|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?
SYS-CON Events announced today that Creative Business Solutions will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Creative Business Solutions is the top stocking authorized HP Renew Distributor in the U.S. Based out of Long Island, NY, Creative Business Solutions offers a one-stop shop for a diverse range of products including Proliant, Blade and Industry Standard Servers, Networking, Server Options and Care Packs. As a trusted supplier, CBS guarantees quality controlled stock levels thanks to an Auto...
Apr. 27, 2015 09:15 AM EDT Reads: 4,363
SOA Software has changed its name to Akana. With roots in Web Services and SOA Governance, Akana has established itself as a leader in API Management and is expanding into cloud integration as an alternative to the traditional heavyweight enterprise service bus (ESB). The company recently announced that it achieved more than 90% year-over-year growth. As Akana, the company now addresses the evolution and diversification of SOA, unifying security, management, and DevOps across SOA, APIs, microservices, and more.
Apr. 27, 2015 09:15 AM EDT Reads: 2,437
GENBAND introduced its Real Time Communications (RTC) Client for Lync* to seamlessly combine real-time communications with Lync Instant Messaging (IM) and Presence. “We’re shaking up the economics of delivering Unified Communications (UC) and offering a compelling way to integrate previously bespoke communications technologies,” said Carl Baptiste, GENBAND’s Senior Vice President, Enterprise Solutions. “We’re offering enterprises the best of both worlds by combining our own high availability voice, video and collaboration with Lync’s IM and Presence; creating a single, web centric, client. O...
Apr. 27, 2015 09:00 AM EDT Reads: 1,732
The list of ‘new paradigm’ technologies that now surrounds us appears to be at an all time high. From cloud computing and Big Data analytics to Bring Your Own Device (BYOD) and the Internet of Things (IoT), today we have to deal with what the industry likes to call ‘paradigm shifts’ at every level of IT. This is disruption; of course, we understand that – change is almost always disruptive.
Apr. 27, 2015 09:00 AM EDT Reads: 1,530
After making a doctor’s appointment via your mobile device, you receive a calendar invite. The day of your appointment, you get a reminder with the doctor’s location and contact information. As you enter the doctor’s exam room, the medical team is equipped with the latest tablet containing your medical history – he or she makes real time updates to your medical file. At the end of your visit, you receive an electronic prescription to your preferred pharmacy and can schedule your next appointment.
Apr. 27, 2015 09:00 AM EDT Reads: 1,443
SYS-CON Events announced today that kintone has been named “Bronze Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY, and the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. kintone promotes cloud-based workgroup productivity, transparency and profitability with a seamless collaboration space, build your own business application (BYOA) platform, and workflow automation system.
Apr. 27, 2015 09:00 AM EDT Reads: 1,724
Can call centers hang up the phones for good? Intuitive Solutions did. WebRTC enabled this contact center provider to eliminate antiquated telephony and desktop phone infrastructure with a pure web-based solution, allowing them to expand beyond brick-and-mortar confines to a home-based agent model. It also ensured scalability and better service for customers, including MUY! Companies, one of the country's largest franchise restaurant companies with 232 Pizza Hut locations. This is one example of WebRTC adoption today, but the potential is limitless when powered by IoT.
Apr. 27, 2015 09:00 AM EDT Reads: 5,294
SYS-CON Events announced today that Optimal Design, an Internet of Things solution provider, will exhibit at SYS-CON's Internet of @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Optimal Design is an award winning product development firm offering industrial design and engineering services to the consumer, medical, and defense markets.
Apr. 27, 2015 09:00 AM EDT Reads: 1,691
Chuck Piluso will present a study of cloud adoption trends and the power and flexibility of IBM Power and Pureflex cloud solutions. Speaker Bio: Prior to Data Storage Corporation (DSC), Mr. Piluso founded North American Telecommunication Corporation, a facilities-based Competitive Local Exchange Carrier licensed by the Public Service Commission in 10 states, serving as the company's chairman and president from 1997 to 2000. Between 1990 and 1997, Mr. Piluso served as chairman & founder of International Telecommunications Corporation, a facilities-based international carrier licensed by t...
Apr. 27, 2015 09:00 AM EDT
There are lots of challenges in IoT around secure, scalable and business friendly infrastructure for enterprises. For large corporations, IoT implementations are one of the top priorities of the decade. All industries are seeing a competitive need to sustain by investing in IoT initiatives. The value addition comes from improved customer service, innovative product and additional revenue streams. The data from these IP-connected devices can be leveraged for a variety of business applications as well as responsive action controls. The various architectural building blocks of an IoT ...
