I have the following two classes. Can I say the first one is a POJO class and the second one as a Bean class?
1) POJO class, since it has only getter and setter method, and all the member are declared as private
public class POJO {
private int id;
private String name;
public int getId() {
return id;
}
public String getName() {
return name;
}
public void setId() {
this.id = id;
}
public void setName() {
this.name = name;
}
}
2) Bean class - all the member variables are private, has getters and setters and implements Serializable interface
public class Bean implements java.io.Serializable {
private String name;
private Integer age;
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public Integer getAge() {
return this.age;
}
public void setAge(Integer age) {
this.age = age;
}
}
It also has a no-arg constructor.
Only difference is bean can be serialized.
From Java docs - http://docs.oracle.com/javase/7/docs/api/java/io/Serializable.html
Serializability of a class is enabled by the class implementing the java.io.Serializable interface. Classes that do not implement this interface will not have any of their state serialized or deserialized. All subtypes of a serializable class are themselves serializable. The serialization interface has no methods or fields and serves only to identify the semantics of being serializable.
the JavaBean class must implement either Serializable or Externalizable, must have a no-arg constructor,all JavaBean properties must public setter and getter methods (as appropriate)
all JavaBean instance variables should be private
Related
I have two classes that implements from two interfaces.
Here is my interfaces:
interface Identifiable {
int getId();
}
interface Greetable {
String helloMessage();
String byeMessage();
}
Here is my classes:
public class Lecturer implements Greetable, Identifiable {
private int employeeId;
private String name;
private String title;
#Override
public String helloMessage() {
return name;
}
#Override
public String byeMessage() {
return title;
}
}
public class Student implements Greetable, Identifiable {
private char examScore;
#Override
public String helloMessage() {
return "Hi";
}
#Override
public String byeMessage() {
return "Whats up";
}
}
I get the error from the classes that it has to abstract the methods from the interfaces? What does that mean?
Non-abstract classes are required to create concrete versions of any methods found in any interfaces that they're implementing, and while your classes implement concrete versions of one of the interface, the Greetable interface, you're not implementing all the methods of both interfaces, here the public int getId() method from the Identifiable interface is missing from both classes.
Solution: give both classes an int id field as well as the getId() method that returns the value held by this field.
e.g. for Student,
public class Student implements Greetable, Identifiable {
private char examScore;
private int id; // **** your classes will need this field ****
// need to set the ID somehow, either with a setter or a constructor
public Student(int id) {
this.id = id;
}
#Override
public String helloMessage() {
return "Hi";
}
#Override
public String byeMessage() {
return "Whats up";
}
#Override // **************** add this method to return the value held by id ******
public int getId() {
return this.id;
}
}
You define to implement both interfaces, but you only implemented the methods of the second interface.
So you have to implement the method getId() in both classes.
You havent implemeted the getId() method in Identifiable. If you are not implementing that method you need to make the Lecturer and Student as abstract or you need to implement the getId() method in both the classes.
In your case I think you will need to create instances of the Student and Lecturer. If so then you cannot make them as abstract, as abstract class instances cannot be created. So better implement the getId() in both the classes.
Your Student and Lecturer classes MUST implement both Greetable, Identifiable interface methods, otherwise they need to be declared as abstract classes i.e., you are missing getId() from Identifiable interface, which is causing the issues, corrected code below.
Lecturer class:
public class Lecturer implements Greetable, Identifiable {
int getId() {
return employeeId;
}
//all other existing methods
}
Student class:
public class Student implements Greetable, Identifiable {
int getId() {
return studentId;
}
//all other existing methods
}
You can look here
I got 2 POJOs that are being passed to a HttpEntity and converted to json.
Before passing them i need a variable which is in both of the POJOs with different names because of the needs of the API so i cant change them.
What is the best way without casting and in terms of OOP also within the POJO definition like mentioned in Wikipedia?
Abstract pojo
public abstract class Pojo{
//some common variables
//setter getters
}
PojoOne
public class PojoOne extends Pojo{
private String id;
//setter getter for id
}
PojoTwo
public class PojoTwo extends Pojo{
private String identifier;
// setter getter for identifier
}
Class that
public class SomeOtherClass {
public void getIdForUse(Pojo pojo){
String s = pojo. // How should this be to have ability to get both id and identifier
}
}
Add a common method to the common superclass:
public abstract class Pojo {
public abstract String getId();
// some common variables
// setter getters
}
public class PojoOne extends Pojo {
private String id;
#Override
public String getId() {
return id;
}
//setter for id
}
public class PojoTwo extends Pojo {
private String identifier;
// setter getter for identifier
#Override
public String getId() {
return identifier;
}
}
I have problem with jackson serialization of object by its interface.
