重写   onKeyDown()方法。

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {

    if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){
        if((System.currentTimeMillis()-exitTime) > 2000){
            Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
            exitTime = System.currentTimeMillis();
        } else {
            finish();
            //释放内存
            System.exit(0);
        }
        return true;
    }
    return super.onKeyDown(keyCode, event);
}