I also see other terms as well: Entity Object, Value Object, etc. Are there other terms out there that I should know, and what do these terms refer to?
Can the differences between them, if any, be identified by reading code?
A class is a template for creating objects. Not all OO languages use classes (see Self, Javascript). Typically classes are implemented as objects.
An object is a bundle of data that is packaged with functions that act on that data (called methods). Calling a class's constructor allocates memory for the object and initializes its member variables.
An entity is an object that represents something that has an identity that the system is interested in tracking. Typical examples are Customers and Accounts.
A value object is a value, it doesn't have an identity, and two instances with the same value are considered to be identical. Typical examples are monetary amounts, locations, payment types.
A data transfer object is used for passing a bunch of data around. Typically they're used in distributed systems to send data as a bundle in order to avoid repeated network calls. Data transfer objects have no identity (or there is no expectation they should have any), they are just containers for data.
Generally you can tell the difference between entities and value objects because entities have a recognizable identity, and the system is concerned with creating them, storing them, and changing them. In cases where objects map to some database, entities have primary keys that are either some kind of composite natural key or an artificial key, while value objects are compared by value.
In general a class is a construct which defines a set of properties and methods/functions while an Object is the actual instance of a class which is created at runtime.
Sample class definition:
public class Example{
...
}
The following will create an instance of the Example class as an Object at runtime;
new Example();
Class defines an entity, while an object is the actualentity
Class is a conceptual modelthat defines allthecharacteristics and actions required of an object, whilean object is a real model
Class is a prototype of an object
All objects belonging to the same class have the samecharacteristics and actions
Related
Have seen some similar questions:
What is the difference between a JavaBean and a POJO?
What is the Difference Between POJO (Plain Old Java Object) and DTO (Data Transfer Object)?
Can you also please tell me the contexts in which they are used? Or the purpose of them?
JavaBeans
A JavaBean is a class that follows the JavaBeans conventions as defined by Sun. Wikipedia has a pretty good summary of what JavaBeans are:
JavaBeans are reusable software components for Java that can be manipulated visually in a builder tool. Practically, they are classes written in the Java programming language conforming to a particular convention. They are used to encapsulate many objects into a single object (the bean), so that they can be passed around as a single bean object instead of as multiple individual objects. A JavaBean is a Java Object that is serializable, has a nullary constructor, and allows access to properties using getter and setter methods.
In order to function as a JavaBean class, an object class must obey certain conventions about method naming, construction, and behavior. These conventions make it possible to have tools that can use, reuse, replace, and connect JavaBeans.
The required conventions are:
The class must have a public default constructor. This allows easy instantiation within editing and activation frameworks.
The class properties must be accessible using get, set, and other methods (so-called accessor methods and mutator methods), following a standard naming convention. This allows easy automated inspection and updating of bean state within frameworks, many of which include custom editors for various types of properties.
The class should be serializable. This allows applications and frameworks to reliably save, store, and restore the bean's state in a fashion that is independent of the VM and platform.
Because these requirements are largely expressed as conventions rather than by implementing interfaces, some developers view JavaBeans as Plain Old Java Objects that follow specific naming conventions.
POJO
A Plain Old Java Object or POJO is a term initially introduced to designate a simple lightweight Java object, not implementing any javax.ejb interface, as opposed to heavyweight EJB 2.x (especially Entity Beans, Stateless Session Beans are not that bad IMO). Today, the term is used for any simple object with no extra stuff. Again, Wikipedia does a good job at defining POJO:
POJO is an acronym for Plain Old Java
Object. The name is used to emphasize
that the object in question is an
ordinary Java Object, not a special
object, and in particular not an
Enterprise JavaBean (especially before
EJB 3). The term was coined by Martin
Fowler, Rebecca Parsons and Josh
MacKenzie in September 2000:
"We wondered why people were so against using regular objects in their
systems and concluded that it was
because simple objects lacked a fancy
name. So we gave them one, and it's
caught on very nicely."
The term continues the pattern of
older terms for technologies that do
not use fancy new features, such as
POTS (Plain Old Telephone Service) in
telephony, and PODS (Plain Old Data
Structures) that are defined in C++
but use only C language features, and
POD (Plain Old Documentation) in Perl.
The term has most likely gained
widespread acceptance because of the
need for a common and easily
understood term that contrasts with
complicated object frameworks. A
JavaBean is a POJO that is
serializable, has a no-argument
constructor, and allows access to
properties using getter and setter
methods. An Enterprise JavaBean is not
a single class but an entire component
model (again, EJB 3 reduces the
complexity of Enterprise JavaBeans).
