Android: onCreate 方法为 Listview 返回空对象

Question

我正在尝试在左侧创建一个抽屉。与此同时，当我尝试执行时，它给出了这个错误并且应用程序崩溃了：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

这是我的代码： MainActivity.java

import android.os.Bundle;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarActivity;
import android.widget.ArrayAdapter;
import android.widget.ListView;


public class MainActivity extends ActionBarActivity {

private String items[];//={"Home","Profile","Requests","Notifications","Search","Track My Tifin","Settings"};
private DrawerLayout dl;
private ListView listView;
//ArrayAdapter<String> adapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    dl=(DrawerLayout)findViewById(R.id.drawerOnleft);
    listView=(ListView) findViewById(R.id.list);
    items=getResources().getStringArray(R.array.list_items);

    //Set adapter for this list
    listView.setAdapter(new ArrayAdapter<>(this,android.R.layout.simple_list_item_1,items));
    setContentView(R.layout.drawer_ex);
}

}

drawer_ex.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout
android:id="@+id/drawerOnleft"
android:layout_height="match_parent"
android:layout_width="match_parent"
xmlns:android="http://schemas.android.com/apk/res/android">
<FrameLayout
    android:id="@+id/mainView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"></FrameLayout>
<ListView
    android:id="@+id/list"
    android:layout_width="240dp"
    android:layout_height="match_parent"
    android:layout_gravity="left"
    android:choiceMode="singleChoice"
    android:divider="@android:color/transparent"
    android:background="#33CCFF"
    android:dividerHeight="0dp"

    ></ListView>

strings.xml

<resources>
<string name="app_name">MyDrawer</string>

<string name="hello_world">Hello world!</string>
<string name="action_settings">Settings</string>
<string-array name="list_items">
    <item>Home</item>
    <item>Profile</item>
    <item>Requests</item>
    <item>Notifications</item>
    <item>Search</item>
    <item>Track My Tifin</item>
    <item>Settings</item>
</string-array>

提前致谢！

Answer 1

在调用 findviewbyId 方法之前设置您的内容视图。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
setContentView(R.layout.drawer_ex);    
dl=(DrawerLayout)findViewById(R.id.drawerOnleft);
    listView=(ListView) findViewById(R.id.list);
    items=getResources().getStringArray(R.array.list_items);

    //Set adapter for this list
    listView.setAdapter(new ArrayAdapter<>(this,android.R.layout.simple_list_item_1,items));

}

Answer 2

Whoilaaa..thank you so much @savepopulation. It worked out like charm. But i got a question that why it was throwing Exception when i've set "setContentView(R.layout.drawer_ex)" in the end. Could you explain a bit more on this? thanks

从布局资源设置 activity 内容。资源将被扩充，将所有顶级视图添加到 activity.

Resource膨胀后才能找到View。这就是为什么你的 findViewByID return null 因为 android 找不到匹配的资源。

Android: onCreate 方法为 Listview 返回空对象

Android: null object returned for Listview on onCreate method

android

android-listview

drawerlayout