Question

亲爱的JAVA程序员，

打开我的一个活动时，我在onCreate方法中遇到了NullPointerException。这就是我的onCreate方法：

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    overridePendingTransition(R.anim.push_left_in, R.anim.fadeout);
    requestWindowFeature(Window.FEATURE_CUSTOM_TITLE);
    setContentView(R.layout.category_page);
    getWindow().setFeatureInt(Window.FEATURE_CUSTOM_TITLE, R.layout.titlebar);

    // get intent data
    Intent i = getIntent();

    // Selected category id
    int position = i.getExtras().getInt("id");
    Categories categories = new Categories();
    Categories.CategoriesFromDatabase textAdapter1 = categories.new CategoriesFromDatabase(this);

    TextView pageTitle = (TextView) findViewById(R.id.category_page_title);
    if (textAdapter1.mCatIds[position] == null){
        pageTitle.setText("error parsing pagename");
    } else {
        pageTitle.setText(textAdapter1.mCatIds[position]);
    }  

    ListView pageContent = (ListView) findViewById(R.id.category_page_content);
    pageContent.setAdapter(new LinksPerCategory(this));

    pageContent.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v,
                int position, long id) {

            // Sending image id to FullScreenActivity
            Intent i = new Intent(getApplicationContext(), viewPageItem.class);
            // passing array index
            i.putExtra("id", position);
            startActivity(i);
        }

    });


}

此活动的主要目的是从过去的活动中获取类别的名称＆mCatIds [position]＆＃39;，并将其置于当前的顶部。我怀疑这里给出了NullPointerException，因此我将if语句放在行周围，但没有效果。由于过去的活动显示内容，因此mCatIds [position]＆＃39;可能有价值。问题可能在其他地方吗？还是我错过了重要的事情？

我期待着你的回复。

编辑：这是堆栈跟踪，抱歉没有直接发布：

05-27 08:37:41.845: E/AndroidRuntime(31413): FATAL EXCEPTION: main
05-27 08:37:41.845: E/AndroidRuntime(31413): java.lang.RuntimeException: Unable to start     activity ComponentInfo{com.example.abnwiki/com.example.abnwiki.viewCategoryPage}:     java.lang.NullPointerException
05-27 08:37:41.845: E/AndroidRuntime(31413):    at     android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1996)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2023)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at android.app.ActivityThread.access$600(ActivityThread.java:127)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1174)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at android.os.Handler.dispatchMessage(Handler.java:99)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at android.os.Looper.loop(Looper.java:137)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at android.app.ActivityThread.main(ActivityThread.java:4503)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at java.lang.reflect.Method.invokeNative(Native Method)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at java.lang.reflect.Method.invoke(Method.java:511)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:809)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:576)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at dalvik.system.NativeStart.main(Native Method)
05-27 08:37:41.845: E/AndroidRuntime(31413): Caused by: java.lang.NullPointerException
05-27 08:37:41.845: E/AndroidRuntime(31413):    at com.example.abnwiki.viewCategoryPage.onCreate(viewCategoryPage.java:38)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at android.app.Activity.performCreate(Activity.java:4479)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1050)
05-27 08:37:41.845: E/AndroidRuntime(31413):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1960)
05-27 08:37:41.845: E/AndroidRuntime(31413):    ... 11 more

Answer 1

是的，请告诉我们您的异常堆栈跟踪！

如果没有看到你的堆栈跟踪，我认为这就是你的问题：

int position = i.getExtras().getInt("id");

i.getExtras（）返回null !!!

Answer 2

您的活动第一次开始时，它不会被意图调用。因此getIntent()将返回NullPointerException。你可以用

解决这个问题

if(getIntent()!=null)
Intent i=getIntent();

或者您可以使用函数onNewIntent()并将当前活动设置为singleTop。

每次意图调用活动时，

onNewIntent()执行任务。如果您包含singleTop，它还会确保只有一个活动实例。

创建android活动时出现NullPointerException

2 个答案: