从片段到活动：

((YourActivityClassName)getActivity()).yourPublicMethod();

从活动到片段：

FragmentManager fm = getSupportFragmentManager();

//if you added fragment via layout xml
YourFragmentClass fragment = (YourFragmentClass)fm.findFragmentById(R.id.your_fragment_id);
fragment.yourPublicMethod();

如果您通过代码添加片段并tag在添加片段时使用字符串，请findFragmentByTag改用：

YourFragmentClass fragment = (YourFragmentClass)fm.findFragmentByTag("yourTag");