android sorting array

Question

I am having some issues sorting an array, I wish to sort the array by day which can be either Monday,Tuesday, Wednesday, Thursday or Friday. I have a class which displays everything within the array but I want to order this by the days above, how can I do this?

I've tried using collections.sort and Arrays.sort but doesn't seem to give me the desired effect, any ideas?

public void sortArray() {



}

04-23 12:55:49.010: E/AndroidRuntime(281): FATAL EXCEPTION: main 04-23 12:55:49.010: E/AndroidRuntime(281): java.lang.RuntimeException: Unable to start activity ComponentInfo{org.assignment.timetable/org.assignment.timetable.Timetable}: java.lang.NullPointerException 04-23 12:55:49.010: E/AndroidRuntime(281): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663) 04-23 12:55:49.010: E/AndroidRuntime(281): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679) 04-23 12:55:49.010: E/AndroidRuntime(281): at android.app.ActivityThread.access$2300(ActivityThread.java:125) 04-23 12:55:49.010: E/AndroidRuntime(281): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033) 04-23 12:55:49.010: E/AndroidRuntime(281): at android.os.Handler.dispatchMessage(Handler.java:99) 04-23 12:55:49.010: E/AndroidRuntime(281): at android.os.Looper.loop(Looper.java:123) 04-23 12:55:49.010: E/AndroidRuntime(281): at android.app.ActivityThread.main(ActivityThread.java:4627) 04-23 12:55:49.010: E/AndroidRuntime(281): at java.lang.reflect.Method.invokeNative(Native Method) 04-23 12:55:49.010: E/AndroidRuntime(281): at java.lang.reflect.Method.invoke(Method.java:521) 04-23 12:55:49.010: E/AndroidRuntime(281): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 04-23 12:55:49.010: E/AndroidRuntime(281): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 04-23 12:55:49.010: E/AndroidRuntime(281): at dalvik.system.NativeStart.main(Native Method) 04-23 12:55:49.010: E/AndroidRuntime(281): Caused by: java.lang.NullPointerException 04-23 12:55:49.010: E/AndroidRuntime(281): at org.assignment.timetable.Timetable$MyComparator.compare(Timetable.java:51) 04-23 12:55:49.010: E/AndroidRuntime(281): at org.assignment.timetable.Timetable$MyComparator.compare(Timetable.java:1) 04-23 12:55:49.010: E/AndroidRuntime(281): at java.util.TimSort.countRunAndMakeAscending(TimSort.java:320) 04-23 12:55:49.010: E/AndroidRuntime(281): at java.util.TimSort.sort(TimSort.java:185) 04-23 12:55:49.010: E/AndroidRuntime(281): at java.util.TimSort.sort(TimSort.java:169) 04-23 12:55:49.010: E/AndroidRuntime(281): at java.util.Arrays.sort(Arrays.java:1907) 04-23 12:55:49.010: E/AndroidRuntime(281): at java.util.Collections.sort(Collections.java:1972) 04-23 12:55:49.010: E/AndroidRuntime(281): at org.assignment.timetable.Timetable.sortModules(Timetable.java:71) 04-23 12:55:49.010: E/AndroidRuntime(281): at org.assignment.timetable.Timetable.onCreate(Timetable.java:40) 04-23 12:55:49.010: E/AndroidRuntime(281): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 04-23 12:55:49.010: E/AndroidRuntime(281): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627) 04-23 12:55:49.010: E/AndroidRuntime(281): ... 11 more

use Arrays.sort() on your array, and create your own comparator. — Jave
– Jave, Commented Apr 23, 2012 at 11:21
Google is your friend, here is one example of creating a comparator: vogella.com/blog/2009/08/04/collections-sort-java I'll throw together a quick example too. — Jave
– Jave, Commented Apr 23, 2012 at 11:31
These both are sorting items when they are added i want to take the array already held and sort by day then display the new output — mitchnufc
– mitchnufc, Commented Apr 23, 2012 at 11:35

Jave · Accepted Answer · 2012-04-23 13:19:25Z

2

Here is an example:

    //Assuming that the Module-class has the day stored as a String named "day"
static class MyComparator implements Comparator<Module>{
    private final static HashMap<String, Integer> order = getMap();

    @Override
    public int compare(Module lhs, Module rhs){
        int leftOrder = order.get(lhs.day.toLowerCase());
        int rightOrder = order.get(rhs.day.toLowerCase());
        return leftOrder - rightOrder;
    }

    private final static HashMap<String, Integer> getMap(){
        HashMap<String, Integer> res = new HashMap<String, Integer>();
        res.put("monday",0);
        res.put("tuesday",1);
        res.put("wednesday",2);
        res.put("thursday",3);
        res.put("friday",4);

        return res;
    }
}

And when you have created your array (any way you want) you can sort it like so:

Arrays.sort(myArray, new MyComparator());

Or if you have a list:

Collections.sort(myList, new MyComparator());

The items in your array will now be sorted.

edited Apr 23, 2012 at 13:19

answered Apr 23, 2012 at 11:40

Jave

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11 Comments

Jave Over a year ago

I made it like an inner class, but technically it could be defined anywhere.

mitchnufc Over a year ago

getting this error and I had to remove the Override within MyComparator: The method sort(T[], Comparator<? super T>) in the type Arrays is not applicable for the arguments (ArrayList<Module>, Timetable.MyComparator)

Jave Over a year ago

@mitchnufc First of all, the Arrays.sort() is for arrays, not lists, for a list (such as ArrayList) you would use Collections.sort() instead. Secondly, You didn't mention that you had a custom class for the days, so I assumed they where strings. Just get the type from your Module class and use that instead of the strings in the map, and replace <String> with <Module>

mitchnufc Over a year ago

sorry for the confussion, so do I just stick the Module and replace the String in the HashMap only?

Jave Over a year ago

@mitchnufc I edited the example a bit. it's a little bit hard to make it good as I don't know how you store the day in your Module-class.

|

Tor P · Accepted Answer · 2012-04-23 11:35:48Z

1

public WeekComparator implements Comparator {
private String[] weeksdays = {"monday", "tuesday", "wednesday", "thursday", "friday", "saturday", "sunday"};
public int compare(Object o1, Object o2) {
  if(!(o1 instanceof String && o2 instanceof String)) {
    throw new ClassCastException();
  }
  int day1 = indexOfDay((String)o1);
  int day2 = indexOfDay((String)o2);
  if(day1==day2) return 0;
  return (day1>day2 ? 1 : -1);
}

private int indexOfDay(String s) {
  for(int i = 0 ; i < weekdays.length;i++) {
    if(weekdays[i].equalsIgnoreCase(s)) return i;
  return 10;
}

}

I haven't tested this, but should work.. on paper (eh...screen)

answered Apr 23, 2012 at 11:35

Tor P

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