Translating Lists to Maps Efficiently with Java 8 Streams

In Java 8, a common task is to convert a list of objects into a map using streams and lambdas. This can be achieved in various ways, depending on the desired behavior and the availability of third-party libraries like Guava.

Java 7 and Below Approach

Traditionally, mapping a list to a map involved manual iteration and a handwritten loop:

private Map nameMap(List choices) {
    final Map hashMap = new HashMap();
    for (final Choice choice : choices) {
        hashMap.put(choice.getName(), choice);
    }

    return hashMap;
}

Guava-Based Solutions

Guava provides a convenient method, Maps.uniqueIndex, for generating maps from lists based on a specified key extractor.

Guava with Java 7

private Map nameMap(List choices) {
    return Maps.uniqueIndex(choices, new Function() {

        @Override
        public String apply(final Choice input) {
            return input.getName();
        }

    });
}

Guava with Java 8 Lambdas

Leveraging Java 8 lambdas simplifies the code even further:

private Map nameMap(List choices) {
    return Maps.uniqueIndex(choices, Choice::getName);
}

Collectors to the Rescue

Java 8's Collectors class offers a robust implementation for mapping lists to maps. The toMap collector takes two arguments:

• A key extractor function (a function that retrieves the key for each element)
• A value extractor function (a function that retrieves the value for each element)

In this case, we can use Choice::getName and Function.identity() to retrieve the key and value, respectively:

Map result =
    choices.stream().collect(Collectors.toMap(Choice::getName,
                                              Function.identity()));