Screenshot of BlueJ
|Original author(s)||David Barnes & Michael Kölling|
|Stable release||3.1.1 / January 27, 2014|
|Type||Integrated development environment|
|License||GNU General Public License|
BlueJ was developed to support the learning and teaching of object-oriented programming, and its design differs from other development environments as a result. The main screen graphically shows the class structure of an application under development (in a UML-like diagram), and objects can be interactively created and tested. This interaction facility, combined with a clean, simple user interface, allows easy experimentation with objects under development. Object-oriented concepts (classes, objects, communication through method calls) are represented visually and in its interaction design in the interface.
The development of BlueJ was started in 1999 by Michael Kölling and John Rosenberg at Monash University, as a successor to the Blue system. BlueJ is an IDE. Blue was an integrated system with its own programming language and environment. BlueJ implements the Blue environment design for the Java programming language.
The features of BlueJ are designed as an aid to learning object-oriented programming concepts, as well as an aid to program development itself. As a result, some commonly available tools differ from other environments, some tools are absent, and other tools are provided not commonly found in development environments. The features include:
- Representation of object orientation – the concepts of classes and objects are presented in a visual manner. Both have distinct visual representations, and it is easy to demonstrate that code is associated directly with a class, that an object has a class type, and that the class type of an object directly determines its available methods and fields as well as its behavior.
- Simplicity of the interface – The user interface is simpler than in full-scale professional environments, and thus easier to learn. The goal is for the development environment to "disappear". Students should be able to concentrate on the programming task without the need to struggle with the environment.
- Object interaction – Programmers are able to experiment with objects by creating objects on the "Object Bench" and calling individual methods interactively (including passing of parameters and inspection of results). This is intended to encourage early and frequent testing, and experimentation to help understand the programming model.
- The "Code Pad" – The code pad is a tool that instantly evaluates arbitrary Java expressions and statements.
- Regression testing – BlueJ supports regression testing via an integration of JUnit. In addition to manual writing of JUnit classes, interactive tests can be recorded and JUnit test cases can be automatically created from these recordings.
- Group work support – BlueJ provides simple support for group work via a subset of CVS and Subversion functionality.
- Java ME support – Java ME (Micro Edition) projects can be developed and deployed from BlueJ.
- Flexible extension system – Extensions (a.k.a. Plug-Ins) can be developed using a public extensions API to extend the functionality of the core environment.
- JAR files and applets – Creation of executable JAR files and applets is built into the system.
- Translations – The BlueJ system interface has been translated into at least 14 different natural languages.
The visual interaction features of BlueJ were designed to allow one to delay the introduction of certain programming concepts considered difficult or problematic by educators. These include:
- Syntax such as
public static void main(String args)– which requires unnecessarily introducing the keywords
static, as well as method arguments and arrays. BlueJ allows arbitrary classes to be instantiated and arbitrary methods to be invoked interactively.
- Program input/output. In BlueJ both the parameter values for and the return values from method calls can be entered/inspected directly, so there is no need for students to deal with terminal I/O or write graphical user interfaces when they are still struggling with the basic concepts of programming.
The program is a fully functional development tool, commonly used by schools, colleges, and universities.
Some of the features BlueJ pioneered in modern development environments, such as interactive instantiation, direct object interaction, unit test recording, the editor "navigation view", and scope highlighting, are generally useful for software development, however, are not usually found in other development environments. Some attempts were made to replicate some of these features: For example, Microsoft's Visual Studio replicated BlueJ's object bench feature, however, it remained somewhat obscure because it is not included in all versions of that environment.
The pedagogical approach represented in BlueJ is based on constructivism and visualisation. Working with the BlueJ environment provides concrete experiences for abstract concepts, such as the class/object relationship, object instantiation, method calling, and parameter passing. These abstract concepts are traditionally hard to understand for novices, and providing concrete representations of them is intended to help the learning process. Visualisations of the processes and structures (such as objects on the object bench, and object inspectors) support this process.
The goal of this approach is for beginners to more easily develop a consistent mental model of object-oriented systems, their properties, and their execution.
Usage in educational institutions
BlueJ is the primary recommended learning software for the Java section of the Computer Application course in ICSE schools all over India, where it is considered the de facto software for learning the basics of Object Oriented Programming and has proven extremely popular due to its ease of use and wide support in schools and educational centers. However, it is not necessary that code that is written in the actual laboratory tests and final examinations be written in BlueJ, instead any IDE that supports Java can be used.