由于自定义 ListView 中的 NullPointerException 导致应用程序崩溃答案

【问题标题】：Application crashing because of a NullPointerException in a custon ListView由于自定义 ListView 中的 NullPointerException 导致应用程序崩溃
【发布时间】：2014-04-19 15:00:17
【问题描述】：

我正在使用一个列表视图，单击它会打开一个对话框，其中包含一个编辑文本和一个按钮。单击按钮时，在 editText 中输入的值将保存在之前按下的 listview 项中的 textView 中。问题是，如果我重新打开应用程序，则不再保存该值。我尝试使用 sharedPrefences 保存它，但它崩溃并显示 nullpointerexception 并且无法处理。

NoteAdapter.java：

 package com.cngcnasaud.orar;

    import java.util.Arrays;

    import android.app.Dialog;
    import android.content.Context;
    import android.util.Log;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.View.OnClickListener;
    import android.view.ViewGroup;
    import android.widget.BaseAdapter;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.ImageView;
    import android.widget.TextView;

    public class NoteAdapter extends BaseAdapter {

        String[] result;
        Context context;
        int[] imageId;
        private static LayoutInflater inflater = null;
        private Dialog dialog;
        String[] savedEntries;
        String[] saved = null;

        public NoteAdapter(Note note, String[] saved, String[] prgmNameList) {
            // TODO Auto-generated constructor stub
            result = prgmNameList;
            context = note;
            inflater = (LayoutInflater) context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

            if (saved == null) {
                savedEntries = new String[result.length];
                Arrays.fill(savedEntries, "");
            } else
                savedEntries = saved;

        }

        @Override
        public int getCount() {
            // TODO Auto-generated method stub
            return result.length;
        }

        @Override
        public Object getItem(int position) {
            // TODO Auto-generated method stub
            return savedEntries[position];
        }

        @Override
        public long getItemId(int position) {
            // TODO Auto-generated method stub
            return position;
        }

        public class Holder {
            TextView tv;
            ImageView img;
            public TextView text;
        }

        @Override
        public View getView(final int position, View convertView, ViewGroup parent) {
            // TODO Auto-generated method stub

            final Holder holder = new Holder();
            View rowView;
            rowView = inflater.inflate(R.layout.note_items, null);
            holder.tv = (TextView) rowView.findViewById(R.id.textView1);
            holder.text = (TextView) rowView.findViewById(R.id.textView2);
            holder.text.setText(savedEntries[position]);
            holder.img = (ImageView) rowView.findViewById(R.id.imageView1);

            rowView.setOnClickListener(new OnClickListener() {
                @Override
                public void onClick(View v) {
                    // TODO Auto-generated method stub
                    dialog = new Dialog(context);
                    dialog.setContentView(R.layout.dialog);
                    dialog.setTitle("Materie:" + result[position]);

                    final EditText txtMode = (EditText) dialog
                            .findViewById(R.id.dialog);
                    Button btnSave = (Button) dialog.findViewById(R.id.bsave);

                    btnSave.setOnClickListener(new OnClickListener() {

                        @Override
                        public void onClick(View v) {

                            String data = txtMode.getText().toString();
                            holder.text.setText(data);

                            savedEntries[position] = data;

                            dialog.dismiss();
                            Log.d("data", data);
                        }
                    });
                    dialog.show();
                }

            });
            return rowView;
        }

    }

logcat：

04-19 04:14:32.130: E/AndroidRuntime(947): FATAL EXCEPTION: main
04-19 04:14:32.130: E/AndroidRuntime(947): Process: com.cngcnasaud.orar, PID: 947
04-19 04:14:32.130: E/AndroidRuntime(947): java.lang.NullPointerException
04-19 04:14:32.130: E/AndroidRuntime(947):  at com.cngcnasaud.orar.NoteAdapter.getView(NoteAdapter.java:76)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.AbsListView.obtainView(AbsListView.java:2263)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.ListView.makeAndAddView(ListView.java:1790)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.ListView.fillDown(ListView.java:691)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.ListView.fillFromTop(ListView.java:752)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.ListView.layoutChildren(ListView.java:1630)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.AbsListView.onLayout(AbsListView.java:2091)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at com.android.internal.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:374)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.View.layout(View.java:14817)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewGroup.layout(ViewGroup.java:4631)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:1987)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1744)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5670)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.Choreographer.doCallbacks(Choreographer.java:574)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.Choreographer.doFrame(Choreographer.java:544)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.os.Handler.handleCallback(Handler.java:733)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.os.Handler.dispatchMessage(Handler.java:95)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.os.Looper.loop(Looper.java:136)
04-19 04:14:32.130: E/AndroidRuntime(947):  at android.app.ActivityThread.main(ActivityThread.java:5017)
04-19 04:14:32.130: E/AndroidRuntime(947):  at java.lang.reflect.Method.invokeNative(Native Method)
04-19 04:14:32.130: E/AndroidRuntime(947):  at java.lang.reflect.Method.invoke(Method.java:515)
04-19 04:14:32.130: E/AndroidRuntime(947):  at com.android.internal.os.

