class - Android pass context causes NullPointerException -

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i trying make methods work other classes current activity. in case there backup mechanism uses toasts , works flawlessly if method in same class activity every time it's called there nullpointerexception.

these essential lines main activity:

backup bckp; private context context = main.this; bckp.copybackup(backupdir, backupdirfile, database, context);

and called method:

public void copybackup(final string backupdir,         final string target, final string source, final context context) {     final file sdcard = environment.getexternalstoragedirectory();     if (sdcard.canwrite()) {         inputstream input = null;         try {             input = new fileinputstream(source);         } catch (filenotfoundexception e) {             toast.maketext(context, "there error finding database", toast.length_short).show();             e.printstacktrace();         }          file dir = new file (backupdir);         if (!dir.exists()) {             dir.mkdirs();         }         outputstream output = null;         try {             output = new fileoutputstream(target);         } catch (filenotfoundexception e) {             toast.maketext(context, "there error creating backup", toast.length_short).show();             e.printstacktrace();         }          byte[] buffer = new byte[1024];         int length;         try {             while ((length = input.read(buffer)) > 0) {                 output.write(buffer, 0, length);             }         } catch (ioexception e) {             toast.maketext(context, "there error copying database", toast.length_short).show();             e.printstacktrace();         }          try {             output.flush();             output.close();             input.close();         } catch (ioexception e) {             toast.maketext(context, "there error ending copy", toast.length_short).show();             e.printstacktrace();         }     } else {         toast.maketext(context, "no sd card found", toast.length_short).show();     } }

it makes no sense me why works if in 1 class not if it's in separate class... help!

edit:

05-08 14:13:24.935: e/androidruntime(636): fatal exception: main 05-08 14:13:24.935: e/androidruntime(636): java.lang.nullpointerexception 05-08 14:13:24.935: e/androidruntime(636):  @ maturaarbeit.nicola_pfister.marks.main$9$1.onclick(main.java:271) 05-08 14:13:24.935: e/androidruntime(636):  @ com.android.internal.app.alertcontroller$buttonhandler.handlemessage(alertcontroller.java:166) 05-08 14:13:24.935: e/androidruntime(636):  @ android.os.handler.dispatchmessage(handler.java:99) 05-08 14:13:24.935: e/androidruntime(636):  @ android.os.looper.loop(looper.java:137) 05-08 14:13:24.935: e/androidruntime(636):  @ android.app.activitythread.main(activitythread.java:4745) 05-08 14:13:24.935: e/androidruntime(636):  @ java.lang.reflect.method.invokenative(native method) 05-08 14:13:24.935: e/androidruntime(636):  @ java.lang.reflect.method.invoke(method.java:511) 05-08 14:13:24.935: e/androidruntime(636):  @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:786) 05-08 14:13:24.935: e/androidruntime(636):  @ com.android.internal.os.zygoteinit.main(zygoteinit.java:553) 05-08 14:13:24.935: e/androidruntime(636):  @ dalvik.system.nativestart.main(native method)

you haven't initialized bckp null when try call function on it. context fine

backup bckp;

make it

backup bckp = new backup();  // send parameters if constructor takes them private context context = main.this; bckp.copybackup(backupdir, backupdirfile, database, context);

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