Putting items in ascending or descending order by key fields. The English language meaning of sort is simply to categorise, e.g. to put all your bills into folders by category, e.g. all the electric bills together, all the phone bills together. An internal sort is done strictly in RAM (Random Access Memory). An external sort uses hard disk temporary files. A sort is stable if it naturally preserves existing order when you have duplicate keys. If you want existing order preserved and you are using an unstable sort, you must add an extra sequence number key to your records and use that as a secondary tie-breaking key.
|Internal Sort||Bad Sorts|
|Comparators||Descending/Inverse/Reverse Order with reverseOrder|
|Predefined Comparators||Ascending/Descending Indicators|
|Sorting Speed||Complete Example|
|External Sort||Learning More|
In the Java version 1.2 or later there are built-in sorts such as Arrays.sort and Collections.sort See java.util.Comparator and java.util.Comparable for different sorting orders. For String sorting in various international orders see java.text.Collator, java.text.RuleBasedCollator and java.text.CollationKey.
import java.util.Arrays; //... // works for: // int myArray; // Object myArray; // Dog myArray; // byte myArray; // etc. Arrays.sort( myArray );or an ArrayList (or any other Collection) like
import java.util.ArrayList; import java.util.Collections; ... Collections.sort( myArrayList );
Note that there is no such method as ArrayList.sort.
return -( (String )a ).compareTo( (String)b );
If you need a Comparator for sorting Strings in a case-insensitive way, you can use the pre-build Comparator String.CASE_INSENSITIVE_ORDER. It is effectively the same as if you had asked the Comparable/Comparator Cutter Amanuensis to
You don’t always have to write your own Comparator. Oracle provides some predefined ones such as String.CASE_INSENSITIVE_ORDER. This is a Comparator, though it is a constant, namely a static final member of the String class that can only be used to sort Strings.
There is also Collections.reverseOrder (Comparator) which can take any Comparator and create one that sorts in reverse order.
In the following table, speed is measured relative to RadixSort on a 100,000 item set of 5-character random keys of the form A9999 where SunSort is arbitrarily assigned 100. Tests done under Java 1.4.1 java.exe on a 233 MHz Pentium 128 MB RAM. Bigger is better.
(bigger is better)
|SunSort (Java’s Built-In Sort)||100||yes|
This is a most excellent sort. It uses a modified QuickSort for primitives and a MergeSort (sometimes called a tournament sort for objects). The merge sort makes a clone of the array (but not the objects) before it starts, so be aware of the memory usage. There is no reason to use any but this sort except for very large sorts with keys of 4 characters or less where RadixSort might beat it out. Of course you can’t use this in the older Java versions before it was available. Invoke with Arrays.sort and Collections.sort. Beware when using the versions with startIndex and endIndex. The endIndex points one past the last element. Here is how you use it:
|RadixSort||86||yes||Good for very large sorts, takes same time per item no matter how many items. Ordinary sorts take more time per item as the number of items goes up. More complicated Comparator routine to write. Likes short keys. If you double the length of the key the sort time doubles.|
|HeapSort||48||no||Works best when data are almost already sorted.|
|ShellSort||34||no||Small and quick-loading. Good for small sorts under 2000 items.|
|QuickSort||14||no||Likes random data to sort. Order in the data can cause it to take a very long time to sort. Not good on very small sorts. Varies wildly in time to sort, sometimes a speed demon, sometimes a pig. It just depends on the luck of the draw. I’m told the version I have could be made much better with a few tweaks.|
Abundance uses Op-Tech sort. It rolls itself out to disk, calls in Opt-Tech sort, giving it all of RAM, then rolls itself back in.
You can find various sorts that can handle data too big to fit in RAM by googling for Java big sort. Usually such big sorts use an N-way Merge Sort algorithm.
You might experiment with a merge sort to use on a multi-core CPU (Central Processing Unit). You split the data into separate datasets and sort each on a separate threads then merge the results in a single pass.
