You should be already aware what is an object and why we use classes, methods and variables (and how to add a new class to your project). Otherwise read previous lesson. To more comfortable jump into Adroid Java developing we need a bit more information about them. Let’s start with class inheritance and class constructors).
As we have already learnt from previous lessons Android apps are written in Java – one of many programming languages (for instance apps for Apple iOS are written in Objective C). We will try to understand some basic concepts of Java.
Android apps are built from activities. Activity is a set of actions visible on the screen (usually it takes whole screen, but it could be in a window). The app could have many activities. For instance a music player has a main menu activity, album list activity, song play activity etc. So activity is a combination of user interface and some functions.
Let’s look again at the project tree (if you can’t see it, just click Project tab on the left of the screen). Previously we were navigating directly to activity_main.xml file, but now we try to look also on other folders and files. Don’t worry only on a few of them, most we would never have to touch.
Android uses standard RGB (red, green and blue) color model. Each primary color value is usually represented by hexadecimal number. At the beginning of such a color definition you have to put a pound character (#).
The simplest is just #RGB format, where #000 is black and #FFF is white. But in this format we have only 16 values per color so it gives 4096 combinations. That’s why #RRGGBB format is mainly used. In this format we have 256 values per primary color, so 16 777 216 colors in total. Black is #000000 and white is #FFFFFF.
