错误 不兼容的类型 ProductFragment无法转换为Fragment transactionreplaceRidmain_content productFragment;

解决方法：

将ProductFragment改为Fragment，即可解决此错误：

Fragment productFragment = new ProductFragment();
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.replace(R.id.main_content, productFragment);
transaction.addToBackStack(null);
transaction.commit();