怎么添加Android返回键的退出功能

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@Overridepublic boolean dispatchKeyEvent(KeyEvent event) { if(event.getKeyCode() == KeyEvent.KEYCODE_BACK){ if (event.getAction() == KeyEvent.ACTION_DOWN && event.getRepeatCount() == 0) { if ((System.currentTimeMillis() - exitTime) > 2000) { Toast.makeText(MAInActivity.this, "Press agAIn to exit the program", Toast.LENGTH_SHORT).show(); exitTime = System.currentTimeMillis(); } else { // Perform logout operation // System.exit(0); finish(); } } return true; } return super.dispatchKeyEvent(event);}