|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?
With the introduction of IoT and Smart Living in every aspect of our lives, one question has become relevant: What are the security implications? To answer this, first we have to look and explore the security models of the technologies that IoT is founded upon. In his session at @ThingsExpo, Nevi Kaja, a Research Engineer at Ford Motor Company, will discuss some of the security challenges of the IoT infrastructure and relate how these aspects impact Smart Living. The material will be delivered i...
Mar. 26, 2017 09:45 PM EDT Reads: 2,027
Apache Hadoop is emerging as a distributed platform for handling large and fast incoming streams of data. Predictive maintenance, supply chain optimization, and Internet-of-Things analysis are examples where Hadoop provides the scalable storage, processing, and analytics platform to gain meaningful insights from granular data that is typically only valuable from a large-scale, aggregate view. One architecture useful for capturing and analyzing streaming data is the Lambda Architecture, represent...
Mar. 26, 2017 08:30 PM EDT Reads: 6,098
As organizations realize the scope of the Internet of Things, gaining key insights from Big Data, through the use of advanced analytics, becomes crucial. However, IoT also creates the need for petabyte scale storage of data from millions of devices. A new type of Storage is required which seamlessly integrates robust data analytics with massive scale. These storage systems will act as “smart systems” provide in-place analytics that speed discovery and enable businesses to quickly derive meaningf...
Mar. 26, 2017 07:45 PM EDT Reads: 9,582
Your homes and cars can be automated and self-serviced. Why can't your storage? From simply asking questions to analyze and troubleshoot your infrastructure, to provisioning storage with snapshots, recovery and replication, your wildest sci-fi dream has come true. In his session at @DevOpsSummit at 20th Cloud Expo, Dan Florea, Director of Product Management at Tintri, will provide a ChatOps demo where you can talk to your storage and manage it from anywhere, through Slack and similar services ...
Mar. 26, 2017 06:45 PM EDT Reads: 4,245
SYS-CON Events announced today that Ocean9will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Ocean9 provides cloud services for Backup, Disaster Recovery (DRaaS) and instant Innovation, and redefines enterprise infrastructure with its cloud native subscription offerings for mission critical SAP workloads.
Mar. 26, 2017 06:30 PM EDT Reads: 2,033
The taxi industry never saw Uber coming. Startups are a threat to incumbents like never before, and a major enabler for startups is that they are instantly “cloud ready.” If innovation moves at the pace of IT, then your company is in trouble. Why? Because your data center will not keep up with frenetic pace AWS, Microsoft and Google are rolling out new capabilities In his session at 20th Cloud Expo, Don Browning, VP of Cloud Architecture at Turner, will posit that disruption is inevitable for c...
Mar. 26, 2017 05:00 PM EDT Reads: 2,120
SYS-CON Events announced today that SoftLayer, an IBM Company, has been named “Gold 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. SoftLayer, an IBM Company, provides cloud infrastructure as a service from a growing number of data centers and network points of presence around the world. SoftLayer’s customers range from Web startups to global enterprises.
Mar. 26, 2017 02:30 PM EDT Reads: 1,762
SYS-CON Events announced today that Conference Guru has been named “Media Sponsor” of SYS-CON's 20th International Cloud Expo, which will take place on June 6–8, 2017, at the Javits Center in New York City, NY. A valuable conference experience generates new contacts, sales leads, potential strategic partners and potential investors; helps gather competitive intelligence and even provides inspiration for new products and services. Conference Guru works with conference organizers to pass great dea...
Mar. 26, 2017 02:15 PM EDT Reads: 4,379
SYS-CON Events announced today that Technologic Systems Inc., an embedded systems solutions company, will exhibit at SYS-CON's @ThingsExpo, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Technologic Systems is an embedded systems company with headquarters in Fountain Hills, Arizona. They have been in business for 32 years, helping more than 8,000 OEM customers and building over a hundred COTS products that have never been discontinued. Technologic Systems’ pr...
Mar. 26, 2017 02:00 PM EDT Reads: 3,384
SYS-CON Events announced today that CA Technologies has been named “Platinum Sponsor” of SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY, and the 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. CA Technologies helps customers succeed in a future where every business – from apparel to energy – is being rewritten by software. From ...
Mar. 26, 2017 01:45 PM EDT Reads: 1,833
With major technology companies and startups seriously embracing Cloud strategies, now is the perfect time to attend @CloudExpo | @ThingsExpo, June 6-8, 2017, at the Javits Center in New York City, NY and October 31 - November 2, 2017, Santa Clara Convention Center, CA. Learn what is going on, contribute to the discussions, and ensure that your enterprise is on the right path to Digital Transformation.
Mar. 26, 2017 01:45 PM EDT Reads: 8,508
SYS-CON Events announced today that Telecom Reseller has been named “Media Sponsor” of SYS-CON's 20th International Cloud Expo, which will take place on June 6–8, 2017, at the Javits Center in New York City, NY. Telecom Reseller reports on Unified Communications, UCaaS, BPaaS for enterprise and SMBs. They report extensively on both customer premises based solutions such as IP-PBX as well as cloud based and hosted platforms.
Mar. 26, 2017 01:15 PM EDT Reads: 2,122
SYS-CON Events announced today that Loom Systems will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Founded in 2015, Loom Systems delivers an advanced AI solution to predict and prevent problems in the digital business. Loom stands alone in the industry as an AI analysis platform requiring no prior math knowledge from operators, leveraging the existing staff to succeed in the digital era. With offices in S...
Mar. 26, 2017 12:45 PM EDT Reads: 1,314
SYS-CON Events announced today that Interoute, owner-operator of one of Europe's largest networks and a global cloud services platform, has been named “Bronze Sponsor” of SYS-CON's 20th Cloud Expo, which will take place on June 6-8, 2017 at the Javits Center in New York, New York. Interoute is the owner-operator of one of Europe's largest networks and a global cloud services platform which encompasses 12 data centers, 14 virtual data centers and 31 colocation centers, with connections to 195 add...
Mar. 26, 2017 12:30 PM EDT Reads: 1,124
SYS-CON Events announced today that T-Mobile will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. As America's Un-carrier, T-Mobile US, Inc., is redefining the way consumers and businesses buy wireless services through leading product and service innovation. The Company's advanced nationwide 4G LTE network delivers outstanding wireless experiences to 67.4 million customers who are unwilling to compromise on ...
Mar. 26, 2017 11:00 AM EDT Reads: 2,206
In his session at @ThingsExpo, Eric Lachapelle, CEO of the Professional Evaluation and Certification Board (PECB), will provide an overview of various initiatives to certifiy the security of connected devices and future trends in ensuring public trust of IoT. Eric Lachapelle is the Chief Executive Officer of the Professional Evaluation and Certification Board (PECB), an international certification body. His role is to help companies and individuals to achieve professional, accredited and worldw...
Mar. 26, 2017 10:45 AM EDT Reads: 632
SYS-CON Events announced today that Infranics will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Since 2000, Infranics has developed SysMaster Suite, which is required for the stable and efficient management of ICT infrastructure. The ICT management solution developed and provided by Infranics continues to add intelligence to the ICT infrastructure through the IMC (Infra Management Cycle) based on mathemat...
Mar. 26, 2017 10:15 AM EDT Reads: 3,015
SYS-CON Events announced today that SD Times | BZ Media has been named “Media Sponsor” of SYS-CON's 20th International Cloud Expo, which will take place on June 6–8, 2017, at the Javits Center in New York City, NY. 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 UAV markets.
Mar. 26, 2017 09:30 AM EDT Reads: 4,315
SYS-CON Events announced today that Cloudistics, an on-premises cloud computing company, has been named “Bronze Sponsor” of SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Cloudistics delivers a complete public cloud experience with composable on-premises infrastructures to medium and large enterprises. Its software-defined technology natively converges network, storage, compute, virtualization, and management into a ...
Mar. 26, 2017 09:15 AM EDT Reads: 1,964
Now that the world has connected “things,” we need to build these devices as truly intelligent in order to create instantaneous and precise results. This means you have to do as much of the processing at the point of entry as you can: at the edge. The killer use cases for IoT are becoming manifest through AI engines on edge devices. An autonomous car has this dual edge/cloud analytics model, producing precise, real-time results. In his session at @ThingsExpo, John Crupi, Vice President and Eng...
Mar. 26, 2017 09:00 AM EDT Reads: 3,888