|By York Davis||
|August 5, 2004 12:00 AM EDT||
Those familiar with the java.util.Comparator interface of the Java API realize its capabilities for sorting a collection of objects based on an attribute of the objects in the collection. This works well when there is only a single field in which sorting is required. When more complex sorting is necessary, the limitations of sorting on a single field become obvious. What about the situation in which a user desires the functionality to sort selectively on any field in object collection? This article describes an implementation of the Comparator interface that along with the reflection API allows an object to be sorted dynamically on any of its publicly accessible fields.
Let's describe the problem a bit more specifically. A collection of employee Transfer Objects (EmployeeTO class) exists. (For a description of the Transfer Object design pattern consult a software design pattern book.) Each EmployeeTO in the collection is a data container object for a single employee's information. For this example, our simplified EmployeeTO object contains only three pieces of data - employee ID, last name, and salary.
A Human Resources application also exists that uses this collection to display a list of all employee data to HR application users. The users of this system have the following requirements:
- Allow sorting on any field on the report
- Allow control of the sort order
Before delving into our dynamic sorting solution that allows sorting on any attribute, let's first look at a simple solution that supports sorting on a single attribute only. This will demonstrate the basic behaviors of Comparator and from this we'll be able to glean the improvements we wish to make. This solution utilizes the more common use of the Comparator interface. There are two classes required to implement this.
EmployeeTO - Simple Version
First, our EmployeeTO can be made sortable by implementing the Comparator interface as shown in Listing 1. In addition to the getters and setters for ID, last name, and salary, EmployeeTO must implement the compare() and equals() methods in order to meet the Comparator's requirements. These methods define how EmployeeTO is to be sorted. The compare() method takes two parameters, both of type Object. Note that compare() returns an int. This return value tells the sort engine the collating sequence equality of two attribute values from each of the object parameters passed to compare(), respectively. We need to write the code that performs this evaluation. The first step in compare() is to cast the two parameters' objects to EmployeeTO objects and extract the employee IDs by calling the getId() method on each object in turn. Now we can compare the values. There are really only three possible outcomes that can result from this comparison. Table 1 describes the results based on these outcomes.
The other method we must code is the equals() method. Although this method is not used for sorting, it must be implemented in order to meet the contract of the Comparator interface.
Sorting the Collection - Simple Version
The SimpleTest class in Listing 2 adds three EmployeeTO objects to a List, then performs a sort on that Collection. (Listings 2-5 can be downloaded from www.sys-con.com/java/sourcec.cfm.) Line 30 of SimpleTest calls the static sort() method of the Collections class to actually perform the sort as follows.
Collections.sort(elements, new EmployeeTO());
The first parameter passed to sort() is the collection object we wish to have sorted. The second parameter is an object of type Comparator that contains the customized sorting logic - in this case EmployeeTO, which implements the Comparator interface. We certainly could have passed any instance of EmployeeTO as the second parameter to the above method call. However, instead of reusing one of the three values initially added to the collection, I chose to pass a new instance of the class for purposes of clarity.
A quick note on encapsulation and responsibility assigning seems to be in order here. In this case, it makes sense to encapsulate the specific compare() method sorting logic within EmployeeTO. With the information we have thus far, EmployeeTO is the only class that requires the knowledge of how it should be sorted. Later, as a dynamic sorting solution is provided, we'll see this sorting logic moved out of the Transfer Object class as the sorting logic becomes less specific to any one particular Transfer Object implementation.
This implementation will run but falls short when it comes to meeting the user's requirements. Remember, we need to be able to sort on any one of the three fields in EmployeeTO based on a user's choice. And let's not forget about the ability to control the sort order.
Let's think about the options that are available to improve what we have and meet the requirements. One option is to code nested if/else statements in our compare() method to allow for sorting on any field in the object based on some field name parameter passed to EmployeeTO. The problem with this solution is that it's difficult to maintain and the code could get rather lengthy as well. If new fields are added to EmployeeTO, we must update compare() appropriately.
What we would really like to do is invoke any given getter method of our Transfer Object at runtime without having to specifically hard code each possible method call in the compare() method. If we could do this, we could use the results of those method calls to dynamically determine equality. In addition, it would be desirable if this dynamic sorting could be reused for any Transfer Object. The good news is that this functionality can be achieved by leveraging the reflection API and the flexibility of the Collections.sort() method.
The java.lang.reflect.Method class provides the ability to invoke a method of a given object based on the value of a string. For example, using a string containing the value "getId", the method getId() of EmployeeTO can be dynamically invoked at runtime. This string value can then be changed as we wish to cause any of the methods of EmployeeTO to be called.
