一、去除List中重复的String
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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:
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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:
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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:
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@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:
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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://www.tuohang.net/article/252195.html
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:
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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:
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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:
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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.tuohang.net/article/252195.html
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原文链接:https://blog.csdn.net/jiaobuchong/article/details/54412094