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();

        }

    }

}