java - Converting 2D ArrayList<String> to 2D String Array -

i want convert 2d arraylist<string> 2d string array. here code

import java.util.arraylist; import android.os.bundle; import android.app.activity; import android.util.log; import android.view.menu;  public class mainactivity extends activity {  @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_main);     arraylist<string> parent = new arraylist<string>();     arraylist<string> child1 = new arraylist<string>();     arraylist<arraylist<string>> child = new arraylist<arraylist<string>>();     (int = 0; < 10; i++) {         parent.add("" + i);         (int j = 0; j <= i; j++) {             child.add(new arraylist<string>());             child.get(i).add("" + j);         }     }     system.out.println("asdasd"+child.size());     string[] parentstring=parent.toarray(new string[parent.size()]);     string[][] childstring=child.toarray(new string[parent.size()][child.size()]);      system.out.println("parent output");     for(int =0; i<parentstring.length ;i++){         system.out.println(parentstring[i]);     }     system.out.println("child output");     for(int i=0;i<parentstring.length;i++){         for(int j=0;j<child.get(i).size();j++){             system.out.println(childstring[i][j]);         }     }     system.out.println("done"); }  @override public boolean oncreateoptionsmenu(menu menu) {     // inflate menu; adds items action bar if present.     getmenuinflater().inflate(r.menu.main, menu);     return true; } 

}

as can see trying convert "child" arraylist "childstring" string. here logcat

08-11 16:03:00.306: e/androidruntime(21581): fatal exception: main 08-11 16:03:00.306: e/androidruntime(21581): java.lang.runtimeexception: unable start activity componentinfo{com.example.darraylist/com.example.darraylist.mainactivity}: java.lang.arraystoreexception: source[0] of type java.util.arraylist cannot stored in destination array of type java.lang.string[][] 08-11 16:03:00.306: e/androidruntime(21581):    @ android.app.activitythread.performlaunchactivity(activitythread.java:2180) 08-11 16:03:00.306: e/androidruntime(21581):    @ android.app.activitythread.handlelaunchactivity(activitythread.java:2230) 08-11 16:03:00.306: e/androidruntime(21581):    @ android.app.activitythread.access$600(activitythread.java:141) 08-11 16:03:00.306: e/androidruntime(21581):    @ android.app.activitythread$h.handlemessage(activitythread.java:1234) 08-11 16:03:00.306: e/androidruntime(21581):    @ android.os.handler.dispatchmessage(handler.java:99) 08-11 16:03:00.306: e/androidruntime(21581):    @ android.os.looper.loop(looper.java:137) 08-11 16:03:00.306: e/androidruntime(21581):    @ android.app.activitythread.main(activitythread.java:5039) 08-11 16:03:00.306: e/androidruntime(21581):    @ java.lang.reflect.method.invokenative(native method) 08-11 16:03:00.306: e/androidruntime(21581):    @ java.lang.reflect.method.invoke(method.java:511) 08-11 16:03:00.306: e/androidruntime(21581):    @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:793) 08-11 16:03:00.306: e/androidruntime(21581):    @ com.android.internal.os.zygoteinit.main(zygoteinit.java:560) 08-11 16:03:00.306: e/androidruntime(21581):    @ dalvik.system.nativestart.main(native method) 08-11 16:03:00.306: e/androidruntime(21581): caused by: java.lang.arraystoreexception: source[0] of type java.util.arraylist cannot stored in destination array of type java.lang.string[][] 08-11 16:03:00.306: e/androidruntime(21581):    @ java.lang.system.arraycopy(native method) 08-11 16:03:00.306: e/androidruntime(21581):    @ java.util.arraylist.toarray(arraylist.java:519) 08-11 16:03:00.306: e/androidruntime(21581):    @ com.example.darraylist.mainactivity.oncreate(mainactivity.java:28) 08-11 16:03:00.306: e/androidruntime(21581):    @ android.app.activity.performcreate(activity.java:5104) 08-11 16:03:00.306: e/androidruntime(21581):    @ android.app.instrumentation.callactivityoncreate(instrumentation.java:1080) 08-11 16:03:00.306: e/androidruntime(21581):    @ android.app.activitythread.performlaunchactivity(activitythread.java:2144) 08-11 16:03:00.306: e/androidruntime(21581):    ... 11 more 

1d arraylist "parent" being converted "child" giving above error

this line:

string[][] childstring=child.toarray(new string[parent.size()][child.size()]); 

is trying create array of arraylists child.toarray() return arraylist[] exception states

source[0] of type java.util.arraylist cannot stored in destination array of type java.lang.string[][]

to need create array , loop populate

string[][] childstring = new string[parent.size()][child.size()]; int = 0; int j = 0; for(arraylist<string> al : child) {     for(string s: al)     {         childstring[i][j] = s;         j++;     }     i++;     j = 0; } 

you have wary if arraylist sizes vary.