Apr. 27, 2015 09:00 AM EDT
@ThingsExpo has been named the Top 5 Most Influential M2M Brand by Onalytica in the ‘Machine to Machine: Top 100 Influencers and Brands.' Onalytica analyzed the online debate on M2M by looking at over 85,000 tweets to provide the most influential individuals and brands that drive the discussion. According to Onalytica the "analysis showed a very engaged community with a lot of interactive tweets. The M2M discussion seems to be more fragmented and driven by some of the major brands present in the M2M space. This really allows some room for influential individuals to create more high value inter...
Apr. 27, 2015 08:45 AM EDT Reads: 5,132
SYS-CON Events announced today that Cisco, the worldwide leader in IT that transforms how people connect, communicate and collaborate, has been named “Gold Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Cisco makes amazing things happen by connecting the unconnected. Cisco has shaped the future of the Internet by becoming the worldwide leader in transforming how people connect, communicate and collaborate. Cisco and our partners are building the platform for the Internet of Everything by connecting the...
Apr. 27, 2015 08:30 AM EDT Reads: 5,840
SYS-CON Events announced today that Liaison Technologies, a leading provider of data management and integration cloud services and solutions, has been named "Silver Sponsor" of SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York, NY. Liaison Technologies is a recognized market leader in providing cloud-enabled data integration and data management solutions to break down complex information barriers, enabling enterprises to make smarter decisions, faster.
Apr. 27, 2015 08:15 AM EDT Reads: 3,902
Participants will reach the final if their IoT solution is liked. A community vote will determine the best solutions submitted in each country, after which an expert jury will select the national winners and the best international IoT solution. Each country's best solution can win a national marketing campaign worth up to €30,000 and become a partner in Deutsche Telekom's participating markets. The winning international solution can become partner of Deutsche Telekom Group across all eight countries and reach out to a potential of 10,8 million business customers. Deutsche Telekom Group has a...
Apr. 27, 2015 08:00 AM EDT Reads: 1,252
Recent technology advances in miniaturization has positioned the wearables as the pinnacle of technology convergence with the human body. We inquire if wearables are mere standard miniaturized devices extended with the connectivity and present our views on considerations like design, applications, performance, efficiency, interoperability, usage scenarios, human device interaction and consequent trade-offs enabling wearables to impart optimal value.
Apr. 27, 2015 08:00 AM EDT Reads: 1,174
WebRTC Summit has announced today that Peter Dunkley has been named summit chair of WebRTC Summit 2015 New York. The 4th International WebRTC Summit will take place on June 9-11, 2015, at the Javits Center in Manhattan, New York. @ThingsExpo anticipates 90% of WebRTC companies & developers will monetize their products & services through IoT by 2016. Peter Dunkley is Technical Director at Acision. He graduated from The University of Edinburgh in 2000 with a BSc (Hons) in Computer Science. After graduation Peter worked on a PSTN switch developing signalling stacks for SS7, ISDN and simi...
Apr. 27, 2015 08:00 AM EDT Reads: 2,194
In this session we look at creating interactive communications via the web by adding messaging, file transfer, and group communication (group chat and audio/video conferencing) into the web experience. We will also discuss potential applications of this technology in areas including B2B, B2C, P2P, and gaming. Peter is Technical Director at Acision. He graduated from The University of Edinburgh in 2000 with a BSc (Hons) in Computer Science. After graduation Peter worked on a PSTN switch developing signalling stacks for SS7, ISDN and similar protocols and creating advanced routing and serv...
Apr. 27, 2015 08:00 AM EDT Reads: 1,232
SYS-CON Events announced today that Windstream, a leading provider of advanced network and cloud communications, has been named “Silver Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9–11, 2015, at the Javits Center in New York, NY. Windstream (Nasdaq: WIN), a FORTUNE 500 and S&P 500 company, is a leading provider of advanced network communications, including cloud computing and managed services, to businesses nationwide. The company also offers broadband, phone and digital TV services to consumers primarily in rural areas.
Apr. 27, 2015 08:00 AM EDT Reads: 5,219
SYS-CON Events announced today that SoftLayer, an IBM company, has been named “Gold Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place June 9-11, 2015 at the Javits Center in New York City, NY, and the 17th International Cloud Expo®, which will take place November 3–5, 2015 at the Santa Clara Convention Center in Santa Clara, CA. SoftLayer operates a global cloud infrastructure platform built for Internet scale. With a global footprint of data centers and network points of presence, SoftLayer provides infrastructure as a service to leading-edge customers ranging from ...
Apr. 27, 2015 08:00 AM EDT Reads: 2,817
The WebRTC Summit 2015 New York, to be held June 9-11, 2015, at the Javits Center in New York, NY, announces that its Call for Papers is open. Topics include all aspects of improving IT delivery by eliminating waste through automated business models leveraging cloud technologies. WebRTC Summit is co-located with 16th International Cloud Expo, @ThingsExpo, Big Data Expo, and DevOps Summit.
Apr. 27, 2015 07:45 AM EDT Reads: 2,468