androidj基础：从网上下载图片

 <ImageView
       android:id="@+id/ImageView"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:layout_centerInParent="true"
       android:background="#ffffff"

       />
<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    >
   <Button
       android:id="@+id/bt1"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:layout_weight="1"
       android:text="下载图片"
       />

   <Button
       android:id="@+id/bt2"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:layout_weight="1"
       android:layout_toRightOf="@+id/bt1"

       android:text="读取图片"
       />

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.administrator.httpexe">
    <uses-permission android:name="android.permission.INTERNET"/>

</manifest>

import...//import语句自动生成，略
public class MainActivity extends AppCompatActivity {

    ImageView imageView;
    Button button;
    Button button1; @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        imageView = (ImageView) findViewById(R.id.ImageView);
        button = (Button) findViewById(R.id.bt1);
        button1=(Button)findViewById(R.id.bt2);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                MyTask task = new MyTask();//创建异步任务对象
                task.execute("https://www.baidu.com/img/bd_logo1.png");//创建下载任务
            }
        });
        button1.setOnClickListener(new View.OnClickListener() {//读取图片

            @Override
            public void onClick(View v) {
                Bitmap bmp=null;
                try{
                    FileInputStream fis= openFileInput("a.png");
                    bmp=BitmapFactory.decodeStream(fis);
                    if(bmp!=null) {
                        imageView.setImageBitmap(bmp);
                    }
                }catch (Exception ex) {
                }
            }
        });
    }
        public class MyTask extends AsyncTask<String, Void, Bitmap> {   //bmp位图,泛型
        @Override
        protected Bitmap doInBackground(String... params) { //真正的下载
            String urlStr = params[0];  //取第一个变参
            Bitmap bmp = null;
            try {
                URL url = new URL(urlStr);
                bmp=BitmapFactory.decodeStream(url.openStream()); //位图工厂,返回bitmap
                InputStream is= url.openStream();
                byte[] buffer=new byte[4096];//byte[]缓冲区
                FileOutputStream fos= MainActivity.this.openFileOutput("a.png",MainActivity.MODE_PRIVATE);
                int hasRead=is.read(buffer);
                while (hasRead>0){
                    fos.write(buffer,0,hasRead);//文件大小，起始位置，读取的量
                    hasRead=is.read(buffer);
                }
                fos.close();//关闭
                is.close();//关闭
            }
            catch (Exception ex) {
            }finally {
            }
            return bmp;
        }
        @Override
        protected void onPostExecute(Bitmap bitmap) {
            if(bitmap!=null){
                imageView.setImageBitmap(bitmap);
            }
        }
    }