首先新建一个实体类Person
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@Datapublic class Person { /** 编码 */ private String code; /** 名字 */ private String name; public Person(String code, String name) { this.code = code; this.name = name; }} |
实例化三个对象放入list集合中
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public static void main(String[] args) { Person person1 = new Person("001", "张三"); Person person2 = new Person("002", "李四"); Person person3 = new Person("002", "王五"); List<Person> personList = new ArrayList<>(); personList.add(person1); personList.add(person2); personList.add(person3); personList.forEach(t -> System.out.println(t.toString()));} |
输出结果为：
Person(code=001, name=张三)
Person(code=002, name=李四)
Person(code=002, name=王五)
1.抽取对象的code作为key，name作为value转化为map集合
方法为
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private static HashMap<String, String> listToMap(List<Person> personList) { return (HashMap<String, String>)personList.stream() .filter(t -> t.getName()!=null) .collect(Collectors.toMap(Person::getCode,Person::getName,(k1,k2)->k2));} |
filter() 方法作用是过滤掉名字为空的对象，当对象的名字为null时，会出现NPE空指针异常
(k1,k2)->k2 意思是遇到相同的key时取第二个值
(k1,k2)->k1 意思是遇到相同的key时取第一个值
调用这个方法
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HashMap<String,String> personMap = listToMap(personList);personMap.forEach((k,v)-> System.out.println(k.toString() + " - " + v.toString())); |
输出结果为：
001 - 张三
002 - 王五
2.抽取对象的name得到name的list集合
方法为
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private static List<String> getNameList(List<Person> personList) { return personList.stream().map(Person::getName).collect(Collectors.toList());} |
调用这个方法
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List<String> nameList = getNameList(personList);nameList.forEach(t -> System.out.println(t.toString())); |
输出结果为：
张三
李四
王五
补充：java8 使用stream将List转成Map,或者从List对象中获取单个属性List,List中根据某个字段排序
1.学生类
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import lombok.Data; @Datapublic class Student{ private String stuId; private String name; private String age; private String sex; } |
2.测试类
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public class Test { public static void main(String[] args) { // 创建学生List List<Student> list = createStudentList(); // 1.获取value为Student对象，key为学生ID的Map getStudentObjectMap(list); // 2.获取value为学生姓名，key为学生ID的Map getStudentNameMap(list); // 3.获取学生姓名List getStudentNameList(list); //4.List中删除学生id = 1的对象 list.removeIf(student -> student.getStuId().equals(1)); //5.如果StudentId为Long类型如何转？ Map<String, String> mapStr = list.stream().collect(Collectors.toMap(student -> student.getStuId().toString(), student -> JSON.toJSONString(student))); //6.根据List中Student的学生姓名排序 Collections.sort(list, (o1, o2) -> { if (o1.getName().compareTo(o2.getName()) > 0) { return 1; } else if (o1.getName().compareTo(o2.getName()) < 0) { return -1; } else { return 0; } }); //7.List遍历 List<String> listStr = new ArrayList<>(); List<Student> listStu = new ArrayList<>(); listStr.forEach(studentStr -> { listStu.add(JSON.parseObject(studentStr, Student.class)); }); //List根据某个字段过滤、排序 listStu.stream() .filter(student -> student.getSex().equals("女")) .sorted(Comparator.comparing(Student::getName)) .collect(Collectors.toList()); //List根据某个字段分组 Map<String,List<Student>> sexGroupMap = listStu.stream() .collect(Collectors.groupingBy(Student::getSex)); //如果Map中多个名称相同，则studentId用逗号间隔 Map<String,String> studentNameIdMap = listStu.stream() .collect