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POJO
POJO stands for "Plain Old Java Object". This is different from Plain Old Javascript Objects. A Plain Old Java Object refers to the Object Oriented Programming (OOP) paradigm that is used in the Java programming language. The OOP model treats data as 'objects'. Each 'object' is an instance of a 'Class', which represents the archetype or template from which all objects inherit their properties and attributes.
A POJO is therefore simply a Java Object. However, it must also satisfy the following additional criteria:
- it must not extend prespecified Java Classes;
public class Foo extends javax.servlet.http.HttpServlet {
...// body ...
}
- it must not implement prespecified Interfaces;
public class Bar implements javax.ejb.EntityBean {
... // body
}
- it must not contain prespecified Annotations.
@javax.persistence.Entity public class Baz {
... // body ...
}
- For any class field which is marked as private it must have getter and setter methods alongside it. Also it must have EMPTY constructor if custom constructor declared.
public class Car {
private int gear;
// default constructor
public Car() { }
// custom constructor
public Car(int car){
this.car = car;
}
public int getGear() {
return this.gear;
}
public void setGear(int gear){
this.gear = gear;
}
}
Therefore a Java Object qualifies as a POJO only when it is free from the above modifications. It therefore follows that a POJO is not 'bound by any restrictions' other those prescribed by the formal Java language specification.
POJO is usually used to describe a class that doesn't need to be a subclass of anything, or implement specific interfaces, or follow a specific pattern. It has properties, getters and setters for respective properties. It may also override Object.toString() and Object.equals().