List<MyStructure> 错误：应用程序崩溃，不知道为什么

Question

我已经建立了一些名为“item”的类，这是它：（这就是整个代码）

public class item {
private int id;
private String title;
private String desc;
private double lat;
private double lon;
private String pub;
private int p;
private int n;

public item(int id, String title, String desc, double lat, double lon, String pub, int p, int n) {
    super();
    this.id = id;
    this.title = title;
    this.desc = desc;
    this.lat = lat;
    this.lon = lon;
    this.pub = pub;
    this.p = p;
    this.n = n;
}

现在我必须为它创建一个 List<item> 和 add() “项目”，但由于某种原因，我的应用程序崩溃了。

这就是活动：

public class MainActivity extends Activity {

    List<item> markers;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        item a = new item(1, "lol", "sdfs", 32.45345, 34.54353, "nir", 0, 0);
        markers.add(a);


    }

LOGCAT：

08-10 16:32:46.710: D/AndroidRuntime(2934): Shutting down VM
08-10 16:32:46.710: W/dalvikvm(2934): threadid=1: thread exiting with uncaught exception (group=0x41a0a930)
08-10 16:32:46.777: E/AndroidRuntime(2934): FATAL EXCEPTION: main
08-10 16:32:46.777: E/AndroidRuntime(2934): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.testitems/com.example.testitems.MainActivity}: java.lang.NullPointerException
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2308)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2358)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.app.ActivityThread.access$600(ActivityThread.java:153)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1247)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.os.Looper.loop(Looper.java:137)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.app.ActivityThread.main(ActivityThread.java:5227)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at java.lang.reflect.Method.invokeNative(Native Method)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at java.lang.reflect.Method.invoke(Method.java:511)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at dalvik.system.NativeStart.main(Native Method)
08-10 16:32:46.777: E/AndroidRuntime(2934): Caused by: java.lang.NullPointerException
08-10 16:32:46.777: E/AndroidRuntime(2934):     at com.example.testitems.MainActivity.onCreate(MainActivity.java:18)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.app.Activity.performCreate(Activity.java:5104)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
08-10 16:32:46.777: E/AndroidRuntime(2934):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2262)
08-10 16:32:46.777: E/AndroidRuntime(2934):     ... 11 more

感谢您的帮助！

Answer 1

初始化你的标记列表

List<item> markers=new ArrayList<item>();

Answer 2

试试这个

public class MainActivity extends Activity {

ArrayList<item> markers;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

     markers = new ArrayList<item>();

    item a = new item(1, "lol", "sdfs", 32.45345, 34.54353, "nir", 0, 0);
    markers.add(a);


}

Answer 3

您必须在使用之前初始化列表。

    setContentView(R.layout.activity_main);

    List<item> markers = new ArrayList<item>();
    item a = new item(1, "lol", "sdfs", 32.45345, 34.54353, "nir", 0, 0);
    markers.add(a);

Answer 4

任何对象都应在使用其方法之前进行初始化，但是您似乎在向列表中添加数据之前忘记了这一步。在添加数据之前初始化markers。

 List<item> markers=new ArrayList<item>();

Answer 5

在代码中你没有初始化 arraylist 对象来消除这些错误 请参阅下面提到的以下步骤：

1. 首先像这样给Arraylist对象分配内存

   公共类 MainActivity 扩展 Activity {

   ArrayList 标记；

   @覆盖 protected void onCreate(Bundle savedInstanceState) {

          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
   
          markers = new ArrayList<item>();
   
          item a = new item(1, "lol", "sdfs", 32.45345, 34.54353, "nir", 0, 0);
          markers.add(a);
   

}

1. 其次使用 try 和 catch 块来处理错误。

公共类 MainActivity 扩展 Activity {

ArrayList 标记；

@覆盖 protected void onCreate(Bundle savedInstanceState) { 尝试 {

           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_main);

           markers = new ArrayList<item>();

           item a = new item(1, "lol", "sdfs", 32.45345, 34.54353, "nir", 0, 0);
           markers.add(a);

    }
    catch(Exception e) 
    {
         //Write to android ddms logger.
            or
          make a Toast Message
    }

}

1. 释放已初始化的资源，如arraylist等。 通过将 finally 块放在 catch 块之后。