listview自己制造bug分析如何解决

网上很多listview加载不同的布局 采用getItemViewType 方式行不通 也不知道他们怎么写的blog.能先自己用一下在发表吗 复制粘贴的东西 不自己分析对错，就这么用吗？

采用所有类型整合到一个item中控制布局显示隐藏的方式，虽然维护不方便，但至少不会出错。

我为了测试，使用了pulltorefreshlistview ,我的blog有讲。

错误源代码：

public class MainActivity extends AppCompatActivity { PullToRefreshListView listView; ImageView imageView; MyAdapter mAdapter; List<MDTO> list = new ArrayList<>(); protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); imageView = (ImageView) findViewById(R.id.iv); imageView.setImageResource(R.drawable.proxyimg); //控件 listView = (PullToRefreshListView) findViewById(R.id.listview); for (int i = 0; i < 50; i++) { MDTO mdto = new MDTO(); mdto.setId(R.drawable.ic_launcher); mdto.setText("text" + i); mdto.setType(i % 3); list.add(mdto); } mAdapter = new MyAdapter(list); listView.setAdapter(mAdapter); //控件模式 listView.setMode(PullToRefreshBase.Mode.PULL_FROM_START); //控件滚动行为 listView.setOnScrollListener(new AbsListView.OnScrollListener() { @Override public void onScrollStateChanged(AbsListView absListView, int i) { } @Override public void onScroll(AbsListView absListView, int i, int i1, int i2) { } }); //控件刷新行为 listView.setOnRefreshListener(new PullToRefreshBase.OnRefreshListener<ListView>() { @Override public void onRefresh(PullToRefreshBase<ListView> refreshView) { } }); } class MyAdapter extends BaseAdapter { List<MDTO> mList = new ArrayList<>(); MyAdapter(List<MDTO> mdtoList) { mList.clear(); this.mList.addAll(mdtoList); for (int i = 0; i < mList.size(); i++) { Log.i("id", mList.get(i).getId() + ""); Log.i("type", mList.get(i).getType() + ""); } notifyDataSetChanged(); } @Override public int getCount() { return mList.size(); } @Override public Object getItem(int i) { return mList.get(i); } @Override public long getItemId(int i) { return i; } @Override public View getView(int i, View view, ViewGroup viewGroup) { int type = mList.get(i).getType(); ViewHolder_One viewHolder_one; if (view == null) { view = LayoutInflater.from(MainActivity.this).inflate(R.layout.item_one, null); viewHolder_one = new ViewHolder_One(); viewHolder_one.iv = (ImageView) view.findViewById(R.id.one_iv); viewHolder_one.tv = (TextView) view.findViewById(R.id.one_tv); } else { viewHolder_one = (ViewHolder_One) view.getTag(); } if (type == 0) { viewHolder_one.iv.setVisibility(View.VISIBLE); viewHolder_one.tv.setVisibility(View.VISIBLE); viewHolder_one.iv.setImageResource(mList.get(i).getId()); viewHolder_one.tv.setText(mList.get(i).getText()); view.setBackgroundColor(Color.RED); } else if (type == 1) { viewHolder_one.tv.setVisibility(View.VISIBLE); viewHolder_one.iv.setVisibility(View.GONE); viewHolder_one.tv.setText(mList.get(i).getText()); view.setBackgroundColor(Color.BLUE); } else { viewHolder_one.iv.setVisibility(View.VISIBLE); viewHolder_one.tv.setVisibility(View.GONE); viewHolder_one.iv.setImageResource(mList.get(i).getId()); view.setBackgroundColor(Color.WHITE); } return view; } class ViewHolder_One { TextView tv; ImageView iv; } } }

当然 熟悉的同学一眼就看出问题所在，如果你忘了那么办呢？

报错处

java.lang.NullPointerException: Attempt to read from field 'android.widget.TextView com.lcldream.www.MainActivity$MyAdapter$ViewHolder_One.tv' on a null object reference at com.lcldream.www.MainActivity$MyAdapter.getView(MainActivity.java:141) at android.widget.HeaderViewListAdapter.getView(HeaderViewListAdapter.java:220) at android.widget.AbsListView.obtainView(AbsListView.java:2575) at android.widget.ListView.makeAndAddView(ListView.java:1956) at android.widget.ListView.fillDown(ListView.java:757) at android.widget.ListView.fillGap(ListView.java:721) at android.widget.AbsListView.trackMotionScroll(AbsListView.java:5733) at android.widget.AbsListView.scrollIfNeeded(AbsListView.java:3780) at android.widget.AbsListView.onTouchMove(AbsListView.java:4277) at android.widget.AbsListView.onTouchEvent(AbsListView.java:4052) at android.widget.ListView.onTouchEvent(ListView.java:4287) at android.view.View.dispatchTouchEvent(View.java:8677) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2517) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2161) at com.handmark.pulltorefresh.library.PullToRefreshListView$InternalListView.dispatchTouchEvent(PullToRefreshListView.java:312) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175) at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523) at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175) at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2539) at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1807) at android.app.Activity.dispatchTouchEvent(Activity.java:2823) at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:71) at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2496) at android.view.View.dispatchPointerEvent(View.java:8883) at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4779) at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4611) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4087) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4140) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4106) at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4243) at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4114) at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4300) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4087) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4140) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4106) at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4114) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4087) at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6598) at a

我们一看 空指针 果断debug,debug是遇到任何问题都首先想到的。

进入了else{ viewHolder_one = (ViewHolder_One) view.getTag(); } 然后执行 对应type 报的空指针 那么至此问题定位就明确了 viewHolder_one 出问题了 我们在看viewholder代码发现没有setTag.添上再运行 很流畅。

那么没有tag为什么会报错呢 第一屏为什么没有报错呢？ 涉及到listview复用机制 listview 为了达到优化目的 使用了复用机制 其实第一屏都是 执行view == null 判断 都是重新加载布局 第二屏开始慢慢移除旧的 此时 view不再为空 所以会重用tag中的内容，类似于日历表 可以上下滑动 但是 中间显示就一个 也就是容量有限 不用的就回收。