Ignore duplicates when producing map using streams

Tackling complex collisions

Key collisions aren't always about choosing the Highlander. Occasionally you'd need to merge values together or perform other sorceries. Imagine summing values using a merge function with Integer::sum:

Map<KeyType, Integer> itemSumMap = items.stream()
    .collect(Collectors.toMap(
        Item::getKey, // Keeper of the key
        Item::getQuantity, // The counter, keeps counting sheep at night
        Integer::sum // Merge function: the eager addition enthusiast
    ));
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Above, our Merge Function becomes an enhusiastic mathematician, summing the quantities of duplicate keys instead of discarding them.

Error 'busting' and debugging

When streaming your data, keep your keyMapper function sharp and vigilant to avoid sneaky null values creating duplicates or null keys. Remember to whitelist your input data before streaming to keep duplicates in check and maintain law and order.

try {
    Map<KeyType, ItemType> map = items.stream()
        // ... other transformations
        .collect(Collectors.toMap(/* arguments */));
} catch (IllegalStateException e) {
    // Handle the error gracefully like a ballerina
}


The art of logging and validation

Sketch a log within your merge function to trace duplicate occurrences, helping you debug and comprehend the peculiarities of your set:

Map<KeyType, ItemType> itemMap = items.stream()
    .collect(Collectors.toMap(
        Item::getKey,
        Function.identity(),
        (first, second) -> {
            // Spotted a repeat offender? Log the key!
            System.out.println("Another one bites the dust: " + first.getKey());
            return first;
        }
    ));


Do remember to regularly verify your keyMapper function to ensure it does not produce "cloned" keys—the usual suspects in debugging nightmares.

Winning strategies

Execution is key in stream processing, so keep the syntax in check:

.collect(Collectors.toMap(/* Key mapper */, /* Value mapper */, /* Merge function */));


Go for a BinaryOperator as your merge function to manage duplicates. It's like a referee in a boxing match, resolving conflicts in your map.

Keep JavaDoc and other learning resources as your dear guide. They hold the torch to lighting up streams in Java!