# Shell Sort Algorithm in Java

This article gives provides an overview on Shell Sort together with an implementation in Java.

Shell Sort is an in-place comparison sort algorithm. Shell Sort is a generalization of insertion sort that allows the exchange of items that are far apart. The algorithm perform preliminary work by sorting pairs of elements far apart from each other. The algorithm progressively reduce the gap between elements to be compared as the goal is to reduce the amount of shifting of elements across the array. As the gap is reduced to 1, the algorithm becomes the same as the insertion sort algorithm.

## Algorithm Classification

The following table contains information about the analysis of the Shell Sort Sort algorithm. It defines the worst, average and best cases in terms of time complexity and also the worst case in space complexity.

AttributeValue
ClassSorting Algorithm
ClassificationInternal, In-place, Unstable Algorithm
Data StructureArray
Time Complexity: BestΩ(n log(n))
Time Complexity: AverageΘ(n log2n)
Time Complexity: WorstO(n2)
Space Complexity: WorstO(1)

Algorithm Classification: Shell Sort

Please use the following link for an explanation on Big-O notation and what is good, fair and bad.

## Shell Sort In Java

The ShellSort class implements the Shell Sort algorithm for sorting an array of integers.

public final class ShellSort {

public void sort(int[] collection) {
if (collection != null) {
shellSort(collection);
} else {
throw new IllegalArgumentException("Input paramenter for array to sort is null.");
}
}

private void shellSort(int[] collection) {
int arrayLength = collection.length;

for (int gap = arrayLength / 2; gap > 0; gap /= 2) {
for (int i = gap; i < arrayLength; i++) {
int newElement = collection[i];

int j = i;
while (j >= gap && collection[j - gap] > newElement) {
collection[j] = collection[j - gap];
j -= gap;
}
collection[j] = newElement;
}
}
}
}