One is a selection sort:
Select the largest of the remaining N-1 keys and move its record to the output area. etc.
Another is the bubble sort, where you pairwise swap elements in a pass through all the elements to percolate the biggest element to the top. Then repeat for the list not containing the biggest elements so far. etc.
A variant of the bubble sort, that is a tiny bit faster is the bi-directional sort that percolates the smallest element to the bottom and the biggest element to the top in alternating direction passes, gradually working toward the middle.
Another is the insertion sort, where you create a new list and gradually add to it, always keeping the new list in sorted order by inserting the new element in the place where it belongs. You find the place by binary search and slide all the subsequent elements down to make room for it. You sometimes have to use this type of sort when you don’t have all the elements at the start of the sort. They are just given to you one at a time and you want a sorted list immediately output after each element is added.
Sorting ascending order means sorting with the small elements first then the big. This is usual ordering. Descending order means sorting with the big elements first then the small.
If you have a Comparator or Comparable of some kind, you can convert it into one that sorts into the reverse of the usual order, e. g. if the original sorts alphabetically, the new one will sort in reverse alphabetical order. Here is how you use it:
If you don’t have a suitable base Comparator, just write an ordinary Comparator from scratch and reverse the operands to each compare inside it, or return - result instead of result.
You can’t use Comparators or Collections.reverseOrder to sort arrays of primitives. How could you sort an array of ints in reverse order? Here are some ideas from the mind of Eric Sosman:
|31 Variations on Ascending/Descending Icons for Sort Order|
|Alpha||29 × 27||4||This technique is clear for alphabetic sorts. You need a fairly big icon to make it readable. It would be paired with the numeric icon, also letting you know if you are getting an alpha or numeric collating sequence, or it could be used for both.|
|Arrow||12 × 16||0||Direction of pointing is opposite to Thunderbird.|
|Arrow and Bars||31 × 30||1||Arrow and bars.|
|Bars||16 × 15||1||The longer bars represent the bigger records.|
|Caret||16 × 12||0||Can be kept small. Direction of pointing is opposite to Thunderbird.|
|Climbing||16 × 16||0||This convention helps break the connection with reading direction and suggests the climbing association.|
|Crystal||21 × 19||0||Option for OpenOffice. Open Office allows you to scale the icons to various sizes. Largest variant shown here.|
|Dice||22 × 64||0||Bigger numbers on dice indicate direction. Needs to be quite large vertically.|
|Exclamation||!||¡||3 × 12||0||This can be done without images, using simple text. ¡ is ¡ or '\u00a1'|
|Dual||14 × 6||0||Direction of pointing is opposite to Thunderbird. It points up for ascending and gets fatter at the bottom to suggest bigger items are at the end of the list. The user will usually assume the ascending icon is on the left on matter what it looks like. You display a pair. The bright half of the pair indicates the current order. You click the icon anywhere to toggle the direction. You don’t have to click just in the dim triangle.|
|Galaxy||22 × 24||0||Default for Open Office. This technique similar to alpha, except the arrow always point down (Tango convention) and the icons are more compact. Open Office allows you to scale the icons to various sizes. Largest variant shown here.|
|Gnome||32 × 32||0||Like arrow with a side decoration.|
|Gradient||16 × 16||1||The more intense/darker side of the icon represents the bigger records. The ascending icon suggests the heavier records settling to the bottom.|
|High Contrast||22 × 26||0||Option for OpenOffice. Open Office allows you to scale the icons to various sizes. Largest variant shown here. Works best on a darker background.|
|Industrial||21 × 19||0||Option for OpenOffice. Open Office allows you to scale the icons to various sizes. Largest variant shown here.|
|Intellij||6 × 16||0||Used in JetBrains Intellij Idea IDE (Integrated Development Environment).|
|Mac OSX||7 × 7||0||Direction of pointing is opposite to Thunderbird. It points up for ascending and gets fatter at the bottom to suggest bigger items are at the end of the list.|
|Music||8 × 16||0||Musical sharp and flat. This can be done without images, using simple text if you have a font that supports them. Sharp is '\u266f' and flat is '\u266d'.|
