android 欢迎界面的淡入效果

package com.example.spinnertest;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.Window;
import android.view.WindowManager;
import android.view.animation.AlphaAnimation;
import android.view.animation.Animation;
import android.view.animation.Animation.AnimationListener;

public class WelcomeActivity extends Activity{

@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE); 
getWindow().setFlags(WindowManager.LayoutParams. FLAG_FULLSCREEN , 
WindowManager.LayoutParams. FLAG_FULLSCREEN);
setContentView(R.layout.welcome_activity);
final View view = findViewById(R.id.welcome_layout_id);
AlphaAnimation aa = new AlphaAnimation(0.3f,1.0f);
aa.setDuration(2000);
view.startAnimation(aa);
aa.setAnimationListener(new AnimationListener()
{
@Override
public void onAnimationEnd(Animation arg0) {
Intent intent = new Intent(WelcomeActivity.this,MainActivity.class);
startActivity(intent);
finish();
}
@Override
public void onAnimationRepeat(Animation animation) {}
@Override
public void onAnimationStart(Animation animation) {}

});
}


}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/welcome_layout_id"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >
    <TextView 
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="Hello World"
        android:textAppearance="@android:style/TextAppearance.Large"/>
    

</LinearLayout>