[Android] Button background change for state. (selector)

color - bg_selector.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">

    <!-- Active tab -->
    <item android:drawable="@drawable/bg_selected" android:state_focused="false" android:state_pressed="false" android:state_selected="true"/>
    <!-- Inactive tab -->
    <item android:drawable="@drawable/bg_unselected" android:state_focused="false" android:state_pressed="false" android:state_selected="false"/>
    <!-- Pressed tab -->
    <item android:drawable="@drawable/bg_focused" android:state_pressed="true"/>
    <!-- Selected tab (using d-pad) -->
    <item android:drawable="@drawable/bg_focused" android:state_focused="true" android:state_pressed="false" android:state_selected="true"/>

</selector>

drawable - bg.xml

<!-- bg_focused.xml -->
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" >

    <corners android:radius="10dp" />

    <gradient
        android:centerColor="#5290c3"
        android:endColor="#6f9fc7"
        android:startColor="#78a4c9" />

</shape>

<!-- bg_selected.xml -->
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" >

    <corners android:radius="10dp" />

    <gradient
        android:centerColor="#72b0e3"
        android:endColor="#8fafe7"
        android:startColor="#98c4e9" />

</shape>

<!-- bg_unselected.xml -->
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" >

    <corners android:radius="10dp" />

    <gradient
        android:centerColor="#7ac19b"
        android:endColor="#588d71"
        android:startColor="#95ceae" />

</shape>

layout - activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" >

        <Button
            android:id="@+id/button1"
            android:layout_width="0dp"
            android:layout_height="48dp"
            android:layout_weight="1"
            android:background="@color/bg_selector"
            android:text="Now" />

        <Button
            android:id="@+id/button2"
            android:layout_width="0dp"
            android:layout_height="48dp"
            android:layout_weight="1"
            android:background="@color/bg_selector"
            android:text="Last" />

        <Button
            android:id="@+id/button3"
            android:layout_width="0dp"
            android:layout_height="48dp"
            android:layout_weight="1"
            android:background="@color/bg_selector"
            android:text="2mnt Ago" />
    </LinearLayout>

    <TextView
        android:id="@+id/text1"
        android:layout_width="fill_parent"
        android:layout_height="0dp"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:layout_weight="1"
        android:padding="@dimen/padding_medium"
        android:text="@string/hello_world"
        tools:context=".MainActivity" />

</LinearLayout>

MainActivity.java

package com.example.sellectorsample;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity implements OnClickListener {
    Button mButton1;
    Button mButton2;
    Button mButton3;
    TextView mText1;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mButton1 = findViewByIdAndCast(R.id.button1);
        mButton2 = findViewByIdAndCast(R.id.button2);
        mButton3 = findViewByIdAndCast(R.id.button3);

        mText1 = findViewByIdAndCast(R.id.text1);

        mButton1.setOnClickListener(this);
        mButton2.setOnClickListener(this);
        mButton3.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        Button button = (Button) v;

//        mButton1.setBackgroundResource(R.color.bg_selector);
//        mButton2.setBackgroundResource(R.color.bg_selector);
//        mButton3.setBackgroundResource(R.color.bg_selector);
//        button.setBackgroundResource(R.drawable.bg_selected);

        // clear state
        mButton1.setSelected(false);
        mButton1.setPressed(false);
        mButton2.setSelected(false);
        mButton2.setPressed(false);
        mButton3.setSelected(false);
        mButton3.setPressed(false);

        // change state
        button.setSelected(true);
        button.setPressed(false);

        mText1.setText("select: " + button.getText());
    }

    @SuppressWarnings("unchecked")
    private <T> T findViewByIdAndCast(int id) {
        return (T) findViewById(id);
    }

}

Thanks for this awesome tutorial, it help me lot to clear my concept. but i'm not able to understand the logic which you done in program to find view of Button and TextView.
when i hover over T , it show me [type parameter T extends object ].
ex. when we call findViewByIdAndCast(int d ). it just passing the id of view in integer. and i could not understand how it know which view is it.
happy christmas and new year in advance ,

Thanks for this tutorial. it help me lot.

tanks