I have class
class Point implements PointView {
private String id;
private String name;
public Point() {
}
public Point(String id, String name) {
this.id = id;
this.name = name;
}
#Override
public String getId() {
return id;
}
public String getName() {
return name;
}
}
which implements
interface PointView {
String getId();
}
and have class
class Map implements MapView {
private String id;
private String name;
private Point point;
public Map() {
}
public Map(String id, String name, Point point) {
this.id = id;
this.name = name;
this.point = point;
}
#Override
public String getId() {
return id;
}
public String getName() {
return name;
}
#JsonSerialize(as = PointView.class)
public Point getPoint() {
return point;
}
}
which implements
interface MapView {
String getId();
Point getPoint();
}
And have class
class Container {
private Map map;
public Container() {
}
public Container(Map map) {
this.map = map;
}
#JsonSerialize(as = MapView.class)
public Map getMap() {
return map;
}
}
I want serialize Container with Jackson and get result
{"map":{"id":"mapId","point":{"id":"pointId"}}}
But in fact I get result
{"map":{"id":"mapId","point":{"id":"pointId","name":"pointName"}}}
that have property "name" in nested object "point" although I specified serializition type of Point in Map (#JsonSerialize(as = PointView.class)). Interface PointView dont have method getName, but in result exists field "name" of Point.
If I remove annotation (#JsonSerialize(as = MapView.class)) from method getMap in class Container I get result
{"map":{"id":"mapId","name":"mapName","point":{"id":"pointId"}}}
Now point dont have property "name", but map have.
How can I get result
{"map":{"id":"mapId","point":{"id":"pointId"}}}
?
To get the desired result also the same method in interface must be annotated by #JsonSerialize
interface MapView {
String getId();
#JsonSerialize(as = PointView.class)
Point getPoint();
}
You can annotate the method like this:
#JsonIgnore
public String getName() {
return name;
}
Or if you want specific serialization in this use case, but normal serialization in others, you can use a #JsonView (see doc).
The reason it's serializing out the name is that the instance has the accessor getName(), even though interface does not.
Yes, you can use
#JsonSerialize(as=MyInterface.class)
public class ConcreteClass implements MyInterface { .... }
either on implementation class (as above), or on property that has value.
basically I created a Person class and a constructor which sets the name,last name,age of the Person.all the properties of the class were set the private as it should be. I have made setters and getters for all the properties. On the main method I tried to override one of the setters just for practice reason. Its did draw an error saying Person.name not visible which means it cannot access private, Why this is happening, I mean if wasn't overriding the method it would have access. but if I set it to protected mode i will work.
Here is the code:
class Person {
private int age;
private String name;
private String last_name;
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getLast_name() {
return last_name;
}
public void setLast_name(String last_name) {
this.last_name = last_name;
}
public Person(int age, String name, String last_name) {
this.age = age;
this.name = name;
this.last_name = last_name;
}
}
public class main {
public static void main(String[] args) {
// TODO Auto-generated method stub
Person per = new Person(15,"bb","Sb") {
public void setName(String name) {
this.name = "aaaa";
}
};
per.setName("asdfaf");
System.out.println(per.getName());
}
}
A private member is only accessible in the class in which it is declared.
You created an anonymous sub-class of Person and tried to access a private member of the super-class from the sub-class. This is never allowed.
When developers of a class wish to allow access to certain members of the class to its sub-classes, they set the acess level to protected.
You have created a class named Person and in the following lines you are trying to create an anonymous subclass:
Person per = new Person(15,"bb","Sb") {
public void setName(String name) {
this.name = "aaaa";
}
};
As mentioned in doc:
A subclass does not inherit the private members of its parent class
Here your anonymous subclass is trying to access private field name directly and so is the error. You can use getter/setter which are public. You can also check this related question on SO.
You cannot access private fields from outside your class, even if you are overriding it. You are basically defining a new subclass of Person in your main(), which isn't allowed access to the private field Person.name. However, it can access a protected field.
