List 如何根據(jù)對(duì)象的屬性去重？Java 8 輕松搞定！

一、去除List中重復(fù)的String

public?List?removeStringListDupli(List?stringList)?{
????Set?set?=?new?LinkedHashSet<>();
????set.addAll(stringList);

????stringList.clear();

????stringList.addAll(set);
????return?stringList;
}

或使用Java8的寫法：

List?unique?=?list.stream().distinct().collect(Collectors.toList());

二、List中對(duì)象去重

比如現(xiàn)在有一個(gè) Person類:

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?+?'\''?+
????????????????'}';
????}
}

重寫Person對(duì)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;
}

下面對(duì)象去重的代碼：

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,?"膠布蟲");

List?persons?=?Arrays.asList(p1,?p2,?p3,?p4,?p5,?p5,?p1,?p2,?p2);

List?personList?=?new?ArrayList<>();
//?去重
persons.stream().forEach(
????p?->?{
????????if?(!personList.contains(p))?{
????????????personList.add(p);
????????}
????}
);
System.out.println(personList);

List 的contains()方法底層實(shí)現(xiàn)使用對(duì)象的equals方法去比較的，其實(shí)重寫equals()就好，但重寫了equals最好將hashCode也重寫了。

我們創(chuàng)建了一個(gè)高質(zhì)量的技術(shù)交流群，與優(yōu)秀的人在一起，自己也會(huì)優(yōu)秀起來，趕緊點(diǎn)擊加群，享受一起成長(zhǎng)的快樂。

可以參見:

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

三、根據(jù)對(duì)象的屬性去重

下面要根據(jù)Person對(duì)象的id去重，那該怎么做呢？

寫一個(gè)方法吧:

??public?static?List?removeDupliById(List?persons)?{
????????Set?personSet?=?new?TreeSet<>((o1,?o2)?->?o1.getId().compareTo(o2.getId()));
????????personSet.addAll(persons);

????????return?new?ArrayList<>(personSet);
????}

通過Comparator比較器，比較對(duì)象屬性，相同就返回0，達(dá)到過濾的目的。

再來看比較炫酷的Java8寫法:

import?static?java.util.Comparator.comparingLong;
import?static?java.util.stream.Collectors.collectingAndThen;
import?static?java.util.stream.Collectors.toCollection;

//?根據(jù)id去重
List?unique?=?persons.stream().collect(
????collectingAndThen(
????????toCollection(()?->?new?TreeSet<>(comparingLong(Person::getId))),?ArrayList::new)
);

這段炫酷的代碼是google的，還不明白是怎么個(gè)原理，等我好好研究一下，再專門寫篇文章好好闡述一下。

還有一種寫法:

public?static??Predicate?distinctByKey(Functionsuper?T,?Object>?keyExtractor)?{
????Map?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 確實(shí)簡(jiǎn)化了很多冗長(zhǎng)的操作，精簡(jiǎn)了代碼，小伙，研究java8去吧！

轉(zhuǎn)自：Balalalalalalalala

鏈接：https://blog.csdn.net/jiaobuchong/article/details/54412094