Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window ObjektIf-root-1-c8b353bcc2203126f0bc19417fd4fbb9fd13d18c.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.