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();

         }

     }

 }