Android Toast 提示按两次返回键退出

 public class MainActivity extends Activity {

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);

     }

     long startTime;

     @Override
     public void onBackPressed() {
         if (System.currentTimeMillis() - startTime < 2000) {
             super.onBackPressed();
         } else {
             startTime = System.currentTimeMillis();

             Toast toast = new Toast(this);

             LinearLayout layout = new LinearLayout(this);
             layout.setOrientation(LinearLayout.VERTICAL);
             layout.setBackgroundColor(Color.YELLOW);

             TextView tv1 = new TextView(this);
             tv1.setText("提示");
             tv1.setGravity(Gravity.CENTER);

             TextView tv2 = new TextView(this);
             tv2.setText("再按一次Back键退出");
             tv2.setGravity(Gravity.CENTER);

             layout.addView(tv1);
             layout.addView(tv2);

             toast.setView(layout);
             toast.setGravity(Gravity.CENTER_VERTICAL | Gravity.FILL_HORIZONTAL, 0, -200);
             toast.setDuration(Toast.LENGTH_SHORT);
             toast.show();

         }

     }

 }

MainActivity