Java List<Object>去掉重复对象-java8

忘是亡心i 2022-07-13 15:07 167阅读 0赞

### 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_stackoverflow.com_questions_30745048_how-to-remove-duplicate-objects-from-java-arraylist]  
[http://blog.csdn.net/growing\_tree/article/details/46622579][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_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][http_stackoverflow.com_questions_29670116_remove-duplicates-from-a-list-of-objects-based-on-property-in-java-8]

[http_stackoverflow.com_questions_30745048_how-to-remove-duplicate-objects-from-java-arraylist]: http://stackoverflow.com/questions/30745048/how-to-remove-duplicate-objects-from-java-arraylist
[http_blog.csdn.net_growing_tree_article_details_46622579]: http://blog.csdn.net/growing_tree/article/details/46622579
[http_www.cnblogs.com_jizha_p_java_arraylist_duplicate.html]: 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]: http://stackoverflow.com/questions/29670116/remove-duplicates-from-a-list-of-objects-based-on-property-in-java-8