一、去除List中重復(fù)的String

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

    stringList.clear();

    stringList.addAll(set);
    return stringList;
}

或使用Java8的寫(xiě)法：

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

二、List中對(duì)象去重

比如現(xiàn)在有一個(gè) Person類(lèi):

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 + '\'' +
                '}';
    }
}

重寫(xiě)Person對(duì)象的 equals() 方法和 hashCode() 方法:

 @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;
}

下面對(duì)象去重的代碼：

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, "膠布蟲(chóng)");

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

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

List 的 contains() 方法底層實(shí)現(xiàn)使用對(duì)象的equals方法去比較的，其實(shí)重寫(xiě) equals() 就好，但重寫(xiě)了equals最好將hashCode也重寫(xiě)了。

可以參見(jiàn):

• http://stackoverflow.com/questions/30745048/how-to-remove-duplicate-objects-from-java-arraylist
• http://blog.csdn.net/growing_tree/article/details/46622579

三、根據(jù)對(duì)象的屬性去重

下面要根據(jù)Person對(duì)象的id去重，那該怎么做呢？

寫(xiě)一個(gè)方法吧:

  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);
    }

通過(guò)Comparator比較器，比較對(duì)象屬性，相同就返回0，達(dá)到過(guò)濾的目的。

再來(lái)看比較炫酷的Java8寫(xiě)法:

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

// 根據(jù)id去重
List<Person> unique = persons.stream().collect(
    collectingAndThen(
        toCollection(() -> new TreeSet<>(comparingLong(Person::getId))), ArrayList::new)
);

這段炫酷的代碼是google的，還不明白是怎么個(gè)原理，等我好好研究一下，再專(zhuān)門(mén)寫(xiě)篇文章好好闡述一下。

還有一種寫(xiě)法:

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 確實(shí)簡(jiǎn)化了很多冗長(zhǎng)的操作，精簡(jiǎn)了代碼，小伙研究java8去吧！