【JAVA】List常用移除、过滤、去重、flatMap、peek等操作

List常用移除，过滤，去重操作

ListItem.class

import lombok.Data;
import lombok.experimental.Accessors;@Data
@Accessors(chain = true)
public class ListItem {
    private String item;private String qwe;
}

移除List中某个的元素

public static void main(String[] args) {
    List<ListItem> listItems = new ArrayList<>();listItems.add(new ListItem().setItem("111111").setQwe("222222"));listItems.add(new ListItem().setItem("111111").setQwe("222222"));listItems.add(new ListItem().setItem("123123").setQwe("321321"));listItems.add(new ListItem().setItem("qweqwe").setQwe("qweqwe"));//jdk8操作方式listItems.removeIf(listItem -> listItem.getItem().equals("111111"));listItems.forEach(System.out::println);//传统写法 1for (int i = 0; i < listItems.size(); i++) {
    if (listItems.get(i).getItem().equals("111111")) {
    listItems.remove(i);//索引回归，why? 因为这个元素移除了，上一个元素会顶上来，不回归就跳过了一个元素的判断i--;}}listItems.forEach(System.out::println);//传统写法 2Iterator<ListItem> iterator = listItems.iterator();while (iterator.hasNext()) {
    if (iterator.next().getItem().equals("111111")) {
    iterator.remove();}}listItems.forEach(System.out::println);
}

List排序

public static void main(String[] args) {
    ReCandidate reCandidate = new ReCandidate();reCandidate.setAge(1);reCandidate.setId(15L);ReCandidate reCandidate2 = new ReCandidate();reCandidate2.setAge(2);reCandidate2.setId(100L);ReCandidate reCandidate3 = new ReCandidate();reCandidate3.setAge(2);reCandidate3.setId(88L);ReCandidate reCandidate4 = new ReCandidate();reCandidate4.setAge(3);reCandidate4.setId(1L);List<ReCandidate> list = Arrays.asList(reCandidate, reCandidate2, reCandidate3, reCandidate4);List<ReCandidate> collect =list.stream().sorted(//按照年龄从小倒大排列Comparator.comparingInt(ReCandidate::getAge)//倒转，也就是变成了从大到小排列.reversed()//根据年龄倒序排完后，年龄相同再根据ID从小到大排.thenComparingLong(ReCandidate::getId)).collect(Collectors.toList());for (ReCandidate candidate : collect) {
    System.out.println(candidate.getAge() + "----" + candidate.getId());}/* 3----12----882----1001----15Process finished with exit code 0*/
}

List去重(如有特殊去重需求，自行重写equals方法)

public static void main(String[] args) {
    List<ListItem> listItems = new ArrayList<>();listItems.add(new ListItem().setItem("111111").setQwe("222222"));listItems.add(new ListItem().setItem("111111").setQwe("222222"));listItems.add(new ListItem().setItem("123123").setQwe("321321"));listItems.add(new ListItem().setItem("qweqwe").setQwe("qweqwe"));List<ListItem> removeRepeatList = listItems.stream().distinct().collect(Collectors.toList());removeRepeatList.forEach(System.out::println);
}

List查找指定元素

public static void main(String[] args) {
    List<ListItem> listItems = new ArrayList<>();listItems.add(new ListItem().setItem("111111").setQwe("222222"));listItems.add(new ListItem().setItem("111111").setQwe("222222"));listItems.add(new ListItem().setItem("123123").setQwe("321321"));listItems.add(new ListItem().setItem("qweqwe").setQwe("qweqwe"));List<ListItem> collect = listItems.stream().filter(item -> item.getItem().equals("111111")).collect(Collectors.toList());collect.forEach(System.out::println);
}

取出List中对象的某个属性构造出新List

public static void main(String[] args) {
    List<ListItem> listItems = new ArrayList<>();listItems.add(new ListItem().setItem("111111").setQwe("222222"));listItems.add(new ListItem().setItem("111111").setQwe("222222"));listItems.add(new ListItem().setItem("123123").setQwe("321321"));listItems.add(new ListItem().setItem("qweqwe").setQwe("qweqwe"));List<String> collect = listItems.stream().map(ListItem::getItem).collect(Collectors.toList());collect.forEach(System.out::println);
}

处理某个字符分隔的List属性，将其拆分构造为新List

public static void main(String[] args) {
    List<String> separateList = new ArrayList<>();separateList.add("aaa,bbb,ccc");separateList.add("ddd,eee,fff");separateList.add("ggg,hhh,iii");//flatMap将流中的每一个元素 T 映射为一个流，再把每一个流连接成为一个流List<String> collect = separateList.stream().map(item -> item.split(",")).flatMap(Arrays::stream).collect(Collectors.toList());collect.forEach(System.out::println);
}