The reflection API provides a variety of interesting features including the ability to pass parameters to methods and the ability to determine method return types. (For a complete list of these capabilities, consult the Java API Javadoc.) We will need the latter capability as the three getter methods of EmployeeTO return different types and it will thus be necessary to be aware of which type we have when doing the comparison in the compare() method. For example, we'll need to code a different sort of equality test on an int return value as opposed to a string.
A dynamic solution will also need to take advantage of the flexibility of the Collections.sort() method. Recall that in our simple sorting solution we passed an instance of an object that implemented the Comparator interface as the second parameter to Collections.sort(). In that case, it was the EmployeeTO object that contained the sorting logic. This worked great for what we needed it to do. Now, however, we want something a bit more sophisticated and flexible. What if we were to create a class that implemented Comparator, which was separate from each Transfer Object? In it we could place our dynamic sorting code and simply pass an instance of this new class as the second parameter to Collections.sort().
Doing this would cause several desirable results. First, we'll have completely decoupled any sorting logic from our Transfer Objects. This is highly desirable as it decreases not only the size of each Transfer Object by essentially eliminating the need for sorting code, but also removes the need for coding individual field comparison logic. Second, we'll have created a reusable utility class that can be used in many different situations where sorting is required. While reusability is not a specifically stated user requirement for our design, it is certainly desirable.
Figure 1 is a UML diagram (with attribute and method details omitted) that shows how the components of both the simple and the dynamic sorting solutions fit together. Really, the only portion that has changed since our simple solution is where the Comparator interface gets implemented. In the first example, EmployeeTO implemented Comparator directly. Now DynamicComparator implements Comparator and contains an intelligent implementation of the compare() method that can be used by any class that requires a collection of objects to be sorted.
Listing 3 shows the completed code listing for DynamicComparator. There are a number of interesting things about this class. First, note the class signature. The class implements two interfaces - Comparator and Serializable. Comparator should come as no surprise since that interface is the essence of the sorting capabilities we desire. Also, although not a requirement, the API docs recommend that any class implementing Comparator implement Serializable as well.
Second, look at the static sort() method. Classes wishing to utilize DynamicComparator will call this method rather than Collections.sort(). I've decided to make sort() static to mimic the Collections.sort() method. Although the DynamicComparator.sort() method is called statically, internally DynamicComparator creates an instance of itself. This is necessary in order for it to provide access to the nonstatic compare() and equals() methods of the Comparator interface that it supports. Next note how this method takes three parameters. The first is the collection object to be sorted. The second is a string that defines the field of each object in the collection on which sorting should be performed. The last parameter is a Boolean specifying the sort order. These three parameters are used as arguments to create a new instance of DynamicComparator that is the second parameter passed to the Collections.sort() method.
Third, take a look at the compare() method. This is where the reflection code really kicks in. One of the first things we need to obtain is a reference to a method object at line 43. We'll use this reference to call methods dynamically. This task is done using the getMethod() helper method, which obtains this value via reflection. Next, we need to determine the return type of the methods we are about to call. Remember that we will need to compare the attribute values of the two objects passed into compare(). If the return type is an int, for instance, we'll certainly have to write different code to do the comparison than if the return type is a string. Once we have the return type, we examine it and, based on its value, dynamically perform the actual method invocation and resulting comparison of the two values.
Note that currently there are three separate "if" test blocks - one each for string, int, and double. The implementation requires comparison logic for any method return type we expect to encounter. For the range of return types in our EmployeeTO example, these three are sufficient. However, additional code would need to be added to DynamicComparator if comparisons of other types are required - short, java.util.Date, java.math.BigDecimal, etc. Coding for each specific return type here is unavoidable as there is no way to dynamically cast Java objects. Similarly, Java-supplied nonobject data types like int, long, and double use entirely different comparison operators than do first-class object types like String or Date.
There are some other important points about this code. First, DynamicComparator fully supports null values. If either or both of the arguments passed to compare() are null, this method knows how to handle the situation accordingly. Second, look at each return statement within compare(). Remember the requirement that the user be able to control not only the sort field but also the sort order? This code supports the latter by essentially reversing the default sort order with a call to getSortOrder(). This is done if the user has decided to sort the result in descending order based on the Boolean value passed into the constructor from the sort() method. Third, the constructMethodName() method converts a Transfer Object attribute name string into a method name by prepending a "get" string and capitalizing the first character of the passed value. For instance, constructMethodName() would convert "salary" to "getSalary".
