CODEWITHSUNDEEP
javaandroidandroid-context
warm
warm

Reputation: 117

In android, when is a context different from "this"?

I am at baby step level of programming on Android (and in Java in general). I do understand that Activity inherits from the Context class. However in every code snippet I have come across, every time a context must be mentionned, it is set to "this".

My question is : when is a context different from "this" ? Could you provide an real life example of context needing to be different from "this"?

Thank you very much.

Upvotes: 2

Views: 176

Answers (5)

Robin Dijkhof
Robin Dijkhof

Reputation: 19288

Example:

class Example{
    int number = 0;

    public Example(int number){
        this.number = number;
    }
}

notice that number in the contructor and number in the class are not the same. Altough they have the same name. Saying number = number doesn't make sense. Be using this you can asses number in the class.

Upvotes: 1

hundeva
hundeva

Reputation: 1058

Typically, you will want to use this when you are "inside" of an Activity. However, when you are using for example a Helper class, the reference this will not work. An example can be something like this:

public class MyActivity extends Activity
{
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_NOSENSOR);
    }
}

A case, where you cannot:

public class MyHelper
{
    /* some code of yours */

    public void lockOrientation(Activity activity)
    {
        activity.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_NOSENSOR);
    }
}

The above code locks the orientation to the current orientation. Notice that you need to supply the method with an Activity parameter, since you cannot use:

this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_NOSENSOR);

In the first example, you could use this to achieve this, because you were "inside" of an Activity.

Another type of example, how do you set onClickListener.

First example, when you use this:

public class MyActivity extends Activity implements View.OnClickListener
{
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        Button btn=(Button)findViewById(R.id.mybutton);
        btn.setOnClickListener(this);
    }

    @Override
    public void onClick(View v)
    {
        //handle the click event
    }
}

In this example, you can use this because in the first line, we wrote implements View.OnClickListener, so the class inherits from the given interface. Without the implements thingie, you couldn't do it. An example of setting the onClickListener without this:

public class MyActivity extends Activity
{
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        Button btn=(Button)findViewById(R.id.mybutton);
        btn.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                //handle the click event
            }
        });
    }
}

In the second example, we are defining an Anonymous Inner Class, which will handle the click event of the button. Notice that in this case, our Activity does NOT implements View.OnClickListener.

Upvotes: 3

Benito Bertoli
Benito Bertoli

Reputation: 25793

this is a reference to the current object — the object whose method or constructor is being called.

Inside an Activity's method this can be used as a Context object because Activity inherits from ContextThemeWrapper, which inherits from ContextWrapper, which inherits from Context.

A Fragment on the other hand does not inherit from Context. So to get the Context inside a Fragment you would have to call getActivity() for example.

This applies to any object you are calling this from.

Consider you are inside the OnClick() method of a View.OnClickListener and you want to start an Activity:

button.setOnClickListener (new View.OnClickListener() {
    public void onClick(View v) {

        Intent intent = new Intent(this, NextActivity.class); // wrong because 'this' is the OnClickListener object

        Intent intent = new Intent(CurrentActivity.this, NextActivity.class); // correct because 'this' is the CurrentActivity object

        startActivity(intent);
    }
});

Upvotes: 0

Hareshkumar Chhelana
Hareshkumar Chhelana

Reputation: 24848

  1. In Outer Class you directly use "this" reference
  2. In Inner Class Or Abstract Class implementation Or Interface implementation use "classname.this" reference

Upvotes: 1

Jon Zangitu
Jon Zangitu

Reputation: 967

For example when you are implementing an OnClickListener the "this" is different.

Upvotes: 0

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