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Android Core Concepts:

1. DecorView & ContentView Concept of view hierachy

DecorView->LinearLayout(optional, depends on whether the actionbar is here)->ContentView + ToolBar(optional)

2. View-> View Group-> View Layout Understand the usage for critical view group classes.

AbsoluteLayout, 
AdapterView<T extends Adapter>, 
CoordinatorLayout, 
DrawerLayout, 
FragmentBreadCrumbs, 
FrameLayout, 
GridLayout, 
LinearLayout, 
LinearLayoutCompat, 
PagerTitleStrip, 
RecyclerView, - Recommended for drawing list view and grid view 
RelativeLayout, 
SlidingDrawer,  - Deprecated
SlidingPaneLayout, 
SwipeRefreshLayout, 
Toolbar, 
TvView, 
ViewPager

3. Basic Application Components

• Activity
• Service
• Fragment