尝试获取全局 ArrayList 时获取 NullPointer

Question

我上了 3 节课。 一个主类、globalVariable 类和一个项目类。

我尝试在主类中设置一个 ArrayList。我想在项目类中读出它。
这是我尝试过的：

主要：

for (int i = data.size()-1; i >= 0; i--){
    if (str_tv.contains(data.get(i).getCaption())){
        //TODO data.remove();
        String str_i = String.valueOf(i);
        checkedArray.add(str_i);

    }
}
System.out.println("MainActivity");
global.setArrList(checkedArray);
System.out.println("size = "+global.getArrList().size());

global.setArrlList 在这里工作。 global.getArrList 也在这里工作（我知道我的尺码）。

现在我尝试从 item 类中获取它：

System.out.println("Item Class");
System.out.println("size = "+global.getArrList().size());   <--gives NullPointer

但是在这里，它不起作用，然后它给出了一个 NullPointer。

这是我的全局类：

ArrayList<String> arrList;

public ArrayList<String> getArrList() {
    return arrList;
}

public void setArrList(ArrayList<String> arrList) {
    this.arrList = arrList;
}

我确实检查了他是否可能首先尝试获取ArrayList 而不是设置它。但他在项目类中显示System.out.println("MainActivity");。

为什么它会给出一个 NullPointer？我错过了什么？

Logcat：

01-24 11:02:04.755: E/AndroidRuntime(30097): FATAL EXCEPTION: main
01-24 11:02:04.755: E/AndroidRuntime(30097): java.lang.NullPointerException
01-24 11:02:04.755: E/AndroidRuntime(30097):    at com.dlv.GewasBescherming3.ItemListAdapter.getView(ItemListAdapter.java:47)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.AbsListView.obtainView(AbsListView.java:2035)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.ListView.makeAndAddView(ListView.java:1772)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.ListView.fillDown(ListView.java:672)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.ListView.fillFromTop(ListView.java:732)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.ListView.layoutChildren(ListView.java:1625)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.AbsListView.onLayout(AbsListView.java:1865)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.View.layout(View.java:11278)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.ViewGroup.layout(ViewGroup.java:4224)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.RelativeLayout.onLayout(RelativeLayout.java:925)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.View.layout(View.java:11278)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.ViewGroup.layout(ViewGroup.java:4224)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.FrameLayout.onLayout(FrameLayout.java:431)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.View.layout(View.java:11278)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.ViewGroup.layout(ViewGroup.java:4224)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1628)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1486)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.LinearLayout.onLayout(LinearLayout.java:1399)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.View.layout(View.java:11278)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.ViewGroup.layout(ViewGroup.java:4224)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.widget.FrameLayout.onLayout(FrameLayout.java:431)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.View.layout(View.java:11278)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.ViewGroup.layout(ViewGroup.java:4224)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1489)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2442)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.os.Handler.dispatchMessage(Handler.java:99)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.os.Looper.loop(Looper.java:137)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at android.app.ActivityThread.main(ActivityThread.java:4575)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at java.lang.reflect.Method.invokeNative(Native Method)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at java.lang.reflect.Method.invoke(Method.java:511)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
01-24 11:02:04.755: E/AndroidRuntime(30097):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

其实

ArrayList<String> arrList; 是null。

只需声明为ArrayList<String> arrList = new ArrayList<String>();

更新：

实际上Main 和Item 类有不同的全局对象，所以它不能引用相同的arrList。

我最好补充一下：

public static ArrayList<String> arrList = new ArrayList<String>();

在Global class 中并像这样使用它：

Global.arrList.size();