Java List去掉重複物件-java8
阿新 • • 發佈:2019-02-17
一、去除List中重複的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的寫法:
List<String> unique = list.stream ().distinct().collect(Collectors.toList());
二、List中物件去重
比如現在有一個 Person類:
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Person物件的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;
}
下面物件去重的程式碼:
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");
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()方法底層實現使用物件的equals方法去比較的,其實重寫equals()就好,但重寫了equals最好將hashCode也重寫了。
三、根據物件的屬性去重
下面要根據Person物件的id去重,那該怎麼做呢?
寫一個方法吧:
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Comparator比較器,比較物件屬性,相同就返回0,達到過濾的目的。
再來看比較炫酷的Java8寫法:
import static java.util.Comparator.comparingLong;
import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.toCollection;
// 根據id去重
List<Person> unique = persons.stream().collect(
collectingAndThen(
toCollection(() -> new TreeSet<>(comparingLong(Person::getId))), ArrayList::new)
);
這段炫酷的程式碼是google的,還不明白是怎麼個原理,等我好好研究一下,再專門寫篇文章好好闡述一下。
還有一種寫法:
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 確實簡化了很多冗長的操作,精簡了程式碼,小夥,研究java8去吧!