Last, the equals() method is needed to complete the interface requirements.
EmployeeTO - Enhanced Version
Listing 4 shows the enhanced version of EmployeeTO. The most obvious change is that EmployeeTO is now even simpler than before. Now that all of the sorting logic has been moved to DynamicComparator and the class no longer implements Comparator, we don't need to implement the compare() and equals() methods.
Sorting the Collection - Dynamic Version
Listing 5 is the code for DynamicTest. The only change between this class and SimpleTest is how the sort is called. Here we pass the three parameters to DynamicComparator.sort() (Collection Object, the decapitalized attribute name, and sort ascending flag) and let the DynamicComparator do the rest.
DynamicTest could just as easily have sorted on last name by passing "lastName" or on employee ID by passing "id" as the second parameter on line 29.
Building a class such as DynamicComparator has many benefits in an application that requires robust sorting capabilities. In this design, we have created a reusable, loosely coupled API that can be used to sort a collection of objects based on getter methods. The sort field is easily configurable and also allows control over the sort order.
This design, however, is not without trade-offs. Although using the reflection API allows us to do lots of cool things, using reflection can slow performance. This is particularly true in applications using pre-1.4 versions of Java. In addition, it's possible that applications wishing to sort very large collections may find DynamicComparator too slow.
Other inadequacies of DynamicComparator might become evident as well. Although it allows sorting on any one attribute of a collection of objects in an easily configurable manner, DynamicComparator does not address the potential need to sort by multiple fields - primary and secondary field sorts like that occur automatically with the ORDER BY clause in Structured Query Language (SQL).
This article introduced a reusable implementation of the Comparator interface that utilizes Java reflection to dynamically sort a collection of objects on any one of any number of fields within that object.
If your application or framework has a need for this specific functionality, perhaps this design will fit your needs.
|Java 04/09/08 09:42:59 AM EDT|
Refer below link for code
|Doug Bell 08/26/04 04:26:21 AM EDT|
I meant to mention one other thing about the articles implementation that nobody else mentioned. The implementation and comment for the DynamicComparator.equals method illustrates confusion about the purpose of the method.
As implemented, the equals method breaks all contracts for equals, including the Comparator contract. It would be better to have left the default implementation of equals rather than override it with a method that always returns true.
The equals method is for comparing the DynamicComparator object to another object to see if two Comparators produce the same results. This would not be too difficult to properly implement by checking for equals between the method and sortAsc fields (and Comparator fields if my previous suggestion was implemented).
One final comment is that the Collection should not be passed to the DynamicComparator constructor. It is only needed by the sort method.
|Doug Bell 08/26/04 04:11:27 AM EDT|
sMyles has the right idea about how to fix the DynamicComparator class. (Some of the other comments and code suggestions missed the mark in one or more areas.) Certainly, you''d rather not have to keep modifying the DynamicComparator class to add new algorithms--this is not the correct design for "encapsulation and responsibility assignment" that the article discusses.
However, instead of sMyles suggestion for option 4, just overload sort() and the DynamicComparator constructor to accept a Comparator for comparing any two fields that don''t implement Comparable. This will also allow fields that do implement Comparable, such as String, to use alternate compare algorithms, such as compareToIgnoreCase.
|David Perelman-Hall 08/11/04 03:26:40 PM EDT|
public final class DynamicComparator implements Comparator
// Object Members
// Private Constructor
// Sort invocation starts here
// Compare for Collections.sort here
// Invoke method to gather two comparable objects
// Sort order getter
EmployeeTO e2 = new EmployeeTO();
EmployeeTO e1 = new EmployeeTO();
EmployeeTO e3 = new EmployeeTO();
DynamicComparator.sort(elements, "getLastName", true);
final Iterator li = elements.iterator();
|sMyles 08/11/04 03:14:36 PM EDT|
It is important to note that invoke() returns an Object, not a primitive.
Knowing this, compare() should do the following:
2. Perform the null comparisons on val1 and val2 (without the introduction of val1Null and val2Null) and return the appropriate value * getSortOrder(), if applicable.
3. If it turns out neither return value is null, return val1.compareTo(val2) * getSortOrder()
4 (optional). Include a special case for String objects and return val1.compareToIgnoreCase(val2) * getSortOrder(). How compare() should treat case sensitivity could be specified as a parameter to sort().