As designs using POJOs have become
more commonly-used, systems have
arisen that give POJOs some of the
functionality used in frameworks and
more choice about which areas of
functionality are actually needed.
Hibernate and Spring are examples.
Value Object
A Value Object or VO is an object such as java.lang.Integer that hold values (hence value objects). For a more formal definition, I often refer to Martin Fowler's description of Value Object:
In Patterns of Enterprise Application Architecture I described Value Object as a small object such as a Money or date range object. Their key property is that they follow value semantics rather than reference semantics.
You can usually tell them because their notion of equality isn't based on identity, instead two value objects are equal if all their fields are equal. Although all fields are equal, you don't need to compare all fields if a subset is unique - for example currency codes for currency objects are enough to test equality.
A general heuristic is that value objects should be entirely immutable. If you want to change a value object you should replace the object with a new one and not be allowed to update the values of the value object itself - updatable value objects lead to aliasing problems.
Early J2EE literature used the term value object to describe a different notion, what I call a Data Transfer Object. They have since changed their usage and use the term Transfer Object instead.
You can find some more good material on value objects on the wiki and by Dirk Riehle.
Data Transfer Object
Data Transfer Object or DTO is a (anti) pattern introduced with EJB. Instead of performing many remote calls on EJBs, the idea was to encapsulate data in a value object that could be transfered over the network: a Data Transfer Object. Wikipedia has a decent definition of Data Transfer Object:
Data transfer object (DTO), formerly known as value objects or VO, is a design pattern used to transfer data between software application subsystems. DTOs are often used in conjunction with data access objects to retrieve data from a database.
The difference between data transfer objects and business objects or data access objects is that a DTO does not have any behaviour except for storage and retrieval of its own data (accessors and mutators).
In a traditional EJB architecture, DTOs serve dual purposes: first, they work around the problem that entity beans are not serializable; second, they implicitly define an assembly phase where all data to be used by the view is fetched and marshalled into the DTOs before returning control to the presentation tier.
So, for many people, DTOs and VOs are the same thing (but Fowler uses VOs to mean something else as we saw). Most of time, they follow the JavaBeans conventions and are thus JavaBeans too. And all are POJOs.
DTO vs VO
DTO - Data transfer objects are just data containers which are used to transport data between layers and tiers.
It mainly contains attributes. You can even use public attributes without getters and setters.
Data transfer objects do not contain any business logic.
Analogy: Simple Registration form with attributes username,
password and email id.
When this form is submitted in RegistrationServlet file you will get all the attributes from view layer to business layer where you pass
the attributes to java beans and then to the DAO or the persistence layer.
DTO's helps in transporting the attributes from view layer to business layer and finally to the persistence layer.
DTO was mainly used to get data transported across the network efficiently, it may be even from JVM to another JVM.
DTOs are often java.io.Serializable - in order to transfer data across JVM.
VO - A Value Object [1][2] represents itself a fixed set of data and is similar to a Java enum. A Value Object's identity is based on their state rather than on their object identity and is immutable. A real world example would be Color.RED, Color.BLUE, SEX.FEMALE etc.
POJO vs JavaBeans
[1]
The Java-Beanness of a POJO is that its private attributes are all accessed via public getters and setters that conform to the JavaBeans conventions. e.g.
private String foo;
public String getFoo(){...}
public void setFoo(String foo){...};
[2]
JavaBeans must implement Serializable and have a no-argument constructor, whereas in POJO does not have these restrictions.
Basically,
DTO: "Data transfer objects " can travel between seperate layers in software architecture.
VO: "Value objects " hold a object such as Integer,Money etc.
POJO: Plain Old Java Object which is not a special object.
Java Beans: requires a Java Class to be serializable, have a no-arg constructor and a getter and setter for each field
Java Beans are not the same thing as EJBs.
The JavaBeans specification in Java 1.0 was Sun's attempt to allow Java objects to be manipulated in an IDE that looked like VB. There were rules laid down for objects that qualified as "Java Beans":
Default constructor
Getters and setters for private data members that followed the proper naming convention
Serializable
Maybe others that I'm forgetting.
EJBs came later. They combine distributed components and a transactional model, running in a container that manages threads, pooling, life cycle, and provides services. They are a far cry from Java Beans.
DTOs came about in the Java context because people found out that the EJB 1.0 spec was too "chatty" with the database. Rather than make a roundtrip for every data element, people would package them into Java Beans in bulk and ship them around.
POJOs were a reaction against EJBs.
POJO :
It is a java file(class) which doesn't extend or implement any other java file(class).
Bean:
It is a java file(class) in which all variables are private, methods are public and appropriate getters and setters are used for accessing variables.
Normal class:
It is a java file(class) which may consist of public/private/default/protected variables and which may or may not extend or implement another java file(class).
Value Object : Use when need to measure the objects' equality based on the objects' value.
Data Transfer Object : Pass data with multiple attributes in one shot from client to server across layer, to avoid multiple calls to remote server.
Plain Old Java Object : It's like simple class which properties, public no-arg constructor. As we declare for JPA entity.
difference-between-value-object-pattern-and-data-transfer-pattern
First Talk About
Normal Class - that's mean any class define that's a normally in java it's means you create different type of method properties etc.
Bean - Bean is nothing it's only a object of that particular class using this bean you can access your java class same as object..
and after that talk about last one POJO
POJO - POJO is that class which have no any services it's have only a default constructor and private property and those property for setting a value corresponding setter and getter methods.
It's short form of Plain Java Object.
I have a Java application where the domain layer is decoupled from the UI by controllers. The problem is that these controllers can return domain objects, and can have domain objects as parameters.
Some of these returned domain objects are mutable, and this is what I want to prevent. I want it impossible for the UI (or future UI's) to directly modify the domain without accessing the controllers.
I tried two options:
In the first one I made sure that each class implemented an 'Unmodifiable' interface which contained only getters. And if I needed to return an object to the UI, I returned its 'Unmodifiable' interface. So that the UI could only view the getters.
The problem with this that they still can be cast easily to the original object and access is gained. At first I thought that this level of security was good enough, but it happend that someone accidentally casted some objects and used them in an incorrect way, and integrity was breached.
In the second one I tried to provide unmodifiable wrappers for each object that could be returned. But the problem is that these returned objects can be used as parameters for methods in the controllers and so they needed to be unwrapped in the controllers. I tried to make the uwrap() methode package-private, but then I have to put every specific wrapper class in the same package with the controllers and this is a bit inconvenient.
EDIT: 3th option:
(With thanks to vic) In the third option the object is wrapped by a unmodifiable wrapper, but can't be unwrapped by this wrapper. Each Unmodifiable is linked to its modifiable object in a Hashmap. So 'unwrapping' is done by getting the modifiable object that is linked to the unmodifiable object.
Does anyone know or has some ideas on how to make objects unmodifiable so they can be returned by a controller, and make it possible to make them modifiable again when they are passed back to the controllers?
What if the controllers store privately the mutable versions of the objects and return immutable versions of it to the "outer world". The mutable versions will have some kind of unique id, and the controllers will be able to unwrap it by looking up in a Collection of some sort.
Go for the State design pattern which is an excellent choice to represent an object's state.
Basically, whenever the UI wants to display a domain object, the controller gives it an immutable DTO object representing a snapshot of the object itself. This will restrict the UI layer to have only immutable snapshots of your domain objects. When the UI wants to make changes to your domain objects, it sends immutable state objects to the controller which use them to internally modify the corresponding domain objects.
Is a Data Transfer Object the same as a Value Object or are they different? If they are different then where should we use a DTO and where should we use a VO?
The programming language we are talking about is Java and the context is - there is a web application, which fetches data from a database and then processes it and ultimately the processed information is displayed on the front-end.
A value object is a simple object whose equality isn't based on identity.
A data transfer object is an object used to transfer data between software application subsystems, usually between business layers and UI. It is focused just on plain data, so it doesn't have any behaviour.
A Data Transfer Object is a kludge for moving a bunch of data from one layer or tier to another, the goal is to minimize the number of calls back and forth by packing a bunch of stuff into the same data structure and sending it together. Some people also use it, like Michael points out in his post here, so that the classes used by one layer are not exposed to the layer calling it. When I refer to DTO as a kludge, I mean there's not a precise abstract concept getting implemented, it's a practical workaround for helping with communication between application layers.
A Value Object is something where we're only interested in its value, like a monetary amount, a date range, or a code from a lookup table. It does not have an identity, meaning you would not be concerned, if you had several of them, of keeping track of which is which, because they are not things in themselves.