NoteAdapter.java 中的第 76 行：

holder.text.setText(savedEntries[position]);

适配器的构造函数 - Note.java：

package com.cngcnasaud.orar;

import java.util.ArrayList;

import android.os.Bundle;
import android.app.Activity;
import android.content.Context;
import android.content.SharedPreferences;
import android.view.Menu;

import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.TextView;

public class Note extends Activity {

    public static final ListAdapter NoteAdapter = null;
    ListView lv;
    Context context;
    ArrayList<?> prgmName;
    TextView text;

    public static String[] prgmNameList = { "Romana   - ", "Matematica   - ",
            "Lb. Engleza   - ", "Lb. Germana/Franceza - ", "Istorie   - ",
            "Geografie   - ", "Biologie   - ", "Fizica   - ", "Ed. Fizica   - " };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.note_listview);

        text = (TextView) findViewById(R.id.textView2);

        context = this;

        lv = (ListView) findViewById(R.id.listView);
        lv.setAdapter(new NoteAdapter(this, prgmNameList, null));

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    protected void onStop() {
        // TODO Auto-generated method stub

        NoteAdapter adapter = (NoteAdapter) lv.getAdapter();

        // Variable is public for clarity.
        String toSave = EncodeDecode.encode(adapter.savedEntries);
        SharedPreferences.Editor editor = getSharedPreferences("LV Data",
                MODE_PRIVATE).edit();
        editor.putString("TVEntries", toSave);
        editor.commit();
    }

    @Override
    protected void onResume() {
        // TODO Auto-generated method stub

        SharedPreferences prefs = getSharedPreferences("LV Data", MODE_PRIVATE);
        String encoded = prefs.getString("TVEntries", "");

        String[] entries;
        if (encoded.equals(""))
            entries = null;
        else
            entries = EncodeDecode.decode(encoded);

        NoteAdapter adapter = (NoteAdapter) lv.getAdapter();
        adapter.savedEntries = entries;
        lv.setAdapter(adapter);

        super.onResume();
    }

}

EncodeDecode.java：

package com.cngcnasaud.orar;

import java.util.ArrayList;
import java.util.Arrays;
import java.lang.StringBuffer;

public class EncodeDecode
{

  public static void main(String[] args)
  {
    //Test functionality.
    String[] toEncode = {"a?b","c,d,", "asdas,dasd?,,,,,", "asdsad*\t      "};
    String encodedString = encode(toEncode);
    String[] decodedArray = decode(encodedString);
    System.out.println("Original Array: " + Arrays.toString(toEncode));
    System.out.println("Encoded String: " + encodedString);
    System.out.println("Decoded Array: " + Arrays.toString(decodedArray));
    System.out.println();
    System.out.println("Testing equality of the arrays.....");
    if(encode(toEncode).equals(encode(decodedArray)))
      System.out.println("Passed!");
    else
       System.out.println("Failed :(!");
  }

  /**  
   *  Function: encode
   *  --------------------
   *  Encode a String array into a single String.
   *  
   *  toEncode: the String array to be encoded.
   *
   *  returns: the encoded string.
   */
  public static String encode(String[] toEncode)
  {
    StringBuffer strBuf = new StringBuffer();
    for(int i = 0; i < toEncode.length; i++)
    {
      /*Retrieve a representation of the current string element that is
      easier to modify.*/
      StringBuffer curString = new StringBuffer(toEncode[i]);
      for(int j = 0; j < curString.length(); j++)
        //If we see a "break" character, prepend another "break" character to it.
        if(curString.charAt(j) == '?')
        {
          curString.insert(j, '?');
          j++;
        }
        //If we see a delimiter character, prepend "break" character to it.
        else if(curString.charAt(j) == ',')
        {
          curString.insert(j, '?');
          j++;
        }

      //Add the delimiter to the encoded string.
      strBuf.append(curString + ",");

    }
    return strBuf.toString();
  }

  /**  
   *   Function: decode
   *  --------------------
   *  Decode an encoded string into its original array..
   *  
   *  toDecode: the String to be decoded.
   *
   *  returns: the decoded String array.
   */
  public static String[] decode(String toDecode)
  {
    /*We utilize an ArrayList, since the exact number of 
    string elements is inderterminate*/
    ArrayList<String> decoded = new ArrayList<String>();

    /*Retrieve a representation of the current string element that is
    easier to modify.*/
    StringBuffer encodedStr = new StringBuffer(toDecode);

    //The starting point of the current isolated string to add/append.
    int currIndex = 0;

    for(int i = 0; i < encodedStr.length(); i++)
    {
      /*If we see the "break character" remove it, and skip
      the following character*/
      if(encodedStr.charAt(i) == '?')
        encodedStr.deleteCharAt(i);

