<uses-permission android:name="android.permission.INTERNET"/>
String PicUrlCogs = "http://latex.codecogs.com/gif.latex?";
Url = new URL(PicUrlCogs + editText.getText().toString().replace(" ",""));
new MyDownloadTask().execute();         // 执行Http请求
while(!finishFlag) {}              // 等待数据接收完毕
imageView.setImageBitmap(pngBM);        // 显示图片
finishFlag = false;               // 标识回位

 protected Void doInBackground(Void... params) {
      try {
        URL picUrl = Url;              // 获取URL地址
         HttpURLConnection conn = (HttpURLConnection) picUrl.openConnection();
//        conn.setConnectTimeout(1000);       // 建立连接
//        conn.setReadTimeout(1000);
        conn.connect();               // 打开连接
        if (conn.getResponseCode() == 200) {     // 连接成功，返回数据
          InputStream ins = conn.getInputStream(); // 将数据输入到数据流中
          pngBM = BitmapFactory.decodeStream(picUrl.openStream()); // 解析数据流
          finishFlag = true;            // 数据传输完毕标识
          ins.close();               // 关闭数据流
        }
      } catch (Exception e) {
        e.printStackTrace();
      }
      return null;
    }

btnPreview.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        try {
          Url = new URL(PicUrlCogs + editText.getText().toString().replace(" ","")); // 转换字符串
          new MyDownloadTask().execute();         // 执行Http请求
          while(!finishFlag) {}              // 等待数据接收完毕
          imageView.setImageBitmap(pngBM);        // 显示图片
          finishFlag = false;               // 标识回位
        } catch (Exception e) {
          e.printStackTrace();
        }
      }
    });

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
public class MainActivity extends Activity {
  private String PicUrlGoogle = "http://chart.apis.google.com/chart?cht=tx&chl=";
  private String PicUrlCogs = "http://latex.codecogs.com/gif.latex?";
  private Button btnPreview;
  private EditText editText;
  private ImageView imageView;
  private Bitmap pngBM;
  private URL Url;
  private boolean finishFlag = false;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    btnPreview = (Button) findViewById(R.id.btnPreview);
    imageView = (ImageView) findViewById(R.id.imageView);
    editText = (EditText) findViewById(R.id.editText);
    btnPreview.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        try {
          Url = new URL(PicUrlCogs + editText.getText().toString().replace(" ","")); // 转换字符串
          new MyDownloadTask().execute();         // 执行Http请求
          while(!finishFlag) {}              // 等待数据接收完毕
          imageView.setImageBitmap(pngBM);        // 显示图片
          finishFlag = false;               // 标识回位
        } catch (Exception e) {
          e.printStackTrace();
        }
      }
    });
  }
  @Override
  public void onDestroy() {
    super.onDestroy();
  }
  class MyDownloadTask extends AsyncTask<Void, Void, Void> {
    protected void onPreExecute() {
      //display progress dialog.
    }
    protected Void doInBackground(Void... params) {
      try {
        URL picUrl = Url;              // 获取URL地址
        HttpURLConnection conn = (HttpURLConnection) picUrl.openConnection();
//        conn.setConnectTimeout(1000);       // 建立连接
//        conn.setReadTimeout(1000);
        conn.connect();               // 打开连接
        if (conn.getResponseCode() == 200) {     // 连接成功，返回数据
          InputStream ins = conn.getInputStream(); // 将数据输入到数据流中
          pngBM = BitmapFactory.decodeStream(picUrl.openStream()); // 解析数据流
          finishFlag = true;            // 数据传输完毕标识
          ins.close();               // 关闭数据流
        }
      } catch (Exception e) {
        e.printStackTrace();
      }
      return null;
    }
    protected void onPostExecute(Void result) {
      // dismiss progress dialog and update ui
    }
  }
}