【Android基础】点击Back键退出应用程序

//第一种方法（弹出对话框）
	@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		// TODO Auto-generated method stub
		switch (keyCode) {
		case KeyEvent.KEYCODE_BACK:
			AlertDialog.Builder build = new AlertDialog.Builder(this);
			build.setTitle("系统提示").setMessage("确定要退出吗？");
			build.setPositiveButton("确定",
					new DialogInterface.OnClickListener() {
						@Override
						public void onClick(DialogInterface dialog, int which) {
							finish();
						}
					});
			build.setNegativeButton("取消",
					new DialogInterface.OnClickListener() {
						@Override
						public void onClick(DialogInterface dialog, int which) {
						}
					}).show();
			break;
		}
		return super.onKeyDown(keyCode, event);
	}

//第二种方法（再按一次退出程序）
	private long exitTime = 0;

	@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();
				System.exit(0);
			}
			return true;
		}
		return super.onKeyDown(keyCode, event);
	}