我在应用程序中获得Null指针异常。我正在创建一个列表应用程序，它从数据库中提取数据并在Android上的ListView上显示。由于此NullPointerException错误，我的应用程序在开始之前崩溃，然后才打开。任何人都可以帮助我。我是Android新手，请帮助我，我非常需要。
public class MainActivity extends AppCompatActivity {
ListView listview;
ArrayAdapter<String> adapter = null;
String address = "http://192.168.0.109/view.php";
InputStream is = null;
String line = null;
String result = null;
String[] data;
@Override
protected void onCreate(Bundle savedInstanceState) {
System.out.println(0);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
System.out.println(1);
listview = (ListView) findViewById(R.id.list_view);
//Allow Netwrok in MainFrame//
StrictMode.setThreadPolicy((new StrictMode.ThreadPolicy.Builder().permitNetwork().build()));
//Retrieve Data//
System.out.println(1.5);
getData();
//Adapter//
System.out.println(2);
//Error Occured java.lang.NullPointerException: storage == null//
adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, data);
listview.setAdapter(adapter);
// getJSON("http://192.168.0.101/view.php");
}
private String getData() {
try {
System.out.println(2.5);
URL url = new URL(address);
HttpURLConnection con = (HttpURLConnection) url.openConnection();
con.setRequestMethod("GET");
is = new BufferedInputStream(con.getInputStream());
} catch (Exception e) {
e.printStackTrace();
}
//Read InputSteam content into String//
try {
System.out.println(3);
BufferedReader br = null;
br = new BufferedReader(new InputStreamReader(is));
StringBuilder sb = null;
sb = new StringBuilder();
while ((line = br.readLine()) != null) {
System.out.println(3.2);
sb.append(line + "\n");
}
is.close();
//Error Occurred java.lang.NullPointerException:
//Attempt to invoke virtual method 'java.lang.String
////java.lang.String.toString()' on a null object reference//
result.toString();
} catch (Exception e) {
e.printStackTrace();
}
//Parse JSON//
try {
System.out.println(3.5);
JSONArray ja = null;
//Error Occurred: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference//
ja = new JSONArray(result);
JSONObject jo = null;
data = new String[ja.length()];
for (int i = 0; i < ja.length(); i++) {
System.out.println(4);
jo = ja.getJSONObject(i);
data[i] = jo.getString("EmpName");
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(4);
return result;
}
}
//这是执行时代码的完整日志崩溃条目。 注意：我正在外部设备上运行调试//
I/InstantRun: starting instant run server: is main process
V/Monotype: SetAppTypeFace- try to flip, app = helloworld.krish.com.listview
V/Monotype: Typeface getFontPathFlipFont - systemFont = default#default
I/System.out: 0
W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
V/Monotype: SetAppTypeFace- try to flip, app = helloworld.krish.com.listview
Typeface getFontPathFlipFont - systemFont = default#default
I/System.out: 1
1.5
2.5
I/DpmTcmClient: RegisterTcmMonitor from: com.android.okhttp.TcmIdleTimerMonitor
I/System.out: 3
I/System.out: 3.2
W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.toString()' on a null object reference
at helloworld.krish.com.listview.MainActivity.getData(MainActivity.java:86)
at helloworld.krish.com.listview.MainActivity.onCreate(MainActivity.java:49)
at android.app.Activity.performCreate(Activity.java:6323)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494)
at android.app.ActivityThread.access$900(ActivityThread.java:153)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5451)
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)
I/System.out: 3.5
W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
W/System.err: at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116)
at org.json.JSONTokener.nextValue(JSONTokener.java:94)
at org.json.JSONArray.<init>(JSONArray.java:92)
at org.json.JSONArray.<init>(JSONArray.java:108)
at helloworld.krish.com.listview.MainActivity.getData(MainActivity.java:96)
at helloworld.krish.com.listview.MainActivity.onCreate(MainActivity.java:49)
at android.app.Activity.performCreate(Activity.java:6323)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494)
at android.app.ActivityThread.access$900(ActivityThread.java:153)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5451)
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)
I/System.out: 4
2
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: helloworld.krish.com.listview, PID: 14294
java.lang.RuntimeException: Unable to start activity ComponentInfo{helloworld.krish.com.listview/helloworld.krish.com.listview.MainActivity}: java.lang.NullPointerException: storage == null
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2434)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494)
at android.app.ActivityThread.access$900(ActivityThread.java:153)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5451)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: storage == null
at java.util.Arrays$ArrayList.<init>(Arrays.java:38)
at java.util.Arrays.asList(Arrays.java:155)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:137)
at helloworld.krish.com.listview.MainActivity.onCreate(MainActivity.java:52)
at android.app.Activity.performCreate(Activity.java:6323)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494)
at android.app.ActivityThread.access$900(ActivityThread.java:153)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5451)
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)
I/Process: Sending signal. PID: 14294 SIG: 9
Application terminated.
答案 0 :(得分：0)
您调用.toString()的结果变量永远不会被赋予null以外的值。
您在代码顶部将其初始化为null
String result = null;
然后永远不要给它一个价值。您无法调用null对象上的方法，您将获得该异常。正如有人在评论中指出的那样，这可能是What is a NullPointerException, and how do I fix it?的重复。
结果也是一个String，你不需要在它上面调用toString（）