Basic idea behind overriding is to redefine existing functionality and give new definition to it. If you refer to documentation, private member variables are only accessible in it own class. That why it is not available in your anonymous sub-class implementation.
Note: Generally we do not override setter methods as they are not a functionality.
This is called encapsulation . You can not access private vars from other classes . you can find more description here
Recently I've started hearing about "POJOs" (Plain Old Java Objects). I googled it, but still don't understand the concept well. Can anyone give me a clear description of a POJO?
Consider a class "Person" with variables "id, name, address, salary" -- how would I create a POJO for this scenario? Is the code below a POJO?
public class Person {
//variables
People people = new People();
private int id;
private String name;
private String address;
private int salary;
public int getId() {
return id;
}
public String getName() {
return name;
}
public String getAddress() {
return address;
}
public int getSalary() {
return salary;
}
public void setId() {
this.id = id;
}
public void setName() {
this.name = name;
}
public void setAddress() {
this.address = address;
}
public void setSalary() {
this.salary = salary;
}
}
A POJO is just a plain, old Java Bean with the restrictions removed. Java Beans must meet the following requirements:
Default no-arg constructor
Follow the Bean convention of getFoo (or isFoo for booleans) and setFoo methods for a mutable attribute named foo; leave off the setFoo if foo is immutable.
Must implement java.io.Serializable
POJO does not mandate any of these. It's just what the name says: an object that compiles under JDK can be considered a Plain Old Java Object. No app server, no base classes, no interfaces required to use.
The acronym POJO was a reaction against EJB 2.0, which required several interfaces, extended base classes, and lots of methods just to do simple things. Some people, Rod Johnson and Martin Fowler among them, rebelled against the complexity and sought a way to implement enterprise scale solutions without having to write EJBs.
Martin Fowler coined a new acronym.
Rod Johnson wrote "J2EE Without EJBs", wrote Spring, influenced EJB enough so version 3.1 looks a great deal like Spring and Hibernate, and got a sweet IPO from VMWare out of it.
Here's an example that you can wrap your head around:
public class MyFirstPojo
{
private String name;
public static void main(String [] args)
{
for (String arg : args)
{
MyFirstPojo pojo = new MyFirstPojo(arg); // Here's how you create a POJO
System.out.println(pojo);
}
}
public MyFirstPojo(String name)
{
this.name = name;
}
public String getName() { return this.name; }
public String toString() { return this.name; }
}
POJO:- POJO is a Java object not bound by any restriction other than those forced by the Java Language Specification.
Properties of POJO
All properties must be public setter and getter methods
All instance variables should be private
Should not Extend prespecified classes.
Should not Implement prespecified interfaces.
Should not contain prespecified annotations.
It may not have any argument constructors
Example of POJO
public class POJO {
private String value;
public String getValue() {
return value;
}
public void setValue(String value) {
this.value = value;
}
}
A POJO is a Plain Old Java Object.
From the wikipedia article I linked to:
In computing software, POJO is an
acronym for Plain Old Java Object. The
name is used to emphasize that a given
object is an ordinary Java Object, not
a special object, and in particular
not an Enterprise JavaBean
Your class appears to already be a POJO.
POJO class acts as a bean which is used to set and get the value.
public class Data
{
private int id;
private String deptname;
private String date;
private String name;
private String mdate;
private String mname;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getDeptname() {
return deptname;
}
public void setDeptname(String deptname) {
this.deptname = deptname;
}
public String getDate() {
return date;
}
public void setDate(String date) {
this.date = date;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getMdate() {
return mdate;
}
public void setMdate(String mdate) {
this.mdate = mdate;
}
public String getMname() {
return mname;
}
public void setMname(String mname) {
this.mname = mname;
}
}
When you aren't doing anything to make your class particularly designed to work with a given framework, ORM, or other system that needs a special sort of class, you have a Plain Old Java Object, or POJO.
Ironically, one of the reasons for coining the term is that people were avoiding them in cases where they were sensible and some people concluded that this was because they didn't have a fancy name. Ironic, because your question demonstrates that the approach worked.
Compare the older POD "Plain Old Data" to mean a C++ class that doesn't do anything a C struct couldn't do (more or less, non-virtual members that aren't destructors or trivial constructors don't stop it being considered POD), and the newer (and more directly comparable) POCO "Plain Old CLR Object" in .NET.
