Java Data Structure How to - Implement a Generic Merge Sorter with generic Comparator

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Question

We would like to know how to implement a Generic Merge Sorter with generic Comparator.

Answer

 // w w  w  .  ja  va2  s  .  c o  m

import java.util.Comparator;


public class AnimationTester {
  public static void main(String[] args) {

    Integer[] values = new Integer[]{1,2,7,3,5};
    Comparator<Integer> comp = new Comparator<Integer>() {
      public int compare(Integer d1, Integer d2) {
        return d1.compareTo(d2);
      }
    };
    MergeSorter.sort(values, comp);
    for (int i = 0; i < values.length; i++){
      System.out.print(values[i]+" ");
    }
    
  }
}

class MergeSorter {

  public static <E> void sort(E[] a, Comparator<? super E> comp) {
    mergeSort(a, 0, a.length - 1, comp);
  }


  private static <E> void mergeSort(E[] a, int from, int to, Comparator<? super E> comp) {
    if (from == to)
      return;
    int mid = (from + to) / 2;
    // Sort the first and the second half
    mergeSort(a, from, mid, comp);
    mergeSort(a, mid + 1, to, comp);
    merge(a, from, mid, to, comp);
  }

  private static <E> void merge(E[] a, int from, int mid, int to, Comparator<? super E> comp) {
    int n = to - from + 1;
    Object[] values = new Object[n];

    int fromValue = from;

    int middleValue = mid + 1;

    int index = 0;

    while (fromValue <= mid && middleValue <= to) {
      if (comp.compare(a[fromValue], a[middleValue]) < 0) {
        values[index] = a[fromValue];
        fromValue++;
      } else {
        values[index] = a[middleValue];
        middleValue++;
      }
      index++;
    }

    while (fromValue <= mid) {
      values[index] = a[fromValue];
      fromValue++;
      index++;
    }
    while (middleValue <= to) {
      values[index] = a[middleValue];
      middleValue++;
      index++;
    }

    for (index = 0; index < n; index++)
      a[from + index] = (E) values[index];
  }
}

The code above generates the following result.

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