List转数组

public static void main(String[] args) {
    List<String> list = new ArrayList<>(2);list.add("123123");list.add("321321");String[] array = list.toArray(new String[0]);for (String s : array) {
    System.out.println(s);}   
}

两个List的交集，并集，差集操作

public static void main(String[] args) {
    List<String> list1 = new ArrayList<>();list1.add("qwe1");list1.add("qwe2");list1.add("qwe3");list1.add("qwe4");list1.add("qwe5");List<String> list2 = new ArrayList<>();list2.add("qwe5");list2.add("qwe6");list2.add("qwe7");list2.add("qwe8");list2.add("qwe9");// 交集List<String> collect = list1.stream().filter(list2::contains).collect(Collectors.toList());collect.forEach(System.out::println);// 差集 List1和List2的差，也就是List1-List2,反之类比即可List<String> collect1 = list1.stream().filter(item -> !list2.contains(item)).collect(Collectors.toList());collect1.forEach(System.out::println);//去重复并集List<String> all = new ArrayList<>();all.addAll(list1);all.addAll(list2);List<String> collect = all.stream().distinct().collect(Collectors.toList());collect.forEach(System.out::println); 
}

flatMap的作用以及使用方式

其主要作用就是将几个小List中内容提取，合并成大List

public static void main(String[] args) {
    List<String> list1 = Arrays.asList("张三", "李四");List<String> list2 = Arrays.asList("王五", "赵六");List<List<String>> list = new ArrayList<>();list.add(list1);list.add(list2);//使用flatmapList<String> collect = list.stream().flatMap(Collection::stream).collect(Collectors.toList());System.out.println(collect.toString());/* [张三, 李四, 王五, 赵六]Process finished with exit code 0 */}

当然，这不仅仅局限于一种类型之上，多种类型也是完全可以的

public static void main(String[] args) {
    ReCandidate reCandidate  = new ReCandidate();reCandidate.setCandidateName("asdasdasda");reCandidate.setId(1111L);reCandidate.setAge(111);ReCandidate reCandidate2  = new ReCandidate();reCandidate2.setCandidateName("12312123123");reCandidate2.setId(33333L);reCandidate2.setAge(33333);//Long 类型ListList<Long> list1 = Arrays.asList(111L, 222L);//String 类型ListList<String> list2 = Arrays.asList("王五", "赵六");//实体类 类型ListList<ReCandidate> list3 = Arrays.asList(reCandidate,reCandidate2);List<List<?>> list = new ArrayList<>();list.add(list1);list.add(list2);list.add(list3);List<?> collect = list.stream().flatMap(Collection::stream).collect(Collectors.toList());System.out.println(collect);/*[111, 222, 王五, 赵六, ReCandidate(id=1111, candidateName=asdasdasda)]Process finished with exit code 0*/}

peek()方法的作用及例子

主要是操作流经管道的数据，未流经管道的数据不做操作

import lombok.Data;import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;public class Test {
    public static void main(String[] args) {
    List<A> list = new ArrayList<>();A a1 = new A();a1.setId("11");a1.setName("zhangsan11");a1.setAge("11");list.add(a1);A a2 = new A();a2.setId("22");a2.setName("lisi22");a2.setAge("22");list.add(a2);A a3 = new A();a3.setId("33");a3.setName("zhangsan33");a3.setAge("33");list.add(a3);list.forEach(System.out::println);System.out.println("----------------------");//先按照id 倒序 排序，在过滤出包含zhangsan的，在把过滤出的数据age替换成666，再输出list.stream().sorted(Comparator.comparing(item -> Integer.parseInt(item.getId()), (a, b) -> {
    return b.compareTo(a);})).filter(item -> item.getName().contains("zhangsan"))//对流经管道的数据进行操作，为流经管道的不做修改.peek(item -> item.setAge("666")).forEach(System.out::println);System.out.println("----------------------");list.forEach(System.out::println);/* A(id=11, name=zhangsan11, age=11)A(id=22, name=lisi22, age=22)A(id=33, name=zhangsan33, age=33)----------------------A(id=33, name=zhangsan33, age=666)A(id=11, name=zhangsan11, age=666)----------------------A(id=11, name=zhangsan11, age=666)A(id=22, name=lisi22, age=22)A(id=33, name=zhangsan33, age=666)Process finished with exit code 0*/}
}@Data
class A {
    private String id;private String name;private String age;
}