原因:

使用 Fragment 的狀態(tài)保存哆姻，當(dāng)系統(tǒng)內(nèi)存不足台谊，F(xiàn)ragment 的宿主 Activity 回收的時(shí)候麻蹋，
Fragment 的實(shí)例并沒(méi)有隨之被回收皂贩。Activity 被系統(tǒng)回收時(shí)菲驴，會(huì)主動(dòng)調(diào)用 onSaveInstance() 
方法來(lái)保存視圖層（View Hierarchy）会傲，所以當(dāng) Activity 通過(guò)導(dǎo)航再次被重建時(shí)混卵，
之前被實(shí)例化過(guò)的 Fragment 依然會(huì)出現(xiàn)在 Activity 中，此時(shí)的 FragmentTransaction 中的
相當(dāng)于又再次 add 了 fragment 進(jìn)去的恶阴，hide()和show()方法對(duì)之前保存的fragment已經(jīng)
失效了诈胜。綜上這些因素導(dǎo)致了多個(gè)Fragment重疊在一起。

解決方法：

第一種

1.給每個(gè) Fragment 加一個(gè) Tag
2.在 onCreate(Bundle savedInstanceState) 中判斷 Bundle savedInstanceState 是否不為空
3.不為空則進(jìn)行 find Tag冯事，重新給幾個(gè) frament 賦值
這樣子仍是對(duì)之前保存的 fragment 操作焦匈，成功解決了重疊的問(wèn)題。

kotlin寫(xiě)法

class MainActivity : AppCompatActivity() {

    private var mHomeFragment: HomeFragment? = null
    private var mMessageFragment: MessageFragment? = null
    private var mMineFragment: MineFragment? = null
    private var mPreFragment: Fragment? = null
    private lateinit var mTransaction: FragmentTransaction


    override fun onCreate(savedInstanceState: Bundle?) {
        mTransaction = supportFragmentManager.beginTransaction()
        //savedInstanceState不為空, 狀態(tài)恢復(fù)時(shí), 重新查找一遍, 重新賦值
        savedInstanceState?.let {
            mHomeFragment = mHomeFragment ?: supportFragmentManager.findFragmentByTag("0") as HomeFragment
            mMessageFragment = mMessageFragment ?: supportFragmentManager.findFragmentByTag("1") as MessageFragment
            mMineFragment = mMineFragment ?: supportFragmentManager.findFragmentByTag("2") as MineFragment
        }
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initData()
    }

    private fun initData() {
        //選中監(jiān)聽(tīng)
        mRgBottomMenu.setOnCheckedChangeListener { _, checkedId ->
            val transaction = supportFragmentManager.beginTransaction()
            mPreFragment?.let {
                transaction.hide(it)
            }
            when (checkedId) {
                R.id.mRbHome -> //home界面
                    mHomeFragment =
                        mHomeFragment?.apply { showFragment(transaction, this) }
                        ?: addFragment(transaction, 0) as HomeFragment

                R.id.mRbMessage ->//消息界面
                    mMessageFragment =
                        mMessageFragment?.apply { showFragment(transaction, this) }
                            ?: addFragment(transaction, 1) as MessageFragment

                R.id.mRbMine -> //我的界面
                    mMineFragment =
                        mMineFragment?.apply { showFragment(transaction, this) }
                            ?: addFragment(transaction, 2) as MineFragment
            }
            transaction.commitAllowingStateLoss()
        }
        mRbHome.isChecked = true
    }

    //顯示已經(jīng)存在的fragment
    private fun showFragment(transaction: FragmentTransaction, fragment: Fragment) {
        transaction.show(fragment)
        mPreFragment = fragment
        Log.d("test:: ", "show: ${fragment::class.java.simpleName} - $fragment")
    }

    //添加第一次創(chuàng)建實(shí)例的fragment
    private fun addFragment(transaction: FragmentTransaction, type: Int): Fragment {
        val fragment = when (type) {
            0 -> HomeFragment()
            1 -> MessageFragment()
            else -> MineFragment()
        }
        transaction.add(R.id.mFlRoot, fragment, "$type")
        mPreFragment = fragment
        Log.d("test:: ", "add: ${fragment::class.java.simpleName} - $fragment")
        return fragment
    }
}

java寫(xiě)法

public class MainActivity extends AppCompatActivity implements RadioGroup.OnCheckedChangeListener {
    private TextView mTvTitle;
    private LinearLayout mLlActionbar;
    private FrameLayout mFlRoot;
    private RadioButton mRbHome;
    private RadioButton mRbMessage;
    private RadioButton mRbMine;
    private RadioGroup mRgBottomMenu;


    private HomeFragment mHomeFragment;
    private MessageFragment mMessageFragment;
    private MineFragment mMineFragment;
    private Fragment mPreFragment;
    private FragmentManager mFragmentManager;


