There are many IDEs available that you can use free of charge: NetBeans, Eclipse, IntelliJ IDEA, BlueJ, DrJava, JDeveloper, JCreator, jEdit, JSource, jCRASP, and jEdit, to name a few. Each of them has followers who are convinced that their choice is the best, so we are not going to argue. It is a matter of preference, after all. We will concentrate on the three most popular ones—NetBeans, Eclipse, and IntelliJ IDEA. We will use the IntelliJ IDEA free Community Edition for our demonstrations.
We recommend reading the documentation about these and other IDEs, and even trying them out, before making your final choice. For your initial research, you can use the Wikipedia article at https://en.wikipedia.org/wiki/Comparison_of_integrated_development_environments#Java, which has a table comparing many modern IDEs.