Java Stream filter duplicate Object field.

In java 8 we cannot filter same field’s object directly.

For example:

Student dart = new Student(“dart”, 20);
Student vecna = new Student(“vecna”, 30);
Student luba = new Student("luba", 20);

If I wanna filter student with identical age. It’s hard to filter without 2 for loop.

Using Java Stream filter with predicate is a good choice to resolve this issue.

First create Java predicate function

public static <T> Predicate<T> distinctBy(Function<? super T, ?> f) {
    Set<Object> objects = new HashSet<>();
    return t -> objects.add(f.apply(t));
}

Then filter the identical age student.

Set<Student> students = students
                       .stream()
                       .filter(distinctBy(Student::age))
                       .collect(Collectors.toSet()));
System.out.println(students);

After that, we should only get these answer.

[{"dart", 20}, {"luba", 30}]

Reference:

collections — Java 8 Distinct by property — Stack Overflow