Also of note:
|Ceri 08/10/04 05:07:56 PM EDT|
Sorry, formatting seems to have been stripped from my previous message.
|Ceri 08/10/04 05:06:17 PM EDT|
The compare method needs to handle primitive types separately. All non-primitives that implement comparable can be easily grouped together into a single if block (to replace the String handling section of Listing 3):
Class interfaces = method.getReturnType().getInterfaces();
|Mike 08/10/04 12:16:23 PM EDT|
York the method "public int compare(Object o1, Object o2)" is long and makes the class look cluddered(like it needs to be refactored), especially if more fields where added requiring more test to be implemented in this method. Wouldn''t separate class implementation for each sorting type help with this, but on second thought this would lead to class proliferation. Okay in the future, would sometype of generics in 1.5.0 work here for the comparing class in addition to the usage of reflection to stream line this code.
|York 08/09/04 01:22:26 PM EDT|
Source code now available for download!
|S Opoku-Boadu 08/09/04 03:26:33 AM EDT|
Cannot locate Listings 2 - 5
|S Opoku-Boadu 08/09/04 03:25:52 AM EDT|
Cannot locate Listings 2 - 5
|Trevor Leach 08/09/04 12:38:46 AM EDT|
The listings are not in the print version either. Perhaps we''ll get emailed when they are found?
|D Polansky 08/05/04 05:41:40 PM EDT|
Where are listings 3, 4, and 5 referenced in the article?
What a difference a year makes. Organizations aren’t just talking about IoT possibilities, it is now baked into their core business strategy. With IoT, billions of devices generating data from different companies on different networks around the globe need to interact. From efficiency to better customer insights to completely new business models, IoT will turn traditional business models upside down. In the new customer-centric age, the key to success is delivering critical services and apps wit...
May. 24, 2016 10:45 AM EDT Reads: 577
SYS-CON Events announced today that ContentMX, the marketing technology and services company with a singular mission to increase engagement and drive more conversations for enterprise, channel and SMB technology marketers, has been named “Sponsor & Exhibitor Lounge Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York City, New York. “CloudExpo is a great opportunity to start a conversation with new prospects, but what happens after the...
May. 24, 2016 10:30 AM EDT Reads: 638
SYS-CON Events announced today that 24Notion has been named “Bronze Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York, New York. 24Notion is full-service global creative digital marketing, technology and lifestyle agency that combines strategic ideas with customized tactical execution. With a broad understand of the art of traditional marketing, new media, communications and social influence, 24Notion uniquely understands how to con...
May. 24, 2016 09:30 AM EDT Reads: 1,664
The essence of data analysis involves setting up data pipelines that consist of several operations that are chained together – starting from data collection, data quality checks, data integration, data analysis and data visualization (including the setting up of interaction paths in that visualization). In our opinion, the challenges stem from the technology diversity at each stage of the data pipeline as well as the lack of process around the analysis.
May. 24, 2016 09:15 AM EDT Reads: 773
The demand for organizations to expand their infrastructure to multiple IT environments like the cloud, on-premise, mobile, bring your own device (BYOD) and the Internet of Things (IoT) continues to grow. As this hybrid infrastructure increases, the challenge to monitor the security of these systems increases in volume and complexity. In his session at 18th Cloud Expo, Stephen Coty, Chief Security Evangelist at Alert Logic, will show how properly configured and managed security architecture can...
May. 24, 2016 09:00 AM EDT Reads: 1,940
When it comes to IoT in the enterprise, namely the commercial building and hospitality markets, a benefit not getting the attention it deserves is energy efficiency, and IoT's direct impact on a cleaner, greener environment when installed in smart buildings. Until now clean technology was offered piecemeal and led with point solutions that require significant systems integration to orchestrate and deploy. There didn't exist a 'top down' approach that can manage and monitor the way a Smart Buildi...
May. 24, 2016 08:30 AM EDT Reads: 2,025
SYS-CON Events announced today BZ Media LLC has been named “Media Sponsor” of SYS-CON's 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. BZ Media LLC is a high-tech media company that produces technical conferences and expositions, and publishes a magazine, newsletters and websites in the software development, SharePoint, mobile development and Commercial Drone markets.
May. 24, 2016 06:00 AM EDT Reads: 3,367
WebRTC is bringing significant change to the communications landscape that will bridge the worlds of web and telephony, making the Internet the new standard for communications. Cloud9 took the road less traveled and used WebRTC to create a downloadable enterprise-grade communications platform that is changing the communication dynamic in the financial sector. In his session at @ThingsExpo, Leo Papadopoulos, CTO of Cloud9, will discuss the importance of WebRTC and how it enables companies to fo...
