Java入门到精通-List集合元素去重的8种方法

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public class ListRmDuplicate {

 private List<String> list;

 private List<Player> playerList;

 @BeforeEach

 public void setup() {

 list = new ArrayList<>();

 list.add("kobe");

 list.add("james");

 list.add("curry");

 list.add("zimug");

 list.add("zimug");

 playerList= new ArrayList<>();

 playerList.add(new Player("kobe","10000")); //科比万岁

 playerList.add(new Player("james","32"));

 playerList.add(new Player("curry","30"));

 playerList.add(new Player("zimug","27")); // 注意这里名字重复

 playerList.add(new Player("zimug","18")); //注意这里名字和年龄重复

 playerList.add(new Player("zimug","18")); //注意这里名字和年龄重复

 }

}

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@Test

void testRemove1() {

 /*Set<String> set = new HashSet<>(list);

 List<String> newList = new ArrayList<>(set);*/

 //去重并排序的方法(如果是字符串，按字母表排序。如果是对象，按Comparable接口实现排序)

 //List<String> newList = new ArrayList<>(new TreeSet<>(list));

 //简写的方法

 List<String> newList = new ArrayList<>(new HashSet<>(list));

 System.out.println( "去重后的集合： " + newList);

}

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@Test

void testRemove2() {

 List<String> newList = list.stream().distinct().collect(Collectors.toList());

 System.out.println( "去重后的集合： " + newList);

}

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//三个集合类list、newList、set，能够保证顺序

@Test

void testRemove3() {

 Set<String> set = new HashSet<>();

 List<String> newList = new ArrayList<>();

 for (String str :list) {

 if(set.add(str)){ //重复的话返回false

 newList.add(str);

 }

 }

 System.out.println( "去重后的集合： " + newList);

}

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//优化 List、newList、set，能够保证顺序

@Test

void testRemove4() {

 List<String> newList = new ArrayList<>();

 for (String cd:list) {

 if(!newList.contains(cd)){ //主动判断是否包含重复元素

 newList.add(cd);

 }

 }

 System.out.println( "去重后的集合： " + newList);

}

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@Test

void testRemove5() {

 //Set<Player> playerSet = new TreeSet<>((o1, o2) -> o1.getName().compareTo(o2.getName()));

 Set<Player> playerSet = new TreeSet<>(Comparator.comparing(Player::getName));

 playerSet.addAll(playerList);

 /*new ArrayList<>(playerSet).forEach(player->{

 System.out.println(player.toString());

 });*/

 //将去重之后的结果打印出来

 new ArrayList<>(playerSet).forEach(System.out::println);

}

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@Test

void testRemove6() {

 List<Player> newList = playerList.stream().collect(Collectors

  .collectingAndThen(

   Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Player::getName))),

   ArrayList::new));

 newList.forEach(System.out::println);

}

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@Test

void testRemove7() {

 List<Player> newList = new ArrayList<>();

 playerList.stream().filter(distinctByKey(p -> p.getName())) //filter保留true的值

  .forEach(newList::add);

 newList.forEach(System.out::println);

}

static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {

 Map<Object,Boolean> seen = new ConcurrentHashMap<>();

 //putIfAbsent方法添加键值对，如果map集合中没有该key对应的值，则直接添加，并返回null，如果已经存在对应的值，则依旧为原来的值。

 //如果返回null表示添加数据成功(不重复)，不重复(null==null :TRUE)

 return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;

}

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@Test

void testRemove8() {

 Set<Player> playerSet = new TreeSet<>(Comparator.comparing(o -> (o.getName() + "" + o.getAge())));

 playerSet.addAll(playerList);

 new ArrayList<>(playerSet).forEach(System.out::println);

}