If you plan to write Java applets using the ECL API, you need a Java development environment, such as JetBrains Inc.'s IntelliJ IDEA, the NetBeans IDE or Eclipse. The development environment you use must also support Java version 1.8 or higher. The ECL documentation has more details about setting up your development environment.
You can learn more about language syntax and how to program in JavaScript and Java from online resources and publications.