Good job in general. Just few comments.

• All the algorithms are implemented on arrays. It is really an overkill; you don't need a random access to sort a collection.

• I am not sure you need to spell out less, equals or greater. Isn't it what Comparable is for?

• I am not sure that isSorted(Comparable[] a, int lo, int hi) needs to exist at all. It doesn't hurt to have it though.

• mergeArrays

A naked loop usually represents an important algorithm. In this case a for (int k = low; k <= high; k++) loop is even attributed with comment; definitely it is copy(), which deserves a method of its own.

Once a i > mid condition is encountered, there is no point to test it over and over again: break the main loop right away and let a copy() method do the rest. Same goes for j > high.

• quickPartition

Unnecessary comments sometimes create very hard to understand problems. It took me a while to realize that greater at line 153 is an unfortunate copy-paste. Yet another reason to convert a naked loop into a method.

• insertionSort

Same naked loop problem. An inner loop is identical to one of the quickPartition's loops.

• selectionSort

See an insertionSort comment

• shellSort

Did I mention naked loops?

• main

Do something with all those Stopwatches!

PS: you already provided arrayCopy - why not use it?

PPS: the Java experts will comment on Java specifics (final especially).

Good job in general. Just few comments.

• All the algorithms are implemented on arrays. It is really an overkill; you don't need a random access to sort a collection.

• I am not sure you need to spell out less, equals or greater. Isn't it what Comparable is for?

• I am not sure that isSorted(Comparable[] a, int lo, int hi) needs to exist at all. It doesn't hurt to have it though.

• mergeArrays()

  A naked loop usually represents an important algorithm. In this case a for (int k = low; k <= high; k++) loop is even attributed with comment; definitely it is copy(), which deserves a method of its own.

  Once a i > mid condition is encountered, there is no point to test it over and over again: break the main loop right away and let a copy() method do the rest. Same goes for j > high.

• quickPartition()

  Unnecessary comments sometimes create very hard to understand problems. It took me a while to realize that greater at line 153 is an unfortunate copy-paste. Yet another reason to convert a naked loop into a method.

• insertionSort()

  Same naked loop problem. An inner loop is identical to one of the quickPartition's loops.

• selectionSort()

  See an insertionSort comment.

• shellSort()

  Did I mention naked loops?

• main()

  Do something with all those Stopwatches! You already provided arrayCopy - why not use it? The Java experts will comment on Java specifics (final especially).