According to Martin Fowler
The term was coined while Rebecca Parsons, Josh MacKenzie and I were preparing for a talk at a conference in September 2000. In the talk, we were pointing out the many benefits of encoding business logic into regular java objects rather than using Entity Beans. 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.
Generally, a POJO is not bound to any restriction and any Java object can be called a POJO but there are some directions. A well-defined POJO should follow below directions.
Each variable in a POJO should be declared as private.
Default constructor should be overridden with public accessibility.
Each variable should have its Setter-Getter method with public accessibility.
Generally POJO should override equals(), hashCode() and toString() methods of Object (but it's not mandatory).
Overriding compare() method of Comparable interface used for sorting (Preferable but not mandatory).
And according to Java Language Specification, a POJO should not have to
Extend pre-specified classes
Implement pre-specified interfaces
Contain pre-specified annotations
However, developers and frameworks describe a POJO still requires the use prespecified annotations to implement features like persistence, declarative transaction management etc. So the idea is that if the object was a POJO before any annotations were added would return to POJO status if the annotations are removed then it can still be considered a POJO.
A JavaBean is a special kind of POJO that is Serializable, has a no-argument constructor, and allows access to properties using getter and setter methods that follow a simple naming convention.
Read more on Plain Old Java Object (POJO) Explained.
there are mainly three options are possible for mapping purpose
serialize
XML mapping
POJO mapping.(Plain Old Java Objects)
While using the pojo classes,it is easy for a developer to map with the database.
POJO classes are created for database and at the same time value-objects classes are created with getter and setter methods that will easily hold the content.
So,for the purpose of mapping in between java with database, value-objects and POJO classes are implemented.
import java.io.Serializable;
public class Course implements Serializable {
protected int courseId;
protected String courseName;
protected String courseType;
public Course() {
courseName = new String();
courseType = new String();
}
public Course(String courseName, String courseType) {
this.courseName = courseName;
this.courseType = courseType;
}
public Course(int courseId, String courseName, String courseType) {
this.courseId = courseId;
this.courseName = courseName;
this.courseType = courseType;
}
public int getCourseId() {
return courseId;
}
public void setCourseId(int courseId) {
this.courseId = courseId;
}
public String getCourseName() {
return courseName;
}
public void setCourseName(String courseName) {
this.courseName = courseName;
}
public String getCourseType() {
return courseType;
}
public void setCourseType(String courseType) {
this.courseType = courseType;
}
#Override
public int hashCode() {
return courseId;
}
#Override
public boolean equals(Object obj) {
if (obj != null || obj instanceof Course) {
Course c = (Course) obj;
if (courseId == c.courseId && courseName.equals(c.courseName)
&& courseType.equals(c.courseType))
return true;
}
return false;
}
#Override
public String toString() {
return "Course[" + courseId + "," + courseName + "," + courseType + "]";
}
}
public class UserInfo {
String LoginId;
String Password;
String FirstName;
String LastName;
String Email;
String Mobile;
String Address;
String DOB;
public String getLoginId() {
return LoginId;
}
public void setLoginId(String loginId) {
LoginId = loginId;
}
public String getPassword() {
return Password;
}
public void setPassword(String password) {
Password = password;
}
public String getFirstName() {
return FirstName;
}
public void setFirstName(String firstName) {
FirstName = firstName;
}
public String getLastName() {
return LastName;
}
public void setLastName(String lastName) {
LastName = lastName;
}
public String getEmail() {
return Email;
}
public void setEmail(String email) {
Email = email;
}
public String getMobile() {
return Mobile;
}
public void setMobile(String mobile) {
Mobile = mobile;
}
public String getAddress() {
return Address;
}
public void setAddress(String address) {
Address = address;
}
public String getDOB() {
return DOB;
}
public void setDOB(String DOB) {
this.DOB = DOB;
}
}
File-setting-plugins-Browse repositories
Search RoboPOJOGenerator and install, Restart Android studio
Open Project and right click on package select on Generate POJO from JSON
Paste JSON in dialogbox and select option according your requirements
Click on Generate button
If a class is not bogged down from a framework or a library, then an object created from that class is recognized as a POJO.
Let's see some examples:
class MyServlet extends HttpServlet{
//....
}
The sole meaning of MyServlet class is given by the HttpServlet class. Therefore the objects created from the MyServlet are not POJOs.
class MyClass implements Serializable{
//...
}
The Serializable interface does not give a meaning to the class MyClass. Therefore the objects created from the MyClass are POJOs.