|Numeric||30 × 28||2||This technique is clear for numeric sorts. You need a fairly big icon to make it readable. It would be paired with the alpha icon, also letting you know if you are getting an alpha or numeric collating sequence.|
|Ramp||16 × 7||0||Ascending/descending ramp. Ties nicely with the words ascending and descending. Also looks a bit like a graph. Might be confused with a volume control.|
|Stairs||15 × 12||2||Ascending/descending staircase. Ties nicely with the words ascending and descending.|
|Tango||31 × 29||1||Proposed Tango standard. Similar to arrow and bars. In Tango, the arrow always points down. Unfortunately, it takes up a fair bit of screen real estate to be legible.|
|Text||ascending||descending||81 × 15||0||ASCII (American Standard Code for Information Interchange) text. Takes up excessive horizontal space.|
|Thunderbird||8 × 7||0||This is the convention Thunderbird email uses to describe how data are
currently sorted. Hard to see on a pale background because it has white
elements. What you see here is an improved anti-aliased version of what
Thunderbird itself uses. Unfortunately it points down for ascending and gets
fatter at the top to suggest bigger items are at the top of the list. I think
the intent is either:
|Thunder Triangle||16 × 14||0||Could be even smaller. Direction of pointing is same as Thunderbird. Unfortunately it points down for ascending and gets fatter at the top to suggest bigger items are at the top of the list.|
|Thunder Triangle (small)||8 × 7||0||Direction of pointing is same as Thunderbird. Unfortunately it points down for ascending and gets fatter at the top to suggest bigger items are at the top of the list.|
|Triangle||16 × 14||2||Direction of pointing is opposite to Thunderbird. It points up for ascending and gets fatter at the bottom to suggest bigger items are at the end of the list. The problem with any of the triangle based icons is there are two established contradictory conventions loose in the world. Neither is about to die soon. You are thus guaranteed to confuse people since they must deal with both in the course of a day. That is too bad. The triangle is easy to read even in tiny renderings.|
|Triangle (small)||8 × 7||0||Direction of pointing is opposite to Thunderbird. It points up for ascending and gets fatter at the bottom to suggest bigger items are at the end of the list.|
|Turtle||30 × 19||0||Descending is associated with inverted. The descending icon has the unfortunate connotation of defunct.|
|Water Drop||16 × 24||0||The water drop falls down, with the fat part at the bottom.|
|Vista||4 × 7||0||Microsoft Windows Vista standard. A bit too small to see clearly. Direction of pointing is same as Thunderbird. Unfortunately it points down for ascending and gets fatter at the top to suggest bigger items are at the top of the list. Unfortunately it points down for ascending and gets fatter at the top to suggest bigger items are at the top of the list.|
Please let me know at † which one you like best so I can record your vote.
Perhaps some day the association and icon choices will be a configurable part of LAF (Look And Feel) so users will have consistent conventions across all the apps they use. Consistency is the problem. You can find variants by googling view-sort-ascending
I have found the majority up application use an upward arrow to indicate that items are currently displayed in ascending order. No matter what you do, you are going do it backwards for some people and irritate them. So you might use indicators that don’t have any arrows or at least no confusing up/down arrows. Let the user figure out what your colours or letters A/D mean for example.
You can clarify the meaning of your icons with tooltip help on the icon to say things like:
My current favourite is the gradient. It avoids the ambiguity of arrow direction conventions and has a nice physical metaphor. Perhaps I will have to make the icons configurable. Any convention would eventually work, if there were consensus, even something completely arbitrary would work. I am hoping this little essay will help form a consensus and encourage standardisation.
However, because of Java’s lack of orthogonality, your List sort won’t work for arrays of such Objects. You need to write very similar code to do that. Even that array version won’t sort an array of primitives such as long, int or byte. You have to write yet another slightly different version of the sort to handle each type of primitive.
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