Hibernate Entity / Persistence LifeCycle States

As you know that Hibernate works with normal Java objects that your application creates with the new operator. In raw form (without annotations), hibernate will not be able to identify your java classes, but when they are properly annotated with required annotations then hibernate will be able to identify them and then work with them e.g. store in DB, update them etc. These objects can be said to mapped with hibernate.

Given an instance of an object that is mapped to Hibernate, it can be in any one of four different states: transient, persistent, detached or removed.

Transient Object

Transient objects exist in heap memory. Hibernate does not manage transient objects or persist changes to transient objects.

Transient objects are independent of Hibernate

To persist the changes to a transient object, you would have to ask the session to save the transient object to the database, at which point Hibernate assigns the object an identifier and marks the object as being in persistent state.

Persistent Object

Persistent objects exist in the database, and Hibernate manages the persistence for persistent objects.

Persistent objects are maintained by Hibernate

If fields or properties change on a persistent object, Hibernate will keep the database representation up to date when the application marks the changes as to be committed.

Detached Object

Detached objects have a representation in the database, but changes to the object will not be reflected in the database, and vice-versa. This temporary separation of the object and the database is shown in image below.

Detached objects exist in the database but are not maintained by Hibernate

A detached object can be created by closing the session that it was associated with, or by evicting it from the session with a call to the session’s evict() method.

In order to persist changes made to a detached object, the application must reattach it to a valid Hibernate session. A detached instance can be associated with a new Hibernate session when your application calls one of the load, refresh, merge, update(), or save() methods on the new session with a reference to the detached object. After the call, the detached object would be a persistent object managed by the new Hibernate session.

Removed Object

Removed objects are objects that are being managed by Hibernate (persistent objects, in other words) that have been passed to the session’s remove() method. When the application marks the changes held in the session as to be committed, the entries in the database that correspond to removed objects are deleted.

Key Points

1. Newly created POJO object will be in the transient state. Transient object doesn’t represent any row of the database i.e. not associated with any session object. It’s plain simple java object.
2. Persistent object represent one row of the database and always associated with some unique hibernate session. Changes to persistent objects are tracked by hibernate and are saved into database when commit call happen.
3. Detached objects are those who were once persistent in past, and now they are no longer persistent. To persist changes done in detached objects, you must reattach them to hibernate session.
4. Removed objects are persistent objects that have been passed to the session’s remove() method and soon will be deleted as soon as changes held in the session will be committed to database.

Hibernate - How to fix “identifier of an instance altered from X to Y”?

Problem :- Getting the error ” identifier of an instance of com.sample.SampleAltered from 1 to 1″

Solution :- Check whether the java type of the identifier field in the Data Object, the type of the field in the hibernate xml file and the type of the column in the database are compatible types or not.

Explanation :-

This problem will occure only when you have an mismatch of data type between your bean-hibername xml mapping & database table column datatype.

For Example,

class SampleAltered{

long sampleId;
String sampleName;


public int getSampleId(){
….
}

public void setSampleId(int i){
…..
}

}

And this was the hbm file declaration

<class name="com.sample.SampleAltered" table="sample_data">
   <id name=”sampleId” type=”int” column=”sample_id”>
       <generator class=”native” />
   </id>
   <property name=”sampleName” type=”string” column=”sample_name” length=”50″/>
</class>

In above example if you note, class SampleAltered was having sampleId with datatype long & hbm xml file contains the entry for same with datatype int.

Whenever you are trying to do database transaction with above mapping you will get the error of "identifier of an instance altered from X to Y".

To fix this issue you need to change datatype of sampleId either in hbm mapping file or in class.

Java Collections Framework

Collections

A data structure is a collection of data organized in some fashion. The structure not only stores data but also supports operations for accessing and manipulating the data. The java.util package contains one of Java’s most powerful subsystems: The Collections Framework. The Collections Framework is a sophisticated hierarchy of interfaces and classes that provide state-of-the-art technology for managing groups of objects.

So What Do You Do with a Collection?

There are a few basic operations you'll normally use with collections

• Add objects to the collection.
• Remove objects from the collection.
• Find out if an object (or group of objects) is in the collection.
• Retrieve an object from the collection (without removing it).
• Iterate through the collection, looking at each element (object) one after another.
• Key Interfaces and Classes of the Collections Framework 

The collections API begins with a group of interfaces, but also gives you a truckload of concrete classes.

Maps	Sets	Lists	Queues	Utilities
HashMap	HashSet	ArrayList	PriorityQueue	Collections
Hashtable	LinkedHashSet	Vector		Arrays
TreeMap	TreeSet	LinkedList
LinkedHashMap

The interface and class hierarchy for collections

Collections come in four basic flavors:

• Lists Lists of things (classes that implement List). 
• Sets Unique things (classes that implement Set). 
• Maps Things with a unique ID (classes that implement Map). 
• Queues Things arranged by the order in which they are to be processed. 

Ordered When a collection is ordered, it means you can iterate through the collection in a specific (not-random) order.

Sorted A sorted collection means that the order in the collection is determined according to some rule or rules, known as the sort order.

List Interface
A List cares about the index. The one thing that List has that non-lists don't have is a set of methods related to the index. Those key methods include things like get(int index), indexOf(Object o), add(int index, Object obj), and so on. All three List implementations are ordered by index position—a position that you determine either by setting an object at a specific index or by adding it without specifying position, in which case the object is added to the end. The three List implementations are described in the following sections.

1 . ArrayList Think of this as a growable array. It gives you fast iteration and fast random access. To state the obvious: it is an ordered collection (by index), but not sorted. You might want to know that as of version 1.4, ArrayList now implements the new RandomAccess interface—a marker interface (meaning it has no methods) that says, "this list supports fast (generally constant time) random access." Choose this over a LinkedList when you need fast iteration but aren't as likely to be doing a lot of insertion and deletion.

