List 根据对象的属性去重

一、去除List中重复的String

1. public List<String> removeStringListDupli(List<String> stringList) {
2. Set<String> set = new LinkedHashSet<>();
3. set.addAll(stringList);
5. stringList.clear();
7. stringList.addAll(set);
8. return stringList;
9. }

或使用Java8的写法：

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

二、List中对象去重

比如现在有一个 Person类:

1. public class Person {
2. private Long id;
4. private String name;
6. public Person(Long id, String name) {
7. this.id = id;
8. this.name = name;
9. }
11. public Long getId() {
12. return id;
13. }
15. public void setId(Long id) {
16. this.id = id;
17. }
19. public String getName() {
20. return name;
21. }
23. public void setName(String name) {
24. this.name = name;
25. }
27. @Override
28. public String toString() {
29. return "Person{" +
30. "id=" + id +
31. ", name='" + name + '\'' +
32. '}';
33. }
34. }

重写Person对象的equals()方法和hashCode()方法:

1. @Override
2. public boolean equals(Object o) {
3. if (this == o) return true;
4. if (o == null || getClass() != o.getClass()) return false;
6. Person person = (Person) o;
8. if (!id.equals(person.id)) return false;
9. return name.equals(person.name);
11. }
13. @Override
14. public int hashCode() {
15. int result = id.hashCode();
16. result = 31 * result + name.hashCode();
17. return result;
18. }

下面对象去重的代码：

1. Person p1 = new Person(1l, "jack");
2. Person p2 = new Person(3l, "jack chou");
3. Person p3 = new Person(2l, "tom");
4. Person p4 = new Person(4l, "hanson");
5. Person p5 = new Person(5l, "胶布虫");
7. List<Person> persons = Arrays.asList(p1, p2, p3, p4, p5, p5, p1, p2, p2);
9. List<Person> personList = new ArrayList<>();
10. // 去重
11. persons.stream().forEach(
12. p -> {
13. if (!personList.contains(p)) {
14. personList.add(p);
15. }
16. }
17. );
18. System.out.println(personList);

List 的contains()方法底层实现使用对象的equals方法去比较的，其实重写equals()就好，但重写了equals最好将hashCode也重写了。 

三、根据对象的属性去重

下面要根据Person对象的id去重，那该怎么做呢？
写一个方法吧:

1. public static List<Person> removeDupliById(List<Person> persons) {
2. Set<Person> personSet = new TreeSet<>((o1, o2) -> o1.getId().compareTo(o2.getId()));
3. personSet.addAll(persons);
5. return new ArrayList<>(personSet);
6. }

通过Comparator比较器，比较对象属性，相同就返回0，达到过滤的目的。
再来看比较炫酷的Java8写法:

1. import static java.util.Comparator.comparingLong;
2. import static java.util.stream.Collectors.collectingAndThen;
3. import static java.util.stream.Collectors.toCollection;
5. // 根据id去重
6. List<Person> unique = persons.stream().collect(
7. collectingAndThen(
8. toCollection(() -> new TreeSet<>(comparingLong(Person::getId))), ArrayList::new)
9. );

还有一种写法:

1. public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
2. Map<Object, Boolean> map = new ConcurrentHashMap<>();
3. return t -> map.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
4. }
6. // remove duplicate
7. persons.stream().filter(distinctByKey(p -> p.getId())).forEach(p -> System.out.println(p));