A reference in XML id, to use @ id / id_name. And if it is necessary to define a id, you need to use @ + id / id_name this syntax.
match_parent represents the current element and let the parent element as high or wide, and wrap_content represents the current element has only just able to contain the contents inside.
When you call setContentView () method to load a layout for the current activity, usually pass in a layout file id.
Activities of the registration statement to be placed in <application> tag within, Android Studio automatically registered.
Configuration Main Activity: adding <activity> tag <intent-filter> tags, and added in the tag <category android: name = "android.intent.category.DEFAULT" /> and <category android: name = "com. example.activitytest.MY_CATEGORY "/> two statements can be. The main role of the primary activity is to click on the desktop when the application is first opened this event.
In the event, the element can be obtained in the layout file defined by the findViewById () method. The created object is instantiated.
By calling setOnClickListener () method for the button to register a listener.
MakeText static method () method requires passing three parameters: Context, i.e. Toast (alert message) requested context, text, display duration
