List 根据对象的属性去重

Java List

一、去除List中重复的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的写法：

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:

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

重写Person对象的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;
}

下面对象去重的代码：

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, "胶布虫");
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:

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:

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

还有一种写法:

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));