2 . Vector Vector is a holdover from the earliest days of Java; Vector and Hashtable were the two original collections, the rest were added with Java 2 versions 1.2 and 1.4. A Vector is basically the same as an ArrayList, but Vector methods are synchronized for thread safety. You'll normally want to use ArrayList instead of Vector because the synchronized methods add a performance hit you might not need. And if you do need thread safety, there are utility methods in class Collections that can help. Vector is the only class other than ArrayList to implement RandomAccess.

3. LinkedList A LinkedList is ordered by index position, like ArrayList, except that the elements are doubly-linked to one another. This linkage gives you new methods (beyond what you get from the List interface) for adding and removing from the beginning or end, which makes it an easy choice for implementing a stack or queue. Keep in mind that a LinkedList may iterate more slowly than an ArrayList, but it's a good choice when you need fast insertion and deletion. As of Java 5, the LinkedList class has been enhanced to implement the java.util.Queue interface. As such, it now supports the common queue methods: peek(), poll(), and offer().

Set Interface
A Set cares about uniqueness—it doesn't allow duplicates. Your good friend the equals() method determines whether two objects are identical (in which case only one can be in the set). The three Set implementations are described in the following sections.

1. HashSet A HashSet is an unsorted, unordered Set. It uses the hashcode of the object being inserted, so the more efficient your hashCode() implementation the better access performance you'll get. Use this class when you want a collection with no duplicates and you don't care about order when you iterate through it.

2. LinkedHashSet A LinkedHashSet is an ordered version of HashSet that maintains a doubly-linked List across all elements. Use this class instead of HashSet when you care about the iteration order. When you iterate through a HashSet the order is unpredictable, while a LinkedHashSet lets you iterate through the elements in the order in which they were inserted.

3. TreeSet The TreeSet is one of two sorted collections (the other being TreeMap). It uses a Red-Black tree structure (but you knew that), and guarantees that the elements will be in ascending order, according to natural order. Optionally, you can construct a TreeSet with a constructor that lets you give the collection your own rules for what the order should be (rather than relying on the ordering defined by the elements' class) by using a Comparable or Comparator.

Map Interface
A Map cares about unique identifiers. You map a unique key (the ID) to a specific value, where both the key and the value are, of course, objects. You're probably quite familiar with Maps since many languages support data structures that use a key/value or name/value pair. The Map implementations let you do things like search for a value based on the key, ask for a collection of just the values, or ask for a collection of just the keys. Like Sets, Maps rely on the equals() method to determine whether two keys are the same or different.

1. HashMap The HashMap gives you an unsorted, unordered Map. When you need a Map and you don't care about the order (when you iterate through it), then HashMap is the way to go; the other maps add a little more overhead. Where the keys land in the Map is based on the key's hashcode, so, like HashSet, the more efficient your hashCode() implementation, the better access performance you'll get.HashMap allows one null key and multiple null values in a collection.

2. Hashtable Like Vector, Hashtable has existed from prehistoric Java times. For fun, don't forget to note the naming inconsistency: HashMap vs. Hashtable. Where's the capitalization of t? Oh well, you won't be expected to spell it. Anyway, just as Vector is a synchronized counterpart to the sleeker, more modern ArrayList, Hashtable is the synchronized counterpart to HashMap. Remember that you don't synchronize a class, so when we say that Vector and Hashtable are synchronized, we just mean that the key methods of the class are synchronized. Another difference, though, is that while HashMap lets you have null values as well as one null key, a Hashtable doesn't let you have anything that's null.

3. LinkedHashMap Like its Set counterpart, LinkedHashSet, the LinkedHash-Map collection maintains insertion order (or, optionally, access order). Although it will be somewhat slower than HashMap for adding and removing elements, you can expect faster iteration with a LinkedHashMap.

4. TreeMap You can probably guess by now that a TreeMap is a sorted Map. And you already know that by default, this means "sorted by the natural order of the elements." Like TreeSet, TreeMap lets you define a custom sort order (via a Comparable or Comparator) when you construct a TreeMap, that specifies how the elements should be compared to one another when they're being ordered.

Queue Interface
A Queue is designed to hold a list of "to-dos," or things to be processed in some way. Although other orders are possible, queues are typically thought of as FIFO (first-in, first-out). Queues support all of the standard Collection methods and they also add methods to add and subtract elements and review queue elements.

1. PriorityQueue This class is new with Java 5. Since the LinkedList class has been enhanced to implement the Queue interface, basic queues can be handled with a LinkedList. The purpose of a PriorityQueue is to create a "priority-in, priority out" queue as opposed to a typical FIFO queue. A PriorityQueue's elements are ordered either by natural ordering (in which case the elements that are sorted first will be accessed first) or according to a Comparator. In either case, the elements' ordering represents their relative priority.

Basic Features of Main Interfaces

Core collection interfaces are the base to Java Collections framework. They include the interfaces Collection, Set, List, Queue and Map on which all the data structures are built.

1. Collection is the root interface for all the hierarchy (except Map).
2. Set interface unique feature is that it does not accept duplicate elements. That is, no two elements will be the same.
3. SortedSet interface is derived from Set interface and adds one more feature that the elements are arranged in sorted order by default.
4. List interface permits duplicate elements.
5. Queue interface holds elements and returns in FIFO order.
6. Map adds the elements in key/value pairs. Duplicate keys are not allowed, but duplicate values are allowed. One key can map one value only.
7. SortedMap interface is a particular case of Map. Keys are sorted by default.