Java 从返回的hashmap获取列表
我有一个类,它有一个名为Java 从返回的hashmap获取列表,java,android,list,hashmap,Java,Android,List,Hashmap,我有一个类,它有一个名为prepareListData的方法。此方法准备数据以在Android中填充ExpandableListView。当该方法被放置在调用它的类中时,它可以完美地工作,并且不需要返回任何东西,因为我使用私有字段。当我将方法移动到它自己的类并使其成为静态时,我会得到一个空指针,因为它显然需要返回它准备的数据 我创建的类具有相同的方法(prepareListData),该方法返回一个HashMap。我试图从另一个类调用该方法,我希望从该类将返回的HashMap中的List赋值给L
prepareListData
的方法。此方法准备数据以在Android中填充ExpandableListView
。当该方法被放置在调用它的类中时,它可以完美地工作,并且不需要返回任何东西,因为我使用私有字段。当我将方法移动到它自己的类并使其成为静态时,我会得到一个空指针,因为它显然需要返回它准备的数据
我创建的类具有相同的方法(prepareListData
),该方法返回一个HashMap
。我试图从另一个类调用该方法,我希望从该类将返回的HashMap
中的List
赋值给List
变量。有没有一种方法可以在Java中实现这一点
我以前从未尝试过,所以我甚至不知道从哪里开始
下面是我使用prepareListData方法的类:
public class PrepareListData {
public static HashMap prepareListData() {
List<String> gpsMenuListDataHeader = new ArrayList<>();
HashMap<String, List<String>> gpsMenuListDataChild = new HashMap<>();
// Main group data
gpsMenuListDataHeader.add("Job Card");
gpsMenuListDataHeader.add("Sign Off");
gpsMenuListDataHeader.add("Test Installation");
// Child Items for Job Card
List<String> jobcard = new ArrayList<>();
jobcard.add("Job Card Sub Item 1");
jobcard.add("Job Card Sub Item 2");
jobcard.add("Job Card Sub Item 3");
jobcard.add("Job Card Sub Item 4");
jobcard.add("Job Card Sub Item 5");
// Child Items fos Sign Off
List<String> signoff = new ArrayList<>();
signoff.add("Sign Off Sub Item 1");
signoff.add("Sign Off Sub Item 2");
signoff.add("Sign Off Sub Item 3");
// Child Items for Test Installation
List<String> testInstallation = new ArrayList<>();
testInstallation.add("Test Installation Sub Item 1");
testInstallation.add("Test Installation Sub Item 2");
testInstallation.add("Test Installation Sub Item 3");
testInstallation.add("Test Installation Sub Item 4");
testInstallation.add("Test Installation Sub Item 5");
testInstallation.add("Test Installation Sub Item 6");
testInstallation.add("Test Installation Sub Item 7");
gpsMenuListDataChild.put(gpsMenuListDataHeader.get(0), jobcard);
gpsMenuListDataChild.put(gpsMenuListDataHeader.get(1), signoff);
gpsMenuListDataChild.put(gpsMenuListDataHeader.get(2), testInstallation);
return gpsMenuListDataChild;
}
}
堆栈跟踪:
08-26 23:12:25.031 32157-32164/za.co.gpsts.gpsjobcard E/art﹕ Failed writing handshake bytes (-1 of 14): Broken pipe
08-26 23:12:25.039 32157-32157/za.co.gpsts.gpsjobcard E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: za.co.gpsts.gpsjobcard, PID: 32157
java.lang.RuntimeException: Unable to start activity ComponentInfo{za.co.gpsts.gpsjobcard/za.co.gpsts.gpsjobcard.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5257)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at za.co.gpsts.gpsjobcard.ExpandableListAdapter.getGroupCount(ExpandableListAdapter.java:45)
at android.widget.ExpandableListConnector.getCount(ExpandableListConnector.java:397)
at android.widget.ListView.setAdapter(ListView.java:487)
at android.widget.ExpandableListView.setAdapter(ExpandableListView.java:603)
at za.co.gpsts.gpsjobcard.MainActivity.onCreate(MainActivity.java:50)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5257)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
我试图从我希望的另一个类调用该方法
将返回的HashMap中的列表分配给列表
变量有没有一种方法可以在Java中实现这一点
hashMap空指针在哪里?请显示错误消息。很难说,但我的第一个猜测是gpsMenuListDataChild
为null
…我已经更新了我的问题并包括堆栈跟踪Hi James,感谢您的回复。这对我有用。我感谢你的帮助。快乐的编码人
08-26 23:12:25.031 32157-32164/za.co.gpsts.gpsjobcard E/art﹕ Failed writing handshake bytes (-1 of 14): Broken pipe
08-26 23:12:25.039 32157-32157/za.co.gpsts.gpsjobcard E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: za.co.gpsts.gpsjobcard, PID: 32157
java.lang.RuntimeException: Unable to start activity ComponentInfo{za.co.gpsts.gpsjobcard/za.co.gpsts.gpsjobcard.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5257)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at za.co.gpsts.gpsjobcard.ExpandableListAdapter.getGroupCount(ExpandableListAdapter.java:45)
at android.widget.ExpandableListConnector.getCount(ExpandableListConnector.java:397)
at android.widget.ListView.setAdapter(ListView.java:487)
at android.widget.ExpandableListView.setAdapter(ExpandableListView.java:603)
at za.co.gpsts.gpsjobcard.MainActivity.onCreate(MainActivity.java:50)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5257)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
HashMap<String, List<String> map = PrepareListData.prepareListData();
List<String> yourlist = map.get("key");