    @Override

    protected void onCreate(@Nullable Bundle savedInstanceState) {
        mFragmentManager = getSupportFragmentManager();
        super.onCreate(savedInstanceState);
        if (savedInstanceState != null) {
            mHomeFragment = (HomeFragment) mFragmentManager.findFragmentByTag("HomeFragment");
            mMessageFragment = (MessageFragment) mFragmentManager.findFragmentByTag("MessageFragment");
            mMineFragment = (MineFragment) mFragmentManager.findFragmentByTag("MineFragment");
        }
        setContentView(R.layout.activity_main);
        initView();
        initData();
        initListener();
    }
    //可要可不要
    @Override
    public void onAttachFragment(@NonNull Fragment fragment) {
//        if (mHomeFragment == null && fragment instanceof HomeFragment) {
//            mHomeFragment = (HomeFragment) fragment;
//        } else if (mMessageFragment == null && fragment instanceof MessageFragment) {
//            mMessageFragment = (MessageFragment) fragment;
//        } else if (mMineFragment == null && fragment instanceof MineFragment) {
//            mMineFragment = (MineFragment) fragment;
//        }
    }

    private void initView() {

        mTvTitle = findViewById(R.id.mTvTitle);
        mLlActionbar = findViewById(R.id.mLlActionbar);
        mFlRoot = findViewById(R.id.mFlRoot);
        mRbHome = findViewById(R.id.mRbHome);
        mRbMessage = findViewById(R.id.mRbMessage);
        mRbMine = findViewById(R.id.mRbMine);
        mRgBottomMenu = findViewById(R.id.mRgBottomMenu);
    }

    private void initData() {

    }

    private void initListener() {
        mRgBottomMenu.setOnCheckedChangeListener(this);
//        mRgBottomMenu.check(R.id.mRbHome);
        mRbHome.setChecked(true);
    }


    @Override
    public void onCheckedChanged(RadioGroup group, int checkedId) {
        FragmentTransaction transaction = mFragmentManager.beginTransaction();
        if (mPreFragment != null) {
            transaction.hide(mPreFragment);
        }
        switch (checkedId) {
            case R.id.mRbHome:
                if (mHomeFragment == null) {
                    mHomeFragment = new HomeFragment();
                    transaction.add(R.id.mFlRoot, mHomeFragment, "HomeFragment");
                } else {
                    transaction.show(mHomeFragment);
                }
                mPreFragment = mHomeFragment;
                break;
            case R.id.mRbMessage:
                if (mMessageFragment == null) {
                    mMessageFragment = new MessageFragment();
                    transaction.add(R.id.mFlRoot, mMessageFragment, "MessageFragment");
                } else {
                    transaction.show(mMessageFragment);
                }
                mPreFragment = mMessageFragment;
                break;
            case R.id.mRbMine:
                if (mMineFragment == null) {
                    mMineFragment = new MineFragment();
                    transaction.add(R.id.mFlRoot, mMineFragment, "MineFragment");
                } else {
                    transaction.show(mMineFragment);
                }
                mPreFragment = mMineFragment;
                break;
        }
        transaction.commitAllowingStateLoss();

    }
}

第二種

Activity 中的 onSaveInstanceState() 里面有一句super.onSaveInstanceState(outState);昵仅，
Google 對(duì)于這句話(huà)的解釋是 “Always call the superclass so it can save the view hierarchy state”缓熟，
大概意思是“總是執(zhí)行這句代碼來(lái)調(diào)用父類(lèi)去保存視圖層的狀態(tài)”。通過(guò)注釋掉這句話(huà)摔笤，
這樣主 Activity 因?yàn)榉N種原因被回收的時(shí)候就不會(huì)保存之前的 fragment state够滑，
也可以成功解決重疊的問(wèn)題。

@Override
    protected void onSaveInstanceState(@NonNull Bundle outState) {
//        super.onSaveInstanceState(outState);
    }

fragment 回退棧有關(guān)問(wèn)題

通常使用fragmentManager.beginTransaction().addToBackStack(null);  

但是也經(jīng)常會(huì)遇到說(shuō)“某個(gè)fragment”已經(jīng)被添加的錯(cuò)誤吕世，這往往是我們?cè)趯?duì)fragment進(jìn)行返回處理時(shí)出現(xiàn)問(wèn)題彰触。這樣的問(wèn)題最常出現(xiàn)在我們點(diǎn)擊界面進(jìn)行跳轉(zhuǎn)的時(shí)候。

一般情況命辖，當(dāng)我們點(diǎn)擊導(dǎo)航欄進(jìn)行界面切換之后况毅，之前back stack中保存的fragment也就不再需要保留了。用下面的語(yǔ)句可以將back stack清空尔艇，同時(shí)也可避免上述錯(cuò)誤的出現(xiàn)尔许。

FragmentManager fragmentManager=getSupportFragmentManager();  
fragmentManager.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);