A free Java Development Environment designed for beginners.
BlueJ has a deliberately smaller and simpler interface than professional environments like NetBeans or Eclipse. This allows beginners to get started more quickly, and without being overwhelmed.
BlueJ is deliberately designed with good pedagogy in mind. There is a popular textbook designed for teaching introductory university/college courses with BlueJ, and a site full of teaching resources.
BlueJ allows you to interact with objects. You can inspect their value, call methods on them, pass them as parameters and more. You can also directly invoke Java expressions without compiling. Thus BlueJ is a powerful graphical shell/REPL for Java.
BlueJ has several features not seen before in other IDEs. Its object bench, code pad, and scope colouring were all original BlueJ features.