There are several Integrated Development Environments (IDE) for Android Java apps. Each offers editing, compiling and testing facilities in a single package. The most widely used IDE is probably still Eclipse with the Android plugin. But Google has stopped supporting this application so the main IDE for development on a computer is now Android Studio. You can read how to use Android Studio here. Android Studio is completely free software.
If you want to develop Android apps on an Android device then again, there are several options. The best of these is AIDE, the Android Integrated Development Environment. You can learn how to use AIDE here. It is commercial software but a free version is available which you can use for relatively small apps. You can download this from the Google Play Store.
You can learn more about developing Android Apps in "Start Programming Mobile Apps with Android Java" by Bill Tait. This is available in Kindle and paperback formats from the Amazon bookstore.