Contrast Value Objects to things that do have a unique identity in your system, which are called Entities. If you have a system where it tracks a customer making a payment, the customer and the payment are entities, because they represent specific things, but the monetary amount on the payment is just a value, it doesn't have an existence by itself, as far as your system is concerned. How something relates to your system determines if it is a Value Object or an Entity.
use a DTO at the boundary of your services if you don't want to send the actual domain object to the service's clients - this helps reduce dependencies between the client and service.
values objects are simply objects whose equality isn't based on identity e.g. java.lang.Integer
DTOs and value objects aren't really alternatives to each other.
They are different, but I've even used the two interchangeably in the past, which is wrong. I read that DTO (Data Transfer Object) was called a VO ( Value Object) in the first edition of the Core J2EE Patterns book, but wasn't able to find that reference.
A DTO, which I've sometimes called a Dumb Transfer Object to help me remember it's a container and shouldn't have any business logic is used to transport data between layers and tiers. It just should be an object with attributes that has getters/setters.
A VO however is similar to a JAVA Enum and represents a fixed set of data. A VO doesn't have object identity (the address of the object instance in memory), it is identified by its value and is immutable.
Martin Fowler, talking about Data Transfer Objects (DTOs):
Many people in the Sun community use the term "Value Object" for this pattern. I use it to mean something else.
So the term "Value Object" has been used to mean DTO, but as of him (and the other posters), its use as a DTO seems discouraged.
Good detailed answer in Matthias Noback article Is it a DTO or a Value Object?
In short a DTO:
Declares and enforces a schema for data: names and types
Offers no guarantees about correctness of values
A value object:
Wraps one or more values or value objects
Provides evidence of the correctness of these values
Maybe because of lack of experience, but I would put it this way: It's the matter of scope.
DTO has word transfer in it so it means some parts of the system will communicate using it.
Value object has smaller scope, you will pass set of data in value object instead in array from one service to the other.
As much as I understood niether of them is "object whose equality isn't based on identity".
We can use a class object for transfering data to another class. what is the speciality of data transfer objects? How to create them ? Is it just like class objects?
The primary difference is that DTOs, by design, don't have any business logic in them. They're just data structures.
For instance: You might have a database in which you store "users", and if using DTOs, you might use a UserBean to store and retrieve user objects. But your business logic may have a User object (possibly derived from the bean, more likely using the bean via aggregation) that not only has the data, but additional methods for things that User can do.
I believe this should be true:
assertTrue(POJO == DTO)
The only special thing about DTO is that they should not contain any behavior.
1, The class object maybe contains too many references to other objects, thus too big to be serialized to transfer. DTO only selects the interesting parts, this can be a performance gain.
2, In Hibernate, the entity object may contain lazy-initialized references, these objects need session context to do initialization. These entity objects looks like "smart objects", DTO here convert these "smart objects" to "plain objects", because transfer "smart objects" is meaningless when the session context is no more existed.
Personally, I don't like DTO, it introduce another layer of redundant, but sometime (especially when working with Hibernate ORM) I can't live without it.
A DTO class is an ordinary Java class with a just special meaning - just like a Observer, a Factory or a Model. The name is coming from a core J2EE design pattern (the Transfer Object pattern) and the pattern proposes a common way to transfer information between a database and java-classes-based model.
In brief, a DTO is a java class where the class name maps to a database table name and each database column maps to a class attribute. Then it contains getter and setter methods.
Here is one explanation of the (Data) Transfer Object pattern.
When we are deserializing an object, its very difficult to understand that, how it is retriving the object in some certain state? Does it contain any Meta data of the object?
When an object is serialized, the object's class is written to the stream along with the contents of the object's non-transient fields. The deserializer will attempt to load that class (and there are several mechanisms for it to do that), then populate the non-transient fields.
The protocol spec is here: http://java.sun.com/javase/6/docs/platform/serialization/spec/protocol.html
If by "metadata" you're referring to annotations on the class, then no, they are not serialized with the object itself, but are available on the class. If you mean something else, please describe what you mean.
At a high level, the serialization stream contains the data inside the object and the name of the classes involved, as well as a version number to ensure the class didn't change. It uses that information to make a new instance of an object and fills it with the same data as the old instance. It does this avoiding all of the usual constraints on object creation (the need to call constructors, for example).
One confusing point people have is that they can think the class definition itself is serialized. It is not, just the data it contains with enough information to know which objects to recreate when deserilalized. When the object is deserialized, it has to match the existing class on the class path, the serialization binary data does not contain the class.