Android Context Class example detailed _android

1. For example, the following code fragment, the first parameter of the Toast class accepts a context object:

@Override protected Dialog oncreatedialog (int id) {switch (ID) {case 0:builder Builder = new Alertdialog.bui 
 Lder (this); 
 Builder.seticon (R.drawable.ic_launcher); 
 Builder.settitle ("This are a dialog with some simple text ..."); 
  Builder.setpositivebutton ("OK", new Dialoginterface.onclicklistener () {public void OnClick (Dialoginterface dialog, 
  int Whichbutton) {Toast.maketext (Getbasecontext (), "OK clicked!", Toast.length_short). Show (); 
 
 } 
  }); Builder.setnegativebutton ("Cancel", new Dialoginterface.onclicklistener () {public void OnClick (Dialoginterface dial 
  OG, int whichbutton) {Toast.maketext (Getbasecontext (), "Cancel clicked!", Toast.length_short). Show (); 
 
 } 
  }); Builder.setmultichoiceitems (items, itemschecked, new Dialoginterface.onmultichoiceclicklistener () {public void onCl 
   Ick (dialoginterface dialog, int which, Boolean ischecked) {Toast.maketext (Getbasecontext (), Items[which] + (ischecked?)"Checked!" 
  : "unchecked!"), Toast.length_short). Show (); 
 } 
  }); 
 
 return Builder.create (); 
 return null; @Override protected Dialog oncreatedialog (int id) {switch (ID) {case 0:builder Builder = new Alertdialog.builde
 R (this);
 Builder.seticon (R.drawable.ic_launcher);
 Builder.settitle ("This are a dialog with some simple text ...");
  Builder.setpositivebutton ("OK", new Dialoginterface.onclicklistener () {public void OnClick (Dialoginterface dialog,
  int Whichbutton) {Toast.maketext (Getbasecontext (), "OK clicked!", Toast.length_short). Show ();

 }
  }); Builder.setnegativebutton ("Cancel", new Dialoginterface.onclicklistener () {public void OnClick (Dialoginterface dialo
  g, int whichbutton) {Toast.maketext (Getbasecontext (), "Cancel clicked!", Toast.length_short). Show ();

 }
  }); Builder.setmultichoiceitems (items, itemschecked, new Dialoginterface.onmultichoiceclicklistener () {public void OnCli CK (dialoginterface dialog, int which, Boolean ischecked) {Toast.maketext (Getbasecontext (), Items[which] + (ischecked?)
    "Checked!"
  : "unchecked!"), Toast.length_short). Show ();
 }
  });

 return Builder.create ();
 return null; }

However, the Toast class is not used directly in the activity, and it is used in the Alertdialog class. So, here you need to get an instance of a context class through the Getbasecontext () method.

2. The context is also met when a view is dynamically created in the activity.

For example, if you want to dynamically create a TextView through hard coding:

TextView TV = new TextView (this); 

The TextView constructor accepts a context object because the activity class is a subclass of the context class, so you can replace the Conext object with the This keyword.

Tips:

Use this to dynamically create a view, such as TextView, Button, which has a potential risk--a memory leak. So, use the Getapplicationcontext () method as much as possible instead of this.