android - fatal error - 空指针异常

Question

我收到一个空指针错误，我找不到它有什么问题。这是 Activity 代码

 package ie.gmit.gmitapp;

 import android.os.Bundle;

 import android.app.ListActivity;
 import android.content.Context;

 import android.view.View;
 import android.view.ViewGroup;
 import android.widget.BaseAdapter;
 import android.widget.LinearLayout;
 import android.widget.ListView;
 import android.widget.TextView;


 public class BusinessActivity extends ListActivity {

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //setContentView(R.layout.activity_bussiness);

    //use our own list adapter
     setListAdapter(new BusinessListAdapter(this));
}

protected void onListItemClick(ListView l, View v, int position, long id) 
{    
   ((BusinessListAdapter)getListAdapter()).toggle(position);
}

  /**
 * A sample ListAdapter that presents content
 * from arrays of text and titles.
 *
 */
private class BusinessListAdapter extends BaseAdapter {
    public  BusinessListAdapter(Context context)
    {
        mContext = context;
    }
    /**
     * The number of items in the list is determined by the number of speeches
     * in our array.
     * 
     * @see android.widget.ListAdapter#getCount()
     */
    public int getCount() {
        return mTitles.length;
    }

    /**
     * Since the data comes from an array, just returning
     * the index is sufficent to get at the data. If we
     * were using a more complex data structure, we
     * would return whatever object represents one 
     * row in the list.
     * 
     * @see android.widget.ListAdapter#getItem(int)
     */
    public Object getItem(int position) {
        return position;
    }

    /**
     * Use the array index as a unique id.
     * @see android.widget.ListAdapter#getItemId(int)
     */
    public long getItemId(int position) {
        return position;
    }

    /**
     * Make a DescriptionView to hold each row.
     * @see android.widget.ListAdapter#getView(int, android.view.View, android.view.ViewGroup)
     */
    public View getView(int position, View convertView, ViewGroup parent) {
        DescriptionView dv;
        if (convertView == null) {
            dv = new DescriptionView(mContext, mTitles[position], mDescription[position], mExpanded[position]);
        } else {
            dv = (DescriptionView)convertView;
            dv.setTitle(mTitles[position]);
            dv.setDescription(mDescription[position]);
            dv.setExpanded(mExpanded[position]);
        }

        return dv;
    }

    public void toggle(int position) {
        mExpanded[position] = !mExpanded[position];
        notifyDataSetChanged();
    }

    /**
     * Remember our context so we can use it when constructing views.
     */
    private Context mContext;

    /**
     * Our data, part 1.
     */
    private String[] mTitles = 
    {
                "Bachelor of Business",   
                "Bachelor of Business (Honours)"


    };

    private String[] mDescription =
    {
        "This programme provides you with a broadly-based business education" +
        " to assist you to work successfully in business. You will study a variety " +
        "of business subjects and develop a general understandng of the administrative, " +
        "economics, legal, and social environment within which business operates.  You will " +
        "study the basics of business and get an insight into many business disciplines" +
        " in years one and two. In year three you can specialise in Marketing, Human" +
        " Resource Management, or Information Systems Management. There is a strong " +
        "emphasis during the programme on practical business projects to develop the" +
        "skills required by graduates in the workplace. (Students may apply to the" +
        " Registrar for a Higher Certificate in Business after successfully completing " +
        "two years of the programme.)",

        "The School of Business offers a four-year honours degree (Level 8) in Business. " +
        "This programme provides students with a broad-based business education to work" +
        " successfully in business and offers students the opportunity to specialise in one " +
        "of five streams. You will study a variety of business subjects and develop a general" +
        " understanding of the administrative, economic, legal and social environment within" +
        " which business operates. You will study the basics of business and get an insight" +
        " into many business disciplines in years one and two. In year three you can specialise " +
        "in marketing, human resource management or information systems management. In year four " +
        "you can specialise in economics and finance, human resource management, management," +
        " marketing or rural enterprise and agri-business. There is a strong emphasis on the" +
        " current business environment and acquiring the skills and knowledge required for " +
        "employment in business. Case studies and practical business projects are used to develop" +
        " students' analysis, communication and professional business skills."
    };
    /**
     * Our data, part 3.
     */
    private boolean[] mExpanded = 
    {
            false,
            false,

    };
}

/**
 * We will use a DescriptionView to display each speech. It's just a LinearLayout
 * with two text fields.
 *
 */
private class DescriptionView extends LinearLayout {
    private TextView mTitle;
    private TextView mDescription;

    public DescriptionView(Context context, String title, String desc, boolean expanded) {
        super(context);

        this.setOrientation(VERTICAL);

        // Here we build the child views in code. They could also have
        // been specified in an XML file.

        mTitle = new TextView(context);
        mTitle.setText(title);
        addView(mTitle, new LinearLayout.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT));

        mDescription.setText(desc);
        addView(mDescription, new LinearLayout.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT));

        mDescription.setVisibility(expanded ? VISIBLE : GONE);
    }

    /**
     * Convenience method to set the title of a DescriptionView
     */
    public void setTitle(String title) {
        mTitle.setText(title);
    }

    /**
     * Convenience method to set the description of a DescriptionView
     */
    public void setDescription(String words) {
        mDescription.setText(words);
    }

    /**
     * Convenience method to expand or hide the description
     */
    public void setExpanded(boolean expanded) {
        mDescription.setVisibility(expanded ? VISIBLE : GONE);
    }



    }
 }

这是日志猫:

03-25 13:34:07.753: E/AndroidRuntime(912): FATAL EXCEPTION: main
03-25 13:34:07.753: E/AndroidRuntime(912): java.lang.NullPointerException
03-25 13:34:07.753: E/AndroidRuntime(912):  at         ie.gmit.gmitapp.BusinessActivity$DescriptionView.<init>(BusinessActivity.java:174)
03-25 13:34:07.753: E/AndroidRuntime(912):  at   ie.gmit.gmitapp.BusinessActivity$BusinessListAdapter.getView(BusinessActivity.java:82)
03-25 13:34:07.753: E/AndroidRuntime(912):  at    android.widget.AbsListView.obtainView(AbsListView.java:2159)
03-25 13:34:07.753: E/AndroidRuntime(912):  at   android.widget.ListView.makeAndAddView(ListView.java:1831)

第174行是

mDescription.setText(desc)

第82行是

dv = new DescriptionView(mContext, mTitles[position], mDescription[position], mExpanded[position]);

Answer 1

您没有创建 mDescription TextView 对象。

因此，mDescription 为空，您得到了 NPE。

在 DescriptionView 构造函数的第 174 行之前添加此行:

mDescription = new TextView(context);

---

下次获得 NPE 时:
当您使用未初始化的对象时，您会得到一个 NPE。因此，检查 NPE 发生的行号并查看您正在使用的对象。在本例中，它只是 mDescription，因此无需查看任何其他代码，很明显 mDescription 未初始化/创建。