返回键处理

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {

        new alertDialog.Builder(this)
                .setMessage("确定退出系统吗？")
                .setNegativeButton("取消",
                        (dialog, which) -> {
                        })
                .setPositiveButton("确定",
                        (dialog, whichButton) -> android.os.Process.killProcess(android.os.Process.myPid())).show();

        return true;
    } else {
        return super.onKeyDown(keyCode, event);
    }
}