Получение JSON URL-ссылки из строк. xml на HTTP-запрос

Question

Я пытаюсь перевести свое приложение, которое извлекает данные JSON из URL. Моя цель - добавить, чтобы получить URL-адреса из строк. xml. Это кусок кода. Когда я пытаюсь использовать

String jsonURL = getApplicationContext().getString(R.string.URL);

, но приложение закрывается принудительно.

MainActivity.class

public class MainActivity extends AppCompatActivity {

String jsonURL = getApplicationContext().getString(R.string.URL);
    @SuppressLint("")

    private void fetchJSON(){
        new AsyncTask<Void, Void, String>(){
            protected String doInBackground(Void[] params) {
                String response="";
                HashMap<String, String> map=new HashMap<>();
                try {
                    HttpRequest req = new HttpRequest(jsonURL);
                    response = req.prepare(HttpRequest.Method.POST).withData(map).sendAndReadString();
            } 
        }.execute();
    }

strings. xml

<string name="URL">http://example.com/data.php</string>

метод onCreate ()

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        fetchJSON();

    }

logcat:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.my.app, PID: 10378
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.my.app/com.my.app.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3194)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
        at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:118)
        at com.my.app.MainActivity.<init>(MainActivity.java:42)
        at java.lang.Class.newInstance(Native Method)
        at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
        at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:43)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1243)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3182)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 
I/Process: Sending signal. PID: 10378 SIG: 9
Process 10378 terminated.

Answer 1

Используйте следующий код.

public class MainActivity extends AppCompatActivity {
    String jsonURL;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        fetchJSON();
    }

    @SuppressLint("")
    private void fetchJSON(){
        jsonURL = getString(R.string.URL);
        new AsyncTask<Void, Void, String>(){
            protected String doInBackground(Void[] params) {
                String response="";
                HashMap<String, String> map=new HashMap<>();
                try {
                    HttpRequest req = new HttpRequest(jsonURL);
                    response = req.prepare(HttpRequest.Method.POST).withData(map).sendAndReadString();
            } 
        }.execute();
    }
}