List集合的常用操作

落日映苍穹つ 2023-10-04 23:34 17阅读 0赞

## **1、list 中的获取、添加、删除、清空元素，判断list 是否相等** ##

添加方法：list.add(list2)

获取方法：list.get(index)

按索引删除：list.remove(index)

按元素内容删除：list.remove(Objo)

清空集合元素数值：list.clear()

判断其他的对象是否和该对象相等：equals(Object o)

import java.util.ArrayList;
    import java.util.List;
    
    public class Demo {
        public static void main(String[] args) {
            List<String> list = new ArrayList<>();
            List<String> newlist = new ArrayList<>();
            System.out.println(list.equals(newlist)); // 是否相等
    
            // 添加
            list.add("A");
            list.add("B");
            list.add("C");
            list.add("D");
            list.add("E");
            System.out.println(list);
    
            // 删除
            list.remove(3);   //.remove(index)
            list.remove("A"); //.remove(Object o)
            System.out.println(list);
    
            // 获取 list 集合中索引为 1 的元素
            String per = list.get(1);
            System.out.println(per);
    
            // 清空
            list.clear();  // 此时 list 集合为空集合，相当于刚开始 new 出来的 list 集合
            System.out.println(list);
        }
    }

![6948a3a214a54c289821d0645f9bb18d.png][]

## **2、list 集合的判断、获取长度** ##

获取长度：list.size()

判断集合是否为空：list.isEmpty()

判断集合中是否包含指定的元素：list.contains(Object object)

![c920e996705f42999d4917974200939e.png][]

## **3、从List中删除空值** ##

从List中删除空值:list.removeIf(Objects::isNull);

或者：

list.stream().filter(Objects::nonNull).collect(Collectors.toList());

import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.List;
    import java.util.Objects;
    import java.util.stream.Collectors;
    
    public class Demo {
        public static void main(String[] args) {
            List<String> list = new ArrayList<>(Arrays.asList("A", null, "B", null));
            System.out.println(list);
    
            list.removeIf(Objects::isNull);
            System.out.println(list);
    
            List<String> list2 = new ArrayList<>(Arrays.asList("A", null, "B", null));
            System.out.println(list2);
            List<String> newList = list2.stream().filter(Objects::nonNull).collect(Collectors.toList());
            System.out.println(newList);
        }
    }

![81dab26be620437f81db88ed87d1699d.png][]

## **4、list 中根据索引将元素值改变(替换)** ##

注意list.set(index, element)；和 list.add(index, element)；的不同。

import java.util.ArrayList;
    import java.util.List;
    
    public class Demo {
        public static void main(String[] args) {
            List<String> list = new ArrayList<>();
            // 添加
            list.add("A");
            list.add("B");
            list.add("C");
            list.add("D");
            list.add("E");
            System.out.println(list);
    
            list.set(0, "d"); //.set(index,element);
            System.out.println(list);
            list.add(1, "e");   //.add(index,element)
            System.out.println(list);
        }
    }

![8d866df068d542de841418c051c21df3.png][]

## **5、list 转为数组** ##

将集合转换为数组：list.toArray()，这里有两种方法：

import java.util.ArrayList;
    import java.util.List;
    
    public class Demo {
        public static void main(String[] args) {
            List<String> list = new ArrayList<>();
            // 添加
            list.add("A");
            list.add("B");
            list.add("C");
            list.add("D");
            list.add("E");
            System.out.println(list);
    
            // 第一种
            String[]  arr1 = new String[list.size()];
            list.toArray(arr1);
            for (int i = 0; i < arr1.length; i++) {
                System.out.println(arr1[i]);
            }
    
            // 第二种
            String[] arr2 = (String[])list.toArray(new String[0]);
            for (int i = 0; i < arr2.length; i++) {
                System.out.println(arr2[i]);
            }
        }
    }

![ab3bc700797845b988ad35d1eb83752e.png][]

## **6、list 排序** ##

*  ### 直接倒序输出：Collections.reverse(list) ###

import java.util.ArrayList;
    import java.util.Collections;
    import java.util.List;
    
    public class Demo {
        public static void main(String[] args) {
            List<String> list = new ArrayList<>();
            // 添加
            list.add("A");
            list.add("B");
            list.add("C");
            list.add("D");
            list.add("E");
            System.out.println(list);
    
            // 需求：直接将集合倒序输出
            Collections.reverse(list);
            // 没有创建新的集合，使用的还是原来的集合list
            System.out.println(list);
        }
    }

![905419f0bf714936a8bc549596655877.png][]

*  ### 按某一字段进行排序：list.sort((x, y) -> Double.compare(x.getObj(), y.getObj())) ###

import java.util.ArrayList;
    import java.util.List;
    
    public class Demo {
        public static void main(String[] args) {
            List<Stu> stuList = new ArrayList<>();
            stuList.add(new Stu(2, 30));
            stuList.add(new Stu(1, 30));
            stuList.add(new Stu(3, 50));
            stuList.add(new Stu(4, 40));
    
            System.out.println(stuList);
    
            // 按某一字段进行排序：list.sort((x, y) -> Double.compare(x.getObj(), y.getObj()))
            // 需求：id 升序 对 stuList 中的学生信息进行排序，然后输出结果
            stuList.sort((x, y) -> Double.compare(x.getId(), y.getId()));
            System.out.println(stuList);
        }
    }
    
    class Stu {
        // 编号
        private Integer id;
        // 分数
        private Integer score;
    
        public Stu(Integer id, Integer score) {
            this.id = id;
            this.score = score;
        }
    
        @Override
        public String toString() {
            return "Stu{" +
                    "id=" + id +
                    ", score=" + score +
                    '}';
        }
    
        public Integer getId() {
            return this.id;
        }
    
        public Integer getScore() {
            return this.score;
        }
    }

![81acdea691a34989934318f168c00f9b.png][]

*  ### 按a、b字段进行排序：list.Comparator.comparing(Stu::getScore).reversed().thenComparing(Stu::getId) ###

import java.util.ArrayList;
    import java.util.Comparator;
    import java.util.List;
    import java.util.stream.Collectors;
    
    public class Demo {
        public static void main(String[] args) {
            List<Stu> stuList = new ArrayList<>();
            stuList.add(new Stu(2, 30));
            stuList.add(new Stu(1, 30));
            stuList.add(new Stu(3, 50));
            stuList.add(new Stu(4, 40));
    
            System.out.println(stuList);
    
            // 需求：score 降序，id 升序 对 stuList 中的学生信息进行排序，然后输出结果
            List<Stu> stuSortList = stuList.stream().sorted(Comparator.comparing(Stu::getScore).reversed().thenComparing(Stu::getId)).collect(Collectors.toList());
            // 打印结果
            System.out.println(stuSortList);
        }
    }
    
    class Stu {
        //编号
        private Integer id;
        //分数
        private Integer score;
    
        public Stu(Integer id, Integer score) {
            this.id = id;
            this.score = score;
        }
    
        @Override
        public String toString() {
            return "Stu{" +
                    "id=" + id +
                    ", score=" + score +
                    '}';
        }
    
        public Integer getId() {
            return this.id;
        }
    
        public Integer getScore() {
            return this.score;
        }
    }

![78e7c31198184f6bb24db1fe1f2b660e.png][]

这里使用到了 java中的 stream 来操作的，stream 中有个 sorted 方法，需传入比较器：Comparator，解释一下代码

//得到 score 升序比较器

Comparator.comparing(Stu::getScore)

//将 score 升序比较器反转，那么就得到 score 降序比较器

Comparator.comparing(Stu::getScore).reversed()

//注意thenComparing，表示前面的比较器得到结果相同的时候，走 thenComparing 函数指定的比较方式，即按照 Stu 中的 id 升序排序。

Comparator.comparing(Stu::getScore).reversed().thenComparing(Stu::getId)

## **7、list 集合过滤** ##

*  ### 单一条件过滤：根据对象中的一个条件过滤（过滤名字不为张三的所有人） ###

import java.util.ArrayList;
    import java.util.List;
    import java.util.stream.Collectors;
    
    public class Demo {
        public static void main(String[] args) {
            List<User> list = new ArrayList<>();
            User user1 = new User("张三", 15, "男");
            User user2 = new User("李四", 22, "男");
            User user3 = new User("王五", 25, "男");
            User user4 = new User("赵六", 30, "男");
            User user5 = new User("李世民", 28, "男");
            User user6 = new User("杨贵妃", 18, "女");
            User user7 = new User("貂蝉", 16, "女");
            list.add(user1);
            list.add(user2);
            list.add(user3);
            list.add(user4);
            list.add(user5);
            list.add(user6);
            list.add(user7);
    
            // 打印结果
            for (int i = 0; i < list.size(); i++)
                System.out.print(list.get(i).getName() + " ");
            System.out.println();
    
            List<User> collect = list.stream()
                    .filter(user -> !"张三".equals(user.getName()))
                    .collect(Collectors.toList());
    
            // 打印结果
            for (int i = 0; i < collect.size(); i++)
                System.out.print(collect.get(i).getName() + " ");
            System.out.println();
        }
    }
    
    class User {
        private String name;
        private int age;
        private String sex;
    
        public User(String name, int age, String sex) {
            this.name = name;
            this.age = age;
            this.sex = sex;
        }
    
        public String getName() {
            return this.name;
        }
    }

![6f04e39d22a74ebda3d9f6846a934cf5.png][]

*  ### 多条件过滤：根据对象中的多个属性过滤（过滤，只剩年龄在30岁以下，并且性别为女的用户） ###

import java.util.ArrayList;
    import java.util.List;
    import java.util.stream.Collectors;
    
    public class Demo {
        public static void main(String[] args) {
            List<User> list = new ArrayList<>();
            User user1 = new User("张三", 15, "男");
            User user2 = new User("李四", 22, "男");
            User user3 = new User("王五", 25, "男");
            User user4 = new User("赵六", 30, "男");
            User user5 = new User("李世民", 28, "男");
            User user6 = new User("杨贵妃", 18, "女");
            User user7 = new User("貂蝉", 16, "女");
            list.add(user1);
            list.add(user2);
            list.add(user3);
            list.add(user4);
            list.add(user5);
            list.add(user6);
            list.add(user7);
    
            // 打印结果
            for (int i = 0; i < list.size(); i++)
                System.out.print(list.get(i).getName() + " ");
            System.out.println();
    
            List<User> collect = list.stream()
                    .filter(user -> user.getAge() < 30 && "女".equals(user.getSex()))
                    .collect(Collectors.toList());
    
            // 打印结果
            for (int i = 0; i < collect.size(); i++)
                System.out.print(collect.get(i).getName() + " ");
            System.out.println();
        }
    }
    
    class User {
        private String name;
        private int age;
        private String sex;
    
        public User(String name, int age, String sex) {
            this.name = name;
            this.age = age;
            this.sex = sex;
        }
    
        public String getName() {
            return this.name;
        }
    
        public int getAge() {
            return this.age;
        }
    
        public String getSex() {
            return this.sex;
        }
    }

![1e5d64dd3e0949e48ab18b4617c164a5.png][]

## **8、list 集合分批（拆分）处理** ##

注意：使用该方法有一个大前提：List中的数据没有重复！

import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.List;
    
    public class Demo {
        public static void main(String[] args) {
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i <= 105; i++) {
                list.add(i);
            }
            doBatch(list);
        }
    
        private static void doBatch(List<Integer> list) {
            if (list.size() == 0) {
                return;
            }
            int perCount = 20;
            int index = 1;
            int times = list.size() / perCount + 1;
            System.out.println("总长度:" + list.size() + ", 共需要执行:" + times + "次");
            do {
                if (list.size() == 0) {
                    break;
                }
                System.out.println("第 " + index + " 轮>>, 剩余数量:" + list.size());
                List<Integer> tempList;
                if (list.size() > perCount) {
                    tempList = list.subList(0, perCount);
                } else {
                    tempList = list.subList(0, list.size());
                }
                deal(tempList);
                index++;
                list.removeAll(tempList);
            } while (index <= times);
    
            System.out.println("结束！");
        }
    
    
        private static void deal(List<Integer> list) {
            System.out.println(Arrays.toString(list.toArray()));
        }
    }

![dcafefeb958f439e98f5d3868edc4b88.png][]

[6948a3a214a54c289821d0645f9bb18d.png]: https://img-blog.csdnimg.cn/img_convert/6948a3a214a54c289821d0645f9bb18d.png
[c920e996705f42999d4917974200939e.png]: https://img-blog.csdnimg.cn/img_convert/c920e996705f42999d4917974200939e.png
[81dab26be620437f81db88ed87d1699d.png]: https://img-blog.csdnimg.cn/img_convert/81dab26be620437f81db88ed87d1699d.png
[8d866df068d542de841418c051c21df3.png]: https://img-blog.csdnimg.cn/img_convert/8d866df068d542de841418c051c21df3.png
[ab3bc700797845b988ad35d1eb83752e.png]: https://img-blog.csdnimg.cn/img_convert/ab3bc700797845b988ad35d1eb83752e.png
[905419f0bf714936a8bc549596655877.png]: https://img-blog.csdnimg.cn/img_convert/905419f0bf714936a8bc549596655877.png
[81acdea691a34989934318f168c00f9b.png]: https://img-blog.csdnimg.cn/img_convert/81acdea691a34989934318f168c00f9b.png
[78e7c31198184f6bb24db1fe1f2b660e.png]: https://img-blog.csdnimg.cn/img_convert/78e7c31198184f6bb24db1fe1f2b660e.png
[6f04e39d22a74ebda3d9f6846a934cf5.png]: https://img-blog.csdnimg.cn/img_convert/6f04e39d22a74ebda3d9f6846a934cf5.png
[1e5d64dd3e0949e48ab18b4617c164a5.png]: https://img-blog.csdnimg.cn/img_convert/1e5d64dd3e0949e48ab18b4617c164a5.png
[dcafefeb958f439e98f5d3868edc4b88.png]: https://img-blog.csdnimg.cn/img_convert/dcafefeb958f439e98f5d3868edc4b88.png