Give an ID to your view in android xml file

Give an ID to your view object is very useful if you want to refer to that view later in source code. For example you want to set text of a TextView in source code. To do that, put [cci]android:id[/cci] attibute to your view.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
         />
</RelativeLayout>

In above code, you can see I have put an [cci]android:id=”@+id/textView1″[/cci]. This line defines an [cci]id[/cci] of our [cci]TextView[/cci] to [cci]textView1[/cci].

Refers view from activity

After you have defined an id, now you can refer to your [cci]TextView[/cci] in an activity like this

TextView textView = (TextView) findViewById(R.id.textView1);

Now, with your [cci]TextView[/cci] in hand, you can set text to it as you want like this

if(textView != null) textView.setText("new text");

Refers view in xml file

Sometimes we need to refers to our view in the xml layout file. For example you want to align top another [cci]TextView[/cci] to other. To achieve this, you can refer to our last [cci]TextView[/cci] like this [cci]@id/textView1[/cci]. Note you must replace ‘+’ sign when referring to already defined [cci]id[/cci]. Use ‘+’ sign only in first time when you define an [cci]id[/cci].

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignTop="@id/textView1"/>