Java List<Object>去掉重复对象-java8

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。

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

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)
);

这段炫酷的代码是google的，还不明白是怎么个原理，等我好好研究一下，再专门写篇文章好好阐述一下。

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));

java8 确实简化了很多冗长的操作，精简了代码，小伙，研究java8去吧！

参考: 
http://www.cnblogs.com/jizha/p/java_arraylist_duplicate.html

http://stackoverflow.com/questions/29670116/remove-duplicates-from-a-list-of-objects-based-on-property-in-java-8