Get Values and Keys as ArrayList From a HashMap

1. Overview

When we manipulate data structures in Java, one common scenario involves extracting values and keys from a HashMap and organizing them into an ArrayList.

In this quick tutorial, we’ll explore various practical approaches to achieving this goal.

2. Introduction to the Problem

First, let’s create a HashMap object as the input example:

static final HashMap<String, String> DEV_MAP;
static {
    DEV_MAP = new HashMap<>();
    DEV_MAP.put("Kent", "Linux");
    DEV_MAP.put("Eric", "MacOS");
    DEV_MAP.put("Kevin", "Windows");
    DEV_MAP.put("Michal", "MacOS");
    DEV_MAP.put("Saajan", "Linux");
}

As the code above shows, we initialized a HashMap using a static block. The map carries a few developers and the primary operating systems they use.

When discussing the extraction of key and value lists from a map, different scenarios can arise based on specific requirements.

One such scenario involves a direct association between the elements keyList[i] and valueList[i] within the original map, given an index i. In essence, elements at corresponding indices in the key and value lists exhibit correlation within the context of the original map:

Map:
    k1 -> v1
    k2 -> v2
    k3 -> v3

index    :  0,  1,  2
KeyList  : k1, k2, k3
ValueList: v1, v2, v3

The second scenario is relatively straightforward. Here, we aim to extract a key list and a value list from the provided map without retaining concern for the initial key-value pair associations.

This tutorial will cover both scenarios. Additionally, to ensure clarity and verification, we’ll use unit test assertions to verify the correctness of each approach’s outcomes.

3. Using HashMap keySet() and values() Methods

First, let’s address the more straightforward case: obtaining the key and value lists from DEV_MAP while disregarding the associations between elements.

The Map interface provides two methods that allow us to solve the problem quickly:

• keySet() – Get all keys from the map as a Set
• values() – Return all values as a Collection

We can pass the Set and the Collection to ArrayList‘s constructor to get expected list objects, for example, to get the key list:

List<String> keyList = new ArrayList<>(DEV_MAP.keySet());
assertEquals(Lists.newArrayList("Kent", "Eric", "Kevin", "Michal", "Saajan"), keyList);

However, running the test is likely to result in failure. This is attributed to the fact that HashMap doesn’t maintain the order of its entries. In other words, the order of elements in the list cannot be predicted.

Next, let’s employ AssertJ‘s containsExactlyInAnyOrder() to check the elements and ignore their order:

assertThat(keyList).containsExactlyInAnyOrder("Kent", "Eric", "Kevin", "Michal", "Saajan");

Similarly, we can obtain a value list using Map.values():

List<String> valueList = new ArrayList<>(DEV_MAP.values());
assertThat(valueList).containsExactlyInAnyOrder("Linux", "MacOS", "Windows", "MacOS", "Linux");

4. Getting Correlated Key and Value Lists

Now, let’s move to the alternate scenario: obtaining a key list and a value list, where the elements keyList[i] and valueList[i] maintain a correlation within the map.

First, let’s create a method to verify the two lists are expected:

void assertKeyAndValueList(List<String> keyList, List<String> valueList) {
    assertThat(keyList).containsExactlyInAnyOrder("Kent", "Eric", "Kevin", "Michal", "Saajan");
    assertThat(valueList).containsExactlyInAnyOrder("Linux", "MacOS", "Windows", "MacOS", "Linux");
    for (int i = 0; i < keyList.size(); i++) {
        assertThat(DEV_MAP).containsEntry(keyList.get(i), valueList.get(i));
    }
}

As the method above shows, apart from verifying that the two lists should contain desired elements, we ensure that their corresponding elements at the same index are associated in DEV_MAP.

One approach to solve this problem is to iterate through the map’s entries and fill two pre-initialized lists with the key and value of each entry:

List<String> keyList = new ArrayList<>();
List<String> valueList = new ArrayList<>();
for (Map.Entry<String, String> entry : DEV_MAP.entrySet()) {
    keyList.add(entry.getKey());
    valueList.add(entry.getValue());
}

assertKeyAndValueList(keyList, valueList);

The test passes if we give it a run. So this approach does the job.

If we work with Java 8 or later, we can replace the for loop with the forEach() call and a lambda expression to improve the code’s readability:

List<String> keyList = new ArrayList<>();
List<String> valueList = new ArrayList<>();
DEV_MAP.forEach((k, v) -> {
    keyList.add(k);
    valueList.add(v);
});

assertKeyAndValueList(keyList, valueList);

5. Conclusion

In this article, we first discussed the two scenarios of the problem: obtaining a key list and a value list from a HashMap. Later, we explored how to solve the problem in each scenario.