Question

我正在尝试从webview保存链接。从我的webview类中，我将页面URL保存如下。

public void AddUrl(String page_url){
    SaveUrlActivity urlactivity = new SaveUrlActivity();
    urlactivity.saveurl(page_url);
}

我的SaveUrlActivity类如下：

public class SaveUrlActivity extends Activity {

public String url;
public ListView lv;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_savedurl);
    lv = (ListView) findViewById(R.id.list);
    saveurl(url);
    lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        public void onItemClick(AdapterView<?> parent, View view,
                                int position, long id) {
            Intent in = new Intent(SaveUrlActivity.this, DisPlayWebPageActivity.class);

            in.putExtra("page_url", url);
            startActivity(in);
        }
    });
}

public void saveurl(String url1){
    url = url1;
    final List<RowItem> ri = new ArrayList<RowItem>();
    RowItem item = new RowItem(url);
    ri.add(item);
    SavedUrlAdapter adapter = new SavedUrlAdapter(SaveUrlActivity.this, ri);
    lv.setAdapter(adapter);
  }
}

每当我运行程序时，我都会收到以下错误。

 FATAL EXCEPTION: main
Process: com.example.smarthelp, PID: 16284
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
        at com.example.smarthelp.SaveUrlActivity.saveurl(SaveUrlActivity.java:46)
        at com.example.smarthelp.DisPlayWebPageActivity.AddUrl(DisPlayWebPageActivity.java:76)
        at com.example.smarthelp.DisPlayWebPageActivity.onOptionsItemSelected(DisPlayWebPageActivity.java:66)

任何人都可以帮我弄明白我犯错误的地方吗？

Answer 1

要了解的几件事情：

确保activity_savedurl布局文件实际上包含带有list ID的视图。如果没有，则lv将为null。
您将url传递给saveurl()，但您从未为其指定过值。

Answer 2

您在初始化之前使用了对ListView lv的引用，如果没有首先启动SaveUrlActivity，只有在setContentView(R.layout.activity_savedurl); lv之后才能初始化{{1} }。

public void saveurl(String url1){
    url = url1;
    final List<RowItem> ri = new ArrayList<RowItem>();
    RowItem item = new RowItem(url);
    ri.add(item);
    SavedUrlAdapter adapter = new SavedUrlAdapter(SaveUrlActivity.this, ri);
    lv.setAdapter(adapter);
  }

如果在SaveUrlActivity类中创建静态var并在启动Activity之前为其赋值，并且在该类内部对saveurl()进行make调用时会更容易你已经初始化了所有的观点。

类似的东西：

public void AddUrl(String page_url){
    SaveUrlActivity.page_url = page_url;
    Intent i = new Intent(this, SaveUrlActivity.class);
    startActivity(i);
}

在你的SaveUrlActivity.java中：

public class SaveUrlActivity extends Activity {

public String url;
public ListView lv;
static page_url;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_savedurl);
    lv = (ListView) findViewById(R.id.list);
    saveurl();
    lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        public void onItemClick(AdapterView<?> parent, View view,
                                int position, long id) {
            Intent in = new Intent(SaveUrlActivity.this, DisPlayWebPageActivity.class);

            in.putExtra("page_url", url);
            startActivity(in);
        }
    });
}

public void saveurl(){
    url = page_url;
    final List<RowItem> ri = new ArrayList<RowItem>();
    RowItem item = new RowItem(url);
    ri.add(item);
    SavedUrlAdapter adapter = new SavedUrlAdapter(SaveUrlActivity.this, ri);
    lv.setAdapter(adapter);
  }
}

如果您计划开始此活动，这是准确的，如果您需要更新已启动的数据，最好使用Handler

Listview无法正确加载

2 个答案: