Apache Commons Collections OrderedMap

Last updated: January 8, 2024

Written by: baeldung

Reviewed by: Kevin Gilmore

Java Map

Apache Commons Collections

Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.

End-to-end testing is a very useful method to make sure that your application works as intended. This highlights issues in the overall functionality of the software, that the unit and integration test stages may miss.

Playwright is an easy-to-use, but powerful tool that automates end-to-end testing, and supports all modern browsers and platforms.

When coupled with LambdaTest (an AI-powered cloud-based test execution platform) it can be further scaled to run the Playwright scripts in parallel across 3000+ browser and device combinations:

>> Automated End-to-End Testing With Playwright

This article is part of a series:

• Apache Commons Collections Bag
• Apache Commons Collections SetUtils

• Apache Commons Collections OrderedMap (current article)

• Apache Commons Collections BidiMap
• A Guide to Apache Commons Collections CollectionUtils
• Apache Commons Collections MapUtils
• Guide to Apache Commons CircularFifoQueue

1. Overview

The Apache Commons Collections library provides useful classes that complement the Java Collections Framework.

In this article, we will review the interface OrderedMap, which extends java.util.Map.

2. Maven Dependency

The first thing we need to do is to add the Maven dependency in our pom.xml :

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.5.0-M2</version>
</dependency>

You can find the latest version of the library on the Maven Central repository.

**3. OrderedMap Properties**

Simply put, a map that implements the OrderedMap interface:

Maintains order in its set of keys, although the set isn’t sorted
Can be iterated over in both directions with methods: firstKey() and nextKey(), or lastKey() and previousKey()
Can be traversed with a MapIterator (also provided by the library)
Provides methods to find, alter, remove, or replace elements

4. Using OrderedMap

Let’s set up an OrderedMap of runners and their ages in a test class. We will use a LinkedMap – one of the OrderedMap implementations provided in the library.

First, let’s set up arrays of runners and ages that we will use to load the map and verify the order of the values:

public class OrderMapUnitTest {
    private String[] names = {"Emily", "Mathew", "Rose", "John", "Anna"};
    private Integer[] ages = {37, 28, 40, 36, 21};
    private LinkedMap<String, Integer> runnersLinkedMap;
 
    //...
}

Now, let’s initialize our map:

@Before
public void createRunners() {
    this.runnersLinkedMap = new LinkedMap<>();
    
    for (int i = 0; i < RUNNERS_COUNT; i++) {
        runners.put(this.names[i], this.ages[i]);
    }
}

4.1. Forward Iteration

Let’s see how the forward iterator is used:

@Test
public void givenALinkedMap_whenIteratedForwards_thenPreservesOrder() {
    String name = this.runnersLinkedMap.firstKey();
    int i = 0;
    while (name != null) {
        assertEquals(name, names[i]);
        name = this.runnersLinkedMap.nextKey(name);
        i++;
    }
}

Note that when we have reached the last key, the method nextKey() will return a null value.

4.2. Backward Iteration

Now let’s iterate back, starting with the last key:

@Test
public void givenALinkedMap_whenIteratedBackwards_thenPreservesOrder() {
    String name = this.runnersLinkedMap.lastKey();
    int i = RUNNERS_COUNT - 1;
    while (name != null) {
        assertEquals(name, this.names[i]);
        name = this.runnersLinkedMap.previousKey(name);
        i--;
    }
}

Once we reach the first key, the previousKey() method will return null.

**4.3. MapIterator Example**

Now let’s use the mapIterator() method to obtain a MapIterator as we show how it preserves the order of runners as defined in the arrays names and ages:

@Test
public void givenALinkedMap_whenIteratedWithMapIterator_thenPreservesOrder() {
    OrderedMapIterator<String, Integer> runnersIterator 
      = this.runnersLinkedMap.mapIterator();
    
    int i = 0;
    while (runnersIterator.hasNext()) {
        runnersIterator.next();
 
        assertEquals(runnersIterator.getKey(), this.names[i]);
        assertEquals(runnersIterator.getValue(), this.ages[i]);
        i++;
    }
}

4.4. Removing Elements

Finally, let’s check how an element can be removed by index or by object:

@Test
public void givenALinkedMap_whenElementRemoved_thenSizeDecrease() {
    LinkedMap<String, Integer> lmap 
      = (LinkedMap<String, Integer>) this.runnersLinkedMap;
    
    Integer johnAge = lmap.remove("John");
 
    assertEquals(johnAge, new Integer(36));
    assertEquals(lmap.size(), RUNNERS_COUNT - 1);

    Integer emilyAge = lmap.remove(0);
 
    assertEquals(emilyAge, new Integer(37));
    assertEquals(lmap.size(), RUNNERS_COUNT - 2);
}

5. Provided Implementations

Currently, in version 4.1 of the library, there are two implementations of the OrderedMap interface – ListOrderedMap and LinkedMap.

ListOrderedMap keeps track of the order of the key set using a java.util.List. It is a decorator of OrderedMap and can be created from any Map by using the static method ListOrderedMap.decorate(Map map).

LinkedMap is based on a HashMap and improves on it by allowing bidirectional iteration and the other methods of the OrderedMap interface.

Both implementations also provide three methods that are outside the OrderedMap interface:

asList() – obtains a list of type List<K> (where K is the type of the keys) preserving the order of the map
get(int index) – gets the element at position index as opposed to method get(Object o) provided in the interface
indexOf(Object o) – gets the index of the object o in the ordered map

We can cast the OrderedMap into a LinkedMap to use asList() method:

@Test
public void givenALinkedMap_whenConvertedToList_thenMatchesKeySet() {
    LinkedMap<String, Integer> lmap 
      = (LinkedMap<String, Integer>) this.runnersLinkedMap;
    
    List<String> listKeys = new ArrayList<>();
    listKeys.addAll(this.runnersLinkedMap.keySet());
    List<String> linkedMap = lmap.asList();
 
    assertEquals(listKeys, linkedMap);
}

Then we can check the functioning of method indexOf(Object o) and get(int index) in the LinkedMap implementation:

@Test
public void givenALinkedMap_whenSearchByIndexIsUsed_thenMatchesConstantArray() {
    LinkedMap<String, Integer> lmap 
      = (LinkedMap<String, Integer>) this.runnersLinkedMap;
    
    for (int i = 0; i < RUNNERS_COUNT; i++) {
        String name = lmap.get(i);
 
        assertEquals(name, this.names[i]);
        assertEquals(lmap.indexOf(this.names[i]), i);
    }
}

6. Conclusion

In this quick tutorial, we’ve reviewed the OrderedMap interface and its primary methods and implementations.

For further information, see the JavaDoc of the Apache Commons Collections library.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.

Apache Commons Collections BidiMap

« Previous

Apache Commons Collections SetUtils

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.