Android中ImageView的使用方法

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:gravity="center">
    <Button
        android:id="@+id/addAlpha"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:text="增加图片透明度"
        />
    <Button
        android:id="@+id/downAlpha"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="减小图片透明度"
        />
    <Button
        android:id="@+id/next"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="切换图片"
        />
</LinearLayout>
    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="280dp"
        android:layout_marginTop="100dp"
        android:scaleType="fitCenter"
        android:src="@drawable/spring"
        />
</LinearLayout>

public class MainActivity extends AppCompatActivity {
    private Button addAlpha;
    private Button downAlpha;
    private Button next;
    private ImageView imageView1;
    int [] images = new int[]{ //用数组存储春，夏，秋，冬四张图片
            R.drawable.spring,
            R.drawable.summer,
            R.drawable.fall,
            R.drawable.winter
    } ;
    int currentImage = 0 ; //定义当前默认显示的图片
    int alpha = 255 ;//定义图片起始透明度
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_imageview);
        addAlpha = (Button) findViewById(R.id.addAlpha);
        downAlpha = (Button) findViewById(R.id.downAlpha);
        next = (Button) findViewById(R.id.next);
        imageView1 = (ImageView) findViewById(R.id.imageView1);
        //对增加图片透明度按钮设置监听事件
        addAlpha.setOnClickListener(new View.OnClickListener() {
            @TargetApi(Build.VERSION_CODES.JELLY_BEAN)
            @Override
            public void onClick(View v) {
                if(alpha >= 255){ //255是透明度上线
                    alpha = 255 ;
                }
                else{ //每点击增加透明度按钮，透明度增加20
                    alpha += 20 ;
                }
                imageView1.setImageAlpha(alpha); //为图片设置透明度
            }
        });
        //对减少透明度按钮设置监听事件
        downAlpha.setOnClickListener(new View.OnClickListener() {
            @TargetApi(Build.VERSION_CODES.JELLY_BEAN)
            @Override
            public void onClick(View v) {
                if(alpha <= 0){ //透明度下限
                    alpha = 0 ;
                }
                else{
                    alpha -= 20 ;
                }
                imageView1.setImageAlpha(alpha); //为图片设置透明度
            }
        });
        //对切换图片按钮设置监听事件
        next.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //控制显示下一张图片
                imageView1.setImageResource(images[++ currentImage % images.length]);
            }
        });
    }
}