MergeSort by subratred8

src/main/java/com/abranhe/allalgorithms/sorting/MergeSort_subratred.java

@@ -0,0 +1,66 @@
+package com.abranhe.allalgorithms.sorting;
+
+import java.util.List;
+
+public class MergeSort_subratred {// Wrapper method for the real algorithm
+	// T is the generic type which will be instantiated at runtime
+	//  elementas are required to be comparable
+	public static <T extends Comparable<T>> void sort(T[] a) {
+		mergesort(a, 0, a.length-1);
+	}
+
+	// Recursive mergesort method, following the pseudocode
+	private static <T extends Comparable<T>> void mergesort (T[] a, int i, int j) {
+		if (j-i < 1) return;
+		int mid = (i+j)/2;
+		mergesort(a, i, mid);
+		mergesort(a, mid+1, j);
+		merge(a, i, mid, j);
+	}
+
+	// Merge method
+	// Here we need to allocate a new array, but Java does not allow allocating arrays of a generic type
+	// As a work-around we allocate an array of type Object[] the use type casting
+	// This would usually generate a warning, which is suppressed
+	@SuppressWarnings("unchecked") private static <T extends Comparable<T>> void  merge(T[] a, int p, int mid, int q) {
+
+		Object[] tmp = new Object[q-p+1]; 
+		int i = p;
+		int j = mid+1;
+		int k = 0;
+		while (i <= mid && j <= q) {
+			if (a[i].compareTo(a[j])<=0)
+				tmp[k] = a[i++];
+			else
+				tmp[k] = a[j++];
+			k++;
+		}
+		if (i <= mid && j > q) {
+			while (i <= mid) 
+				tmp[k++] = a[i++];
+		} else {
+			while (j <= q)
+				tmp[k++] = a[j++];
+		}
+		for (k = 0; k < tmp.length; k++) {
+			a[k+p] = (T)(tmp[k]); // this is the line that woudl generate the warning 
+		}
+	}
+
+	// Main methos to test the code, using Integer Objects
+	public static void main(String[] args) {
+		Integer[] a = new Integer[5];
+		a[0] = new Integer(2);
+		a[1] = new Integer(1);
+		a[2] = new Integer(4);
+		a[3] = new Integer(3);
+		a[4] = new Integer(-1);
+
+		// T will be instantiated to Integer as a resutl of this call
+		MergeSort_subratred.sort(a);
+
+		// Print the result after the sorting
+		for (int i = 0; i < a.length; i++)
+			System.out.println(a[i].toString());
+	}
+}