importance of Context in android

Possible Duplicate:
What is Context in Android?

i read context in android current state of the application/object

   Intent intent=new Intent(this,SecondaryActivity.class);
   startActivity(intent);

in the place of this we can use getApplicationContext() but in an

@Override
public void onClick(View view) {

Toast.makeText(getApplicationContext(), "Hai",Toast.LENGTH_LONG).show();
}

we can't use this here i mean in getApplicationContext() why this also refers to the current object ,hence am confused about this and context help me to study this,an reference share me..

---

When you pass this to method, you mean that this reference is an instance of Context , so if you are in Activity you can pass this instead of Context. But when you are in anonymous class:

button.setOnClickListener(new OnClickListener(){
     @Override
     public void onClick(){
         //here this is a reference to OnClickListener instance
     }

To pass Context in onClick method you can write

MyActivity.this

---

Activity is a sublcass of context so all Activity objects are also a Context :

android.content.Context
   ↳    android.content.ContextWrapper
       ↳    android.view.ContextThemeWrapper
           ↳    android.app.Activity

So if you can't use this than it means this is not an instance of an Context class or its subclass.

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