Thread: Merge Sort

I have a lab that requires me to sort type T arrays with assertion sort and merge sort. But my merge sort is giving me some problems and I'm not sure why. Thank you in advance for any help!

ArraySort.java

 
/**
 * @param <T>
 *
 */
public class ArraySort<T> {
 
	/**
	 * @param a
	 * @param length
	 */
	public static<T extends Comparable<T>> void insertionSort(T[] a, int length){
		int j;
		T temp; 
		for(int i=1;i<length;i++){
			temp = a[i]; 
			j=i;
				while(j>0 && (a[j-1].compareTo(temp)>0)) { 
					a[j] = a[j-1];
					j--; 
				}
			a[j] = temp; 
		} 
	}
 
 
	/**
	 * @param a
	 * @param length
	 */
	@SuppressWarnings("javadoc")
	public static<T extends Comparable<T>> void mergeSort(T[] a, int length){
 
			SortMerge(a, length, 0, length-1);
 
	}
 
	/**
	 * @param a
	 * @param length
	 * @param first
	 * @param last 
	 */
	@SuppressWarnings("javadoc")
	private static<T extends Comparable<T>> void SortMerge(T[] a, int length, int first, int last ){
		if(first < last){
			int middle = first + (last-first)/2;
			SortMerge(a, length, first, middle);
			SortMerge(a, length, middle+1, last);
			merge(a, length, first, middle, last);
		}
	}
 
	/**
	 * @param a
	 * @param length
	 * @param first
	 * @param middle
	 * @param last
	 */
	@SuppressWarnings({ "null", "javadoc" })
	private static<T extends Comparable<T>> void merge(T[] a, int length, int first, int middle, int last){
		int i = first;
		int j = middle + 1;
		int k = first;
		T[] temp = null;
		for (int x = first; x < length; x++){
			temp[x] = a[x];
		}
		while(i <= middle && j <= last){
			if(temp[i].compareTo(temp[j]) > 0){
				a[k] = temp[i];
				i++;
			}
			else{
				a[k] = temp[k];
				j++;
			}
			k++;
		}
		while(i <= middle){
			a[k]=temp[i];
			k++;
			i++;
		}
	}
}

and the driver just in case you needed it

SortDriver.java

 
public class SortDriver {
	/**
	 * @param args
	 */
	@SuppressWarnings("javadoc")
	public static void main(String[] args){
	String[] test1 = {"a","b","c","d","z","w","x","t","q","r","m","n","i","f"};
 
 
		ArraySort.mergeSort(test1, 14);
 
		for(int i=0; i < 14; i++)
			System.out.print(test1[i]+" ");
 
	}
}

and the output

Exception in thread "main" java.lang.NullPointerException
	at ArraySort.merge(ArraySort.java:69)
	at ArraySort.SortMerge(ArraySort.java:51)
	at ArraySort.SortMerge(ArraySort.java:49)
	at ArraySort.SortMerge(ArraySort.java:49)
	at ArraySort.SortMerge(ArraySort.java:49)
	at ArraySort.mergeSort(ArraySort.java:35)
	at SortDriver.main(SortDriver.java:15)

I think it has something to do with my temp array because once i start trying to put a in to temp that causes the error but i cant create t without setting it to null at first