      /*If we see a delimiter, add what we have amassed
      so far to the list.*/
      else if(encodedStr.charAt(i) == ',')
      {
        decoded.add(encodedStr.substring(currIndex, i));

        //Update the starting point.
        currIndex = i + 1;
      }
    }

    String[] toReturn = new String[decoded.size()];
    return decoded.toArray(toReturn);
  }
}

EncodeDecode:java: 中的第 39 行：

for(int i = 0; i < toEncode.length; i++)

【问题讨论】：

你应该看看 note_items.xml：是否有一个 id 为 textView2 的文本视图？之前的代码中也可能已将 savedEntries 设置为 null
当然我检查了 id 是否存在并且它是正确的。它存在并且是正确的。我也尝试设置 savedEntries string = null，但仍然是同样的崩溃和错误。更明确地说，应用程序启动时 textView 应该为 null，然后它应该由用户填充值。这不是问题吗？谢谢！！！
id 在 note_items 布局内？无论如何在抛出 NPE 的行之前添加一个日志并检查所有值。
是的。你能解释一下如何添加日志吗？
您可以使用 android Log 或使用 System.out.println 或者在错误行放置断点并运行调试器。

标签： android android-listview nullpointerexception sharedpreferences android-dialog

【解决方案1】：

根据你的调试savedEntries == null。这就是这里出现 NullPointerException 的原因：savedEntries[position]

既然您说这些数组值将由用户稍后提供，您可以例如添加一个空检查来避免此问题，如下所示：

if (savedEntries != null) {
    holder.text.setText(savedEntries[position]);
} else {
    holder.text.setText(""); 
}

更新：

看起来您尝试在 2 个不同的布局 xml 中查找 ID 为 textView2 的文本视图！一个在note_items 布局中，另一个在note_listview 中。

参见笔记活动：

    setContentView(R.layout.note_listview);

    text = (TextView) findViewById(R.id.textView2);

参见NoteAdapter：

    rowView = inflater.inflate(R.layout.note_items, null);
    holder.tv = (TextView) rowView.findViewById(R.id.textView1);
    holder.text = (TextView) rowView.findViewById(R.id.textView2);

==> 甚至不看 xml 布局（你没有包括在内）我可以说这不好。原因是您在不同的布局中有 textviews id textView2，这很糟糕，因为 ids 必须是唯一的，或者在 note_items 布局中根本没有 textView2。

【讨论】：

在 else 中仍然崩溃和空指针：} else { holder.text.setText("");
Uy :-) 我是否理解您的评论错误，savedEntries == null while text & holder != null？为了确保您可以在错误行之前添加它：Log.d("debugging NPE", "text: " + holder.text + " savedEntries: " + savedEntries); 并让我知道它打印的内容。因为最后看起来 text 是空的，虽然你说 id 在 note_items 布局中是正确的.. 很奇怪。
看起来它可以工作，但我仍然在 EncodeDecode 活动的第 39 行得到 1 个 NPE。我不能改变我的共享偏好代码而不是使用这个类吗？你不能帮我解决这个问题吗？非常感谢！ ** 我在第一篇文章中编辑并添加了 EncodeDecode.java。谢谢！

【解决方案2】：

lv.setAdapter(new NoteAdapter(this, prgmNameList, null)) 是问题所在。 NoteAdapter 的构造函数期望 第三个 参数是程序列表。

此外，我建议在Note 中创建一个空数组，并将其作为NoteAdapter 的第二个参数传入。然后，在你的onStop 中，你可以简单地说String toSave = EncodeDecode.encode(the_array_you_created); 而不是String toSave = EncodeDecode.encode(adapter.savedEntries);

编辑

public class Note extends Activity {

...
private String[] savedArray;
...

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    savedArray = new String[prgmNameList.length];
    lv.setAdapter(new NoteAdapter(this, savedArray, prgmNameList));

}

...

@Override
protected void onStop() {
    String toSave = EncodeDecode.encode(savedArray);
    SharedPreferences.Editor editor = getSharedPreferences("LV Data",
            MODE_PRIVATE).edit();
    editor.putString("TVEntries", toSave);
    editor.commit();
}
....

}

【讨论】：

我不明白你的意思。您能否更具体地说明如何创建该空数组并将其传递给 NoteAdapter 的第二个参数？
@Andrei，添加了代码。我的观点是，在我看来，您保存内容的数组是作为 NoteAdapter 的第二个参数传入的，而您在 Activity 中初始化的程序列表是第三个参数。
@Andrei，你的意思是不工作还是现在工作？ :)
我在 textView 解决了 NPE，但现在我在 EncodeDecode 类的第 39 行得到了另一个 NPE。我更新了第一个帖子，你可以看看吗？谢谢！
嗯，看起来您传递给 encode 方法的数组仍然为空。您能告诉我您对 Note 类做了哪些更改吗？