//首先需要先实例好n个全局Fragment

//private  Fragment  currentFragment=new Fragment();（全局）

private  FragmentTransaction switchFragment(Fragment targetFragment) {

    FragmentTransaction transaction = getSupportFragmentManager()
            .beginTransaction();
    if (!targetFragment.isAdded()) {
        //第一次使用switchFragment()时currentFragment为null，所以要判断一下
        if (currentFragment != null) {
            transaction.hide(currentFragment);
            }
        transaction.add(R.id.fragment, targetFragment,targetFragment.getClass().getName());

        } else {
            transaction
                    .hide(currentFragment)
                    .show(targetFragment);


        }
        currentFragment = targetFragment;
       return   transaction;
    }

在点击切换Fragment时调用：

@Override
public void onTabSelected(@IdRes int tabId) {
        if (tabId == R.id.tab_one){
            switchFragment(first).commit();
        }
        if (tabId == R.id.tab_two){
            switchFragment(second).commit();
        }
        if (tabId == R.id.tab_three){
            switchFragment(third).commit();
        }
    }

如果fragment数据更新了怎么办?
onHiddenChanged()