May. 24, 2016 04:30 AM EDT Reads: 2,382
SYS-CON Events announced today TechTarget has been named “Media Sponsor” of SYS-CON's 18th International Cloud Expo, which will take place on June 7–9, 2016, at the Javits Center in New York City, NY, and the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. TechTarget is the Web’s leading destination for serious technology buyers researching and making enterprise technology decisions. Its extensive global networ...
May. 24, 2016 04:00 AM EDT Reads: 2,997
With major technology companies and startups seriously embracing IoT strategies, now is the perfect time to attend @ThingsExpo 2016 in New York and Silicon Valley. Learn what is going on, contribute to the discussions, and ensure that your enterprise is as "IoT-Ready" as it can be! Internet of @ThingsExpo, taking place Nov 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 17th Cloud Expo and will feature technical sessions from a rock star conference faculty ...
May. 23, 2016 05:00 PM EDT Reads: 4,607
The 19th International Cloud Expo has announced that its Call for Papers is open. Cloud Expo, to be held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, Big Data, Internet of Things, DevOps, Containers, Microservices and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportunity. Submit y...
May. 23, 2016 04:00 PM EDT Reads: 1,815
Internet of @ThingsExpo, taking place November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with the 19th International Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world and ThingsExpo New York Call for Papers is now open.
May. 23, 2016 03:00 PM EDT Reads: 1,659
In his keynote at 18th Cloud Expo, Andrew Keys, Co-Founder of ConsenSys Enterprise, will provide an overview of the evolution of the Internet and the Database and the future of their combination – the Blockchain. Andrew Keys is Co-Founder of ConsenSys Enterprise. He comes to ConsenSys Enterprise with capital markets, technology and entrepreneurial experience. Previously, he worked for UBS investment bank in equities analysis. Later, he was responsible for the creation and distribution of life ...
May. 23, 2016 02:30 PM EDT Reads: 1,701
There are several IoTs: the Industrial Internet, Consumer Wearables, Wearables and Healthcare, Supply Chains, and the movement toward Smart Grids, Cities, Regions, and Nations. There are competing communications standards every step of the way, a bewildering array of sensors and devices, and an entire world of competing data analytics platforms. To some this appears to be chaos. In this power panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists will discuss the vast to...
May. 23, 2016 02:00 PM EDT Reads: 2,333
SYS-CON Events announced today that Enzu, a leading provider of cloud hosting solutions, will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. Enzu’s mission is to be the leading provider of enterprise cloud solutions worldwide. Enzu enables online businesses to use its IT infrastructure to their competitive advantage. By offering a suite of proven hosting and management services, Enzu wants companies to foc...
May. 23, 2016 01:45 PM EDT Reads: 2,056
Customer experience has become a competitive differentiator for companies, and it’s imperative that brands seamlessly connect the customer journey across all platforms. With the continued explosion of IoT, join us for a look at how to build a winning digital foundation in the connected era – today and in the future. In his session at @ThingsExpo, Chris Nguyen, Group Product Marketing Manager at Adobe, will discuss how to successfully leverage mobile, rapidly deploy content, capture real-time d...
May. 23, 2016 12:45 PM EDT Reads: 1,330
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, wh...
May. 23, 2016 12:30 PM EDT Reads: 1,790
IoT generates lots of temporal data. But how do you unlock its value? How do you coordinate the diverse moving parts that must come together when developing your IoT product? What are the key challenges addressed by Data as a Service? How does cloud computing underlie and connect the notions of Digital and DevOps What is the impact of the API economy? What is the business imperative for Cognitive Computing? Get all these questions and hundreds more like them answered at the 18th Cloud Expo...
May. 23, 2016 12:30 PM EDT Reads: 2,029
Whether your IoT service is connecting cars, homes, appliances, wearable, cameras or other devices, one question hangs in the balance – how do you actually make money from this service? The ability to turn your IoT service into profit requires the ability to create a monetization strategy that is flexible, scalable and working for you in real-time. It must be a transparent, smoothly implemented strategy that all stakeholders – from customers to the board – will be able to understand and comprehe...
May. 23, 2016 08:15 AM EDT Reads: 2,381
Korean Broadcasting System (KBS) will feature the upcoming 18th Cloud Expo | @ThingsExpo in a New York news documentary about the "New IT for the Future." The documentary will cover how big companies are transmitting or adopting the new IT for the future and will be filmed on the expo floor between June 7-June 9, 2016, at the Javits Center in New York City, New York. KBS has long been a leader in the development of the broadcasting culture of Korea. As the key public service broadcaster of Korea...
May. 23, 2016 03:45 AM EDT Reads: 1,669