What is meaning of Plain Old Java Object (POJO)?

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What does the term Plain Old Java Object (POJO) mean? I couldn't find anything explanatory enough.

POJO's Wikipedia page says that POJO is an ordinary Java Object and not a special object. Now, what makes or what doesn't make and object special in Java?

The above page also says that a POJO should not have to extend prespecified classes, implement prespecified Interfaces or contain prespecified Annotations. Does that also mean that POJOs are not allowed to implement interfaces like Serializable, Comparable or classes like Applets or any other user-written Class/Interfaces?

Also, does the above policy (no extending, no implementing) means that we are not allowed to use any external libraries?

Where exactly are POJOs used?

EDIT: To be more specific, am I allowed to extend/implement classes/interfaces that are part of the Java or any external libraries?

pojo terminology java
9 Answers

There is an abundance of posts that are half correct and half incorrect. The best example of the correct interpretation is given by Rex M in their answer here.

[POJO are classes] that doesn't require any significant "guts" to make it work. The idea is in contrast with very dependent objects that have a hard time being (or can't be) instantiated and manipulated on their own - they require other services, drivers, provider instances, etc. to also be present.

Unfortunately, these very same answers often come along with the misunderstanding that they are somehow simple or often have a simple structure. This is not necessarily true and the confusion seems to stem from the fact that in the Java (POJO) and C# world (POCO) business logic is relatively easily modeled especially in the web application world.

POJO's can have multiple levels of inheritance, generic types, abstractions, etc. It just so happens that this isn't required in the majority of web applications as business logic doesn't necessitate it - alot of the effort goes into databases, queries, data transfer objects and repositories.

As soon as you step out of line with simple web apps, your POJO's start looking a lot more complex. E.g. Make a web app that assigns taxi's to user schedules. To do this, you need a graph coloring algorithm. To color the graphs, you need a graph object. Each node in the graph is a schedule object. Now what if we want to make it generic so that coloring the graph can be done not only with schedules but other things as well. We can make it generic, abstract and add levels of inheritance - almost to the point of making it a mini library.

At this point though, no matter its complexity, its still a POJO because it doesn't rely on the guts of other frameworks.

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