List根据对象属性去重
一、去除List中重复的String
List<String> stringList = Arrays.asList("1", "2","3","1","1","1","1");
Set<String> set = new LinkedHashSet<>();
set.addAll(stringList);
stringList.clear();
stringList.addAll(set);
System.out.println(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
这里根据对象的id属性去重
List<Person> persons = Arrays.asList(p1, p2, p3, p4, p5, p5, p1, p2, p2);
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。
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)
);