relational data model.

you have to work existing and complex schema's.

And simply use Hibernate if:

Your environment is driven by object model and wants generates SQL automatically.

To count there are few differences:

iBATIS is:
Simpler

Faster development time

Flixable

Much smaller in package size

Hibernate:
Generates SQL for you which means you don't spend time on SQL

Provides much more advance cache

Highly scalable

Other difference is that iBATIS makes use of SQL which could be database dependent where as Hibernate makes use of HQL which is relatively independent of databases and it is easier to change db in Hibernate.

Hibernate maps your Java POJO objects to the Database tables where as iBatis maps the ResultSet from JDBC API to your POJO Objets.

If you are using stored procedures, well you can do it in Hibernate but it is little difficult in comparision of iBATIS. As an alternative solution iBATIS maps results sets to objects, so no need to care about table structures. This works very well for stored procedures, works very well for reporting applications, etc

Finally, Hibernate and iBATIS both are open source Object Relational Mapping(ORM) tools available in the industry. Use of each of these tools depends on the context you are using them. Hibernate and iBatis both also have good support from SPRING framework so it should not be a problem to chose one of them.

What are the different types of references in java?

Ans) Java has a more expressive system of reference than most other garbage-collected programming languages, which allows for special behavior for garbage collection. A normal reference in Java is known as a strong reference. The java.lang.ref package defines three other types of references—soft, weak, and phantom references. Each type of reference is designed for a specific use.

A SoftReference can be used to implement a cache. An object that is not reachable by a strong reference (that is, not strongly reachable), but is referenced by a soft reference is called softly reachable. A softly reachable object may be garbage collected at the discretion of the garbage collector. This generally means that softly reachable objects will only be garbage collected when free memory is low, but again, it is at the discretion of the garbage collector. Semantically, a soft reference means "keep this object unless the memory is needed."

A WeakReference is used to implement weak maps. An object that is not strongly or softly reachable, but is referenced by a weak reference is called weakly reachable. A weakly reachable object will be garbage collected during the next collection cycle. This behavior is used in the class java.util.WeakHashMap. A weak map allows the programmer to put key/value pairs in the map and not worry about the objects taking up memory when the key is no longer reachable anywhere else. Another possible application of weak references is the string intern pool. Semantically, a weak reference means "get rid of this object when nothing else references it."

A PhantomReference is used to reference objects that have been marked for garbage collection and have been finalized, but have not yet been reclaimed. An object that is not strongly, softly or weakly reachable, but is referenced by a phantom reference is called phantom reachable. This allows for more flexible cleanup than is possible with the finalization mechanism alone. Semantically, a phantom reference means "this object is no longer needed and has been finalized in preparation for being collected."

When to use abstract class and interface?

Consider a scenario where all Cars will have 4 tyres and other features can be different.
In this case any subclass of Car has to have 4 tyres. This is a case where abstract class will be used and a default implementaion for tyres will be provided.

public abstract class Car{

public abstract String getCarName();

public final int getNoOfTyres(){
   return 4;
}

}
Consider a scenario where Cars can have any number of tyres and other features can also be different. In this case interface will be created.

public interface Car{

public abstract String getCarName();
public abstract int getNoOfTyres();
}

How Semaphore work in java ?

A Counting semaphore with one permit is known as binary semaphore because it has only two state permit available or permit unavailable. Binary semaphore can be used to implement mutual exclusion or critical section where only one thread is allowed to execute. Thread will wait on acquire() until Thread inside critical section release permit by calling release() on semaphore.

here is a simple example of counting semaphore in Java where we are using binary semaphore to provide mutual exclusive access on critical section of code in java:

import java.util.concurrent.Semaphore;

public class SemaphoreTest {

    Semaphore binary = new Semaphore(1);
 
    public static void main(String args[]) {
        final SemaphoreTest test = new SemaphoreTest();
        new Thread(){
            @Override
            public void run(){
              test.mutualExclusion();
            }
        }.start();
     
        new Thread(){
            @Override
            public void run(){
              test.mutualExclusion();
            }
        }.start();
     
    }
 
    private void mutualExclusion() {
        try {
            binary.acquire();

            //mutual exclusive region
            System.out.println(Thread.currentThread().getName() + " inside mutual exclusive region");
            Thread.sleep(1000);

        } catch (InterruptedException i.e.) {
            ie.printStackTrace();
        } finally {
            binary.release();
            System.out.println(Thread.currentThread().getName() + " outside of mutual exclusive region");
        }
    }
 
}

Output:
Thread-0 inside mutual exclusive region
Thread-0 outside of mutual exclusive region
Thread-1 inside mutual exclusive region
Thread-1 outside of mutual exclusive region

Some Scenario where Semaphore can be used:
1) To implement better Database connection pool which will block if no more connection is available instead of failing and handover Connection as soon as its available.

