How Can I Filter a Java 8 Collection by a Specific Property's Distinctness Using Streams?

Java 8: Filtering Collections with Stream API Based on Property Distinctness

In Java 8, the distinct() method is commonly used to remove duplicate elements from a stream. However, by default, it considers objects equal if they are equal according to the default equality check. For cases where distinctness should be determined based on a specific property, a more sophisticated approach is necessary.

One solution without modifying the Person class is to use the distinctByKey() method, which can be implemented as follows:

public static <t> Predicate<t> distinctByKey(Function<t> keyExtractor) {
    Set<object> seen = ConcurrentHashMap.newKeySet();
    return t -> seen.add(keyExtractor.apply(t));
}</object></t></t></t>

To filter a collection of Person objects by their names using this method, we can write:

persons.stream().filter(distinctByKey(Person::getName))

This approach uses a stateful filter to keep track of previously seen key values, ensuring that only the first occurrence of each unique name is preserved. It's important to note that in the case of parallel processing, the distinctByKey() method may not guarantee the ordering of distinct elements, unlike the default distinct() method.

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