Android编程实现图片透明的方法

<?xml version="1.0" encoding="utf-8" ?>
<LinearLayout xmlns:Android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="fill_parent"
android:layout_height="fill_parent">
  <ImageView android:id="@+id/imageView" android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_gravity="center_horizontal"
  android:src="@drawable/icon" />
  <Button android:id="@+id/alpha_plus"
  android:layout_width="wrap_content" android:layout_height="wrap_content"
  android:text="透明度增加" android:layout_gravity="center_horizontal"
  />
  <Button android:id="@+id/alpha_minus"
  android:layout_width="wrap_content" android:layout_height="wrap_content"
  android:text="透明度减少" android:layout_gravity="center_horizontal"
  />
</LinearLayout>

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
public class TOUMINGDU extends Activity {
  private ImageView imageView=null;
  private Button alpha_plus=null; //透明度增加
  private Button alpha_minus=null; //透明度减少
  private int alpha=255; //记录ImageView的透明度
  Boolean flag=true;
  @Override
    public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    imageView=(ImageView)findViewById(R.id.imageView);
    alpha_plus=(Button)findViewById(R.id.alpha_plus);
    alpha_minus=(Button)findViewById(R.id.alpha_minus);
    alpha_plus.setOnClickListener(listener);
    alpha_minus.setOnClickListener(listener);
  }
  private View.OnClickListener listener = new View.OnClickListener(){
    public void onClick(View v) {
      //加加
      if(v==alpha_plus){
        alpha+=10;
        if(alpha>255){
          alpha=255;
        }
        imageView.setAlpha(alpha);
      }
      //减减
      if(v==alpha_minus){
        alpha-=10;
        if(alpha<0){
          alpha=0;
        }
        imageView.setAlpha(alpha);
      }
    }
  };
}