2) To put a bound on collection classes. by using semaphore you can implement bounded collection whose bound is specified by counting semaphore.

That's all on Counting semaphore example in Java. Semaphore is real nice concurrent utility which can greatly simply design and implementation of bounded resource pool. Java 5 has added several useful  concurrent utility and deserve a better attention than casual look.

How Garbage Collection works in Java

before moving ahead let's recall few important points about garbage collection in java:

1) objects are created on heap in Java  irrespective of there scope e.g. local or member variable. while its worth noting that class variables or static members are created in method area of Java memory space and both heap and method area is shared between different thread.
2) Garbage collection is a mechanism provided by Java Virtual Machine to reclaim heap space from objects which are eligible for Garbage collection.
3) Garbage collection relieves java programmer from memory management which is essential part of C++ programming and gives more time to focus on business logic.
4) Garbage Collection in Java is carried by a daemon thread called Garbage Collector.
5) Before removing an object from memory Garbage collection thread invokes finalize () method of that object and gives an opportunity to perform any sort of cleanup required.
6) You as Java programmer can not force Garbage collection in Java; it will only trigger if JVM thinks it needs a garbage collection based on Java heap size.
7) There are methods like System.gc () and Runtime.gc () which is used to send request of Garbage collection to JVM but it’s not guaranteed that garbage collection will happen.
8) If there is no memory space for creating new object in Heap Java Virtual Machine throws OutOfMemoryError or java.lang.OutOfMemoryError heap space
9) J2SE 5(Java 2 Standard Edition) adds a new feature called Ergonomics goal of ergonomics is to provide good performance from the JVM with minimum of 

Database Index architecture or type .

Non-clustered

The data is present in arbitrary order, but the logical ordering is specified by the index. The data rows may be spread throughout the table regardless of the value of the indexed column or expression. The non-clustered index tree contains the index keys in sorted order, with the leaf level of the index containing the pointer to the record (page and the row number in the data page in page-organized engines; row offset in file-organized engines).

In a non-clustered index:

• The physical order of the rows is not the same as the index order.
• Typically created on non-primary key columns used in JOIN, WHERE, and ORDER BY clauses.

There can be more than one non-clustered index on a database table.

[edit]Clustered

Clustering alters the data block into a certain distinct order to match the index, resulting in the row data being stored in order. Therefore, only one clustered index can be created on a given database table. Clustered indices can greatly increase overall speed of retrieval, but usually only where the data is accessed sequentially in the same or reverse order of the clustered index, or when a range of items is selected.

Since the physical records are in this sort order on disk, the next row item in the sequence is immediately before or after the last one, and so fewer data block reads are required. The primary feature of a clustered index is therefore the ordering of the physical data rows in accordance with the index blocks that point to them. Some databases separate the data and index blocks into separate files, others put two completely different data blocks within the same physical file(s). Create an object where the physical order of rows is the same as the index order of the rows and the bottom (leaf) level of clustered index contains the actual data rows .

A Strategy for Defining Immutable Objects

The following rules define a simple strategy for creating immutable objects. Not all classes documented as "immutable" follow these rules. This does not necessarily mean the creators of these classes were sloppy — they may have good reason for believing that instances of their classes never change after construction. However, such strategies require sophisticated analysis and are not for beginners.

1. Don't provide "setter" methods — methods that modify fields or objects referred to by fields.
2. Make all fields final and private.
3. Don't allow subclasses to override methods. The simplest way to do this is to declare the class as final. A more sophisticated approach is to make the constructor privateand construct instances in factory methods.
4. If the instance fields include references to mutable objects, don't allow those objects to be changed:
   • Don't provide methods that modify the mutable objects.
   • Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.