eclipse - Simple MergeSort Java -

i have class lab due tomorrow , absolutely stumped. requirement simply: make mergesort algorithm sorts given arraylist using comparables. "a" have final sorted list. throwing following:

exception in thread "main" java.lang.indexoutofboundsexception: index: 1, size: 1 @ java.util.arraylist.rangecheck(arraylist.java:653) @ java.util.arraylist.get(arraylist.java:429) @ sorts.merge(sorts.java:142) @ sorts.mergesort(sorts.java:161) @ sorts.mergesort(sorts.java:160) @ sorts.mergesort(sorts.java:159) @ sortstep.sortmenu(sortstep.java:65) @ sortstep.main(sortstep.java:168) 

anything helps! thanks! (i know line numbers don't guys. sorry!)

/**  * takes in entire vector, merge following sections together:  * left sublist a[first]..a[mid], right sublist a[mid+1]..a[last].  * precondition: each sublist in ascending order  *  * @param  *            reference array of integers sorted  * @param first  *            starting index of range of values sorted  * @param mid  *            midpoint index of range of values sorted  * @param last  *            last index of range of values sorted  */ private void merge(arraylist<comparable> a, int first, int mid, int last) {     arraylist<comparable> temp = new arraylist<comparable>();      while (first <= mid && mid <= last) {         if (a.get(first).compareto(a.get(mid)) > 0) {             temp.add(a.get(first));             first++;         } else {             temp.add(a.get(mid));             mid++;         }     }      while (first < mid) {         temp.add(a.get(first));         first++;     }      while (mid <= last) {         temp.add(a.get(mid));         mid++;     }      (int = 0; <= last; i++)         a.set(i, temp.get(i)); }  /**  * recursive mergesort of array of integers  *  * @param  *            reference array of integers sorted  * @param first  *            starting index of range of values sorted  * @param last  *            ending index of range of values sorted  */ public void mergesort(arraylist<comparable> a, int first, int last) {     if (first < last) {         int mid = (first + last) / 2;         mergesort(a, first, mid);         mergesort(a, mid + 1, last);         merge(a, first, last, mid);     } } 

• the method merge expect arguments first, mid, last, passed first, last, mid.
• mid moving ahead, elements may stored in temp.
• the last loop of merge wrong , should start in first when method called, not 0.
• the range of each section wrong.

corrected code:

private void merge(arraylist<comparable> a, int first, int mid, int last) {     arraylist<comparable> temp = new arraylist<comparable>();     mid++;      int firstfirst = first;     int firstend = mid;     while (first < firstend && mid <= last) {         if (a.get(first).compareto(a.get(mid)) > 0) {             temp.add(a.get(first));             first++;         } else {             temp.add(a.get(mid));             mid++;         }     }      while (first < firstend) {         temp.add(a.get(first));         first++;     }      while (mid <= last) {         temp.add(a.get(mid));         mid++;     }      (int = firstfirst; <= last; i++)         a.set(i, temp.get(i - firstfirst)); }  public void mergesort(arraylist<comparable> a, int first, int last) {     if (first < last) {         int mid = (first + last) / 2;         mergesort(a, first, mid);         mergesort(a, mid + 1, last);         merge(a, first, mid, last);     } }