boolean doubleBackToExitPressedOnce = false;@Overridepublic void onBackPressed() {    if (doubleBackToExitPressedOnce) {        super.onBackPressed();        return;    }    this.doubleBackToExitPressedOnce = true;    Toast.makeText(this, "Please click BACK again to exit", Toast.LENGTH_SHORT).show();    new Handler().postDelayed(new Runnable() {        @Override        public void run() {            doubleBackToExitPressedOnce=false;                               }    }, 2000);}

　　感觉此方法比较简单，也很好理解！