JADX will feel very similar to jd-gui, and it will eliminate the steps of unpacking the .apk and converting the .dex file to a .jar before loading into the tool, which allows the examiner to view the Java source code. The tool is available for Windows, Linux, and macOS.

Navigating through the decompiled code, you will likely see one or more packages that consist of many different classes that provide the blueprint for the objects that make up the Android application. Classes (and the objects derived from them) will consist of both a state and a behavior. A state is stored in a field or variable, and behaviors are exposed through methods or functions.¹

Classes

  •  State: (stored as a field/variable)

  •  Behavior: (exposed in method/function)

Classes can be public, private, and/or protected, and these are denoted by different colors within the tool interface.

  •  Green Circle—Public: can be used outside of this class by any other class/function

  •  Red Square—Private: can only be used inside this class

  •  Yellow Diamond—Protected: class members and functions can only be used inside the class or by classes derived from this class

Reference:

[1] https://for585.com/javadefinition