Interview: how do lists duplicate objects according to their attributes?

One 、 Remove List Repeated in String

public List<String> removeStringListDupli(List<String> stringList) {
    Set<String> set = new LinkedHashSet<>();
    set.addAll(stringList);

    stringList.clear();

    stringList.addAll(set);
    return stringList;
}

Or use Java8 Writing ：

List<String> unique = list.stream().distinct().collect(Collectors.toList());

Two 、List Medium object de duplication

For example, now there is a Person class :

public class Person {
    private Long id;

    private String name;

    public Person(Long id, String name) {
        this.id = id;
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

rewrite Person Object's equals() Methods and hashCode() Method :

 @Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    Person person = (Person) o;

    if (!id.equals(person.id)) return false;
    return name.equals(person.name);
}

@Override
public int hashCode() {
    int result = id.hashCode();
    result = 31 * result + name.hashCode();
    return result;
}

The following object de duplication code ：

Person p1 = new Person(1l, "jack");
Person p2 = new Person(3l, "jack chou");
Person p3 = new Person(2l, "tom");
Person p4 = new Person(4l, "hanson");
Person p5 = new Person(5l, " Tape worm ");

List<Person> persons = Arrays.asList(p1, p2, p3, p4, p5, p5, p1, p2, p2);

List<Person> personList = new ArrayList<>();
//  duplicate removal 
persons.stream().forEach(
    p -> {
        if (!personList.contains(p)) {
            personList.add(p);
        }
    }
);
System.out.println(personList);

List Of contains() Method to implement the object equals Method to compare , Actually rewrite equals() Just fine , But rewrite equals It is best to hashCode Also rewrite the .

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You can see :

• http://stackoverflow.com/questions/30745048/how-to-remove-duplicate-objects-from-java-arraylist

• http://blog.csdn.net/growing_tree/article/details/46622579

3、 ... and 、 According to the properties of the object

The following is based on Person Object's id duplicate removal , What should I do ？

Write a method :

  public static List<Person> removeDupliById(List<Person> persons) {
        Set<Person> personSet = new TreeSet<>((o1, o2) -> o1.getId().compareTo(o2.getId()));
        personSet.addAll(persons);

        return new ArrayList<>(personSet);
    }

adopt Comparator The comparator , Compare object properties , The same goes back to 0, To achieve the purpose of filtration .

Let's look at the cool Java8 How to write it :

import static java.util.Comparator.comparingLong;
import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.toCollection;

//  according to id duplicate removal 
List<Person> unique = persons.stream().collect(
    collectingAndThen(
        toCollection(() -> new TreeSet<>(comparingLong(Person::getId))), ArrayList::new)
);

This cool code is google Of , I don't understand how it works , Wait for me to study , Write another article specifically to elaborate .

There's another way to write it :

public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
    Map<Object, Boolean> map = new ConcurrentHashMap<>();
    return t -> map.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}

// remove duplicate
persons.stream().filter(distinctByKey(p -> p.getId())).forEach(p -> System.out.println(p));

java8 It does simplify a lot of lengthy operations , Streamlined the code , Boy , Research java8 Go for it ！