How Can I Efficiently Convert a Java List to a Map?

How to Convert a List to a Map in Java: Optimal Approaches

Converting a List to a Map in Java is a common task that can be achieved in various ways. One approach is to iterate through the List and populate a Map manually. However, there are more efficient methods to perform this conversion using built-in Java functionality.

Stream-Based Approach (Java 8 or Later)

With Java 8, the Stream API provides a concise and performant way to convert a List to a Map. This approach utilizes the Collectors.toMap() method:

Map<Integer, String> map = 
    list.stream().collect(Collectors.toMap(Item::getKey, item -> item));

Here, the Item::getKey lambda function retrieves the key for each element in the list, while the item -> item lambda retrieves the value.

Manual Approach Using a Loop

For pre-Java 8 versions or if streaming is not suitable, a manual approach can be used:

List<Object[]> results;
Map<Integer, String> resultsMap = new HashMap<>();
for (Object[] o : results) {
    resultsMap.put((Integer) o[0], (String) o[1]);
}

This approach involves iterating through the List and manually extracting key and value pairs, which can be less efficient than the stream-based approach.

Using a Binary Operator for Non-Bijective Functions

If the mapping function is not bijective (i.e., multiple values can map to the same key), a binary operator can be used to merge duplicate key values. For example, if we want to find the sum of values for each unique key:

Map<String, Integer> map = 
    intList.stream().collect(toMap(i -> String.valueOf(i % 3), 
                                   i -> i, 
                                   Integer::sum));

Here, the binary operator Integer::sum provides the logic for merging duplicate key values.

Conclusion

While the optimal approach for converting a List to a Map depends on the specific requirements, using the Stream API with Collectors.toMap() is recommended for its efficiency and conciseness. The manual approach can be used in older versions of Java or when streaming is not applicable.

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