我有很多关于它的主题和问题，但无法找到任何解决方案。我也是Android和Java的新手。
我尝试做的是解析我的本地JSON文件并设置适配器以在ListView中查看我的字符串。不应该太复杂，但我收到了这个错误：
01-10 10:04:38.109 29962-29962/com.thkoeln.stucked E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.thkoeln.stucked, PID: 29962
java.lang.RuntimeException: Unable to start activity ComponentInfo{com../com...MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.AssetManager android.content.Context.getAssets()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.AssetManager android.content.Context.getAssets()' on a null object reference
at android.content.ContextWrapper.getAssets(ContextWrapper.java:81)
at com.thkoeln.stucked.JSONParser.loadJSONFromAsset(JSONParser.java:28)
at com.thkoeln.stucked.JSONParser.parseJson(JSONParser.java:52)
at com.thkoeln.stucked.MainActivity.onCreate(MainActivity.java:24)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
这是我的MainActivity.java：
package com.thkoeln.stucked;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.ListView;
import android.widget.ListAdapter;
import android.widget.SimpleAdapter;
public class MainActivity extends AppCompatActivity {
ListView activityList;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
activityList = (ListView) findViewById(R.id.activityList);
String fromArray[] = {"uid", "uname", "ustart"};
int to[] = {R.id.uid, R.id.uname, R.id.ustart};
JSONParser jp = new JSONParser();
jp.parseJson();
/*ListAdapter adapter = new SimpleAdapter(this, jp.userList, R.layout.user_item, fromArray, to);
activityList.setAdapter(adapter);*/
}
}
这是我的JSONParser.java：
package com.thkoeln.stucked;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.widget.ListView;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.HashMap;
public class JSONParser extends AppCompatActivity {
ArrayList<HashMap<String, String>> userList;
public String loadJSONFromAsset() {
String json = null;
try {
InputStream is = getAssets().open("users.json");
int size = is.available();
byte[] buffer = new byte[size];
is.read(buffer);
is.close();
json = new String(buffer, "UTF-8");
} catch (IOException ex) {
ex.printStackTrace();
}
return json;
}
public JSONObject parseJson() {
//ArrayList<HashMap<String, String>> userList
try {
JSONObject reader = new
JSONObject(String.valueOf(loadJSONFromAsset()));
JSONArray users = reader.getJSONArray("users");
Log.d("AnzahlUser", String.valueOf(users.length()));
for (int i = 0; i < users.length(); i++) {
JSONObject u = users.getJSONObject(i);
Log.d("Details-->", u.toString());
String uid = u.getString("uid");
String uname = u.getString("uname");
String ustart = u.getString("ustart");
//String uend = u.getString("u.end");
HashMap<String, String> user = new HashMap<>();
user.put("uid", uid);
user.put("uname", uname);
user.put("ustart", ustart);
//user.put("u.end", uend);
userList.add(user);
}
} catch (final JSONException e) {
e.printStackTrace();
}
return null;
}
}
如果有人可以帮助我，我会非常感激...
答案 0 :(得分：-1)
您不应将AppCompatActivity扩展为上下文。您应该将上下文传递给JSONParser类以获取资产。
如果活动附加到Android生命周期，您只能获取活动的上下文。
您的JSONParser应该是这样的。
public class JSONParser{
private Context context;
public JSONParser(Context context){
this.context = context;
}
ArrayList<HashMap<String, String>> userList;
public String loadJSONFromAsset() {
String json = null;
try {
InputStream is = context.getAssets().open("users.json");
int size = is.available();
byte[] buffer = new byte[size];
is.read(buffer);
is.close();
json = new String(buffer, "UTF-8");
} catch (IOException ex) {
ex.printStackTrace();
}
return json;
}
public JSONObject parseJson() {
//ArrayList<HashMap<String, String>> userList
try {
JSONObject reader = new
JSONObject(String.valueOf(loadJSONFromAsset()));
JSONArray users = reader.getJSONArray("users");
Log.d("AnzahlUser", String.valueOf(users.length()));
for (int i = 0; i < users.length(); i++) {
JSONObject u = users.getJSONObject(i);
Log.d("Details-->", u.toString());
String uid = u.getString("uid");
String uname = u.getString("uname");
String ustart = u.getString("ustart");
//String uend = u.getString("u.end");
HashMap<String, String> user = new HashMap<>();
user.put("uid", uid);
user.put("uname", uname);
user.put("ustart", ustart);
//user.put("u.end", uend);
userList.add(user);
}
} catch (final JSONException e) {
e.printStackTrace();
}
return null;
}
}
您的MainActivity应该是这样的。
public class MainActivity extends AppCompatActivity {
ListView activityList;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
activityList = (ListView) findViewById(R.id.activityList);
String fromArray[] = {"uid", "uname", "ustart"};
int to[] = {R.id.uid, R.id.uname, R.id.ustart};
JSONParser jp = new JSONParser(this);
jp.parseJson();
/*ListAdapter adapter = new SimpleAdapter(this, jp.userList, R.layout.user_item, fromArray, to);
activityList.setAdapter(adapter);*/
}
}