我收到NullPointerException但我不确定为什么因为这段代码应该正常工作。下面是调用方法，我需要从'raw'文件夹中的文件创建JSONObject。
JSONObject currenciesJsonObject = MiscHelper.getJsonObjectFromResource(R.raw.app_currencies, getActivity());
以下是由DialogFragment扩展的完整类文件：
package uk.co.example.calculator.ui.fragments;
import android.app.Dialog;
import android.app.Fragment;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.View;
import android.widget.LinearLayout;
import org.json.JSONException;
import org.json.JSONObject;
import java.util.ArrayList;
import uk.co.example.calculator.R;
import uk.co.example.calculator.adapters.SimpleItemRecyclerViewAdapter;
import uk.co.example.calculator.ui.activities.DividerItemDecoration;
import uk.co.example.calculator.ui.activities.ExchangeRate;
import uk.co.example.utils.MiscHelper;
/**
* A simple {@link Fragment} subclass.
*/
public class CurrencyConverterDialogFrag extends android.app.DialogFragment {
// Declare UI elements
private RecyclerView recyclerView;
public CurrencyConverterDialogFrag() {
// Required empty public constructor
}
@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = getActivity().getLayoutInflater().inflate(R.layout.fragment_currency_converter_dialog_calc, new LinearLayout(getActivity()), false);
recyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
initializeRecyclerView();
Dialog dialog = new Dialog(getActivity());
dialog.setTitle("Choose currency");
dialog.setContentView(view);
return dialog;
}
JSONObject currenciesJsonObject = MiscHelper.getJsonObjectFromResource(R.raw.app_currencies, getActivity());
private ArrayList<ExchangeRate> getSampleArrayList() {
ArrayList<ExchangeRate> items = new ArrayList<>();
try {
items.add(new ExchangeRate(currenciesJsonObject.getJSONObject("GBP").getString("name"), "Valyria"));
items.add(new ExchangeRate("Rob Stark", "Winterfell"));
items.add(new ExchangeRate("Jon Snow", "Castle Black"));
items.add(new ExchangeRate("Tyrion Lanister", "King's Landing"));
} catch (JSONException e) {
e.printStackTrace();
}
return items;
}
private void initializeRecyclerView() {
// Setup layout manager for items
LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity());
// Control orientation of the items
layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
//Customize the position you want to default scroll to
layoutManager.scrollToPosition(0);
// Attach layout manager to the RecyclerView
recyclerView.setLayoutManager(layoutManager);
// allows for optimizations if all item views are of the same size:
recyclerView.setHasFixedSize(true);
RecyclerView.ItemDecoration itemDecoration = new DividerItemDecoration(getActivity(), DividerItemDecoration.VERTICAL_LIST);
recyclerView.addItemDecoration(itemDecoration);
bindDataToAdapter();
}
private void bindDataToAdapter() {
// Bind adapter to recycler view object
recyclerView.setAdapter(new SimpleItemRecyclerViewAdapter(getSampleArrayList()));
}
}
下面是MiscHelper类中发生异常的方法：
public static JSONObject getJsonObjectFromResource(int resource, Context context) {
// PROBLEM OCCURS ON THIS NEXT LINE AS STATED BY THE LOGCAT. LINE:252
InputStream inputStream = context.getResources().openRawResource(resource);
BufferedReader r = new BufferedReader( new InputStreamReader( inputStream ) );
StringBuilder stringBuilder = new StringBuilder();
String line;
String jsonString = null;
JSONObject jsonObject = null;
try {
while (( line = r.readLine() ) != null) {
stringBuilder.append( line );
}
jsonString = stringBuilder.toString();
}
catch (Exception e) {
Log.e("GetJsonFromResource", Log.getStackTraceString(e));
}
try {
jsonObject = new JSONObject(jsonString);
} catch (JSONException e) {
Log.e("GetJsonFromResource", Log.getStackTraceString(e));
}
return jsonObject;
}
以下是LogCat，但有例外：
07-12 12:38:01.999: E/AndroidRuntime(6793): FATAL EXCEPTION: main
07-12 12:38:01.999: E/AndroidRuntime(6793): Process: uk.co.codepix.calculator, PID: 6793
07-12 12:38:01.999: E/AndroidRuntime(6793): java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
07-12 12:38:01.999: E/AndroidRuntime(6793): at uk.co.codepix.utils.MiscHelper.getJsonObjectFromResource(MiscHelper.java:252)
07-12 12:38:01.999: E/AndroidRuntime(6793): at uk.co.codepix.calculator.ui.fragments.CurrencyConverterDialogFrag.<init>(CurrencyConverterDialogFrag.java:56)
07-12 12:38:01.999: E/AndroidRuntime(6793): at uk.co.codepix.calculator.ui.fragments.CurrencyConverterInputCalcFrag$1.onClick(CurrencyConverterInputCalcFrag.java:73)
07-12 12:38:01.999: E/AndroidRuntime(6793): at android.view.View.performClick(View.java:4780)
07-12 12:38:01.999: E/AndroidRuntime(6793): at android.view.View$PerformClick.run(View.java:19866)
07-12 12:38:01.999: E/AndroidRuntime(6793): at android.os.Handler.handleCallback(Handler.java:739)
07-12 12:38:01.999: E/AndroidRuntime(6793): at android.os.Handler.dispatchMessage(Handler.java:95)
07-12 12:38:01.999: E/AndroidRuntime(6793): at android.os.Looper.loop(Looper.java:135)
07-12 12:38:01.999: E/AndroidRuntime(6793): at android.app.ActivityThread.main(ActivityThread.java:5254)
07-12 12:38:01.999: E/AndroidRuntime(6793): at java.lang.reflect.Method.invoke(Native Method)
07-12 12:38:01.999: E/AndroidRuntime(6793): at java.lang.reflect.Method.invoke(Method.java:372)
07-12 12:38:01.999: E/AndroidRuntime(6793): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
07-12 12:38:01.999: E/AndroidRuntime(6793): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
任何人都可以告诉我我做错了什么吗？任何关于如何解决问题的详细解释都将受到高度赞赏。
答案 0 :(得分：1)
据我所知，您在实例初始化程序中初始化变量xlink:href，这意味着当您执行此操作时，currenciesJsonObject并未附加到Fragment。
移动此电话：
Activity到生命周期回调，其中MiscHelper.getJsonObjectFromResource(R.raw.app_currencies, getActivity());
不会返回null。 getActivity或onAttach()可能是一个好地方。
答案 1 :(得分：0)
我没有足够的声誉来评论，所以这里只是一个快速输入：
尝试使用getBaseContext()代替getActivity()。
getBaseContext()是ContextWrapper的一种方法。可以在不更改原始上下文的情况下修改此ContextWrapper。在Javadoc中：“代理实现Context，简单地将其所有调用委托给另一个Context。可以子类化以修改行为而不改变原始Context。”
还有一种方法可行：
getApplicationContext()。另一方面，此方法返回应用程序上下文（整个应用程序生命周期）。这意味着当调用onDestroy()时，此方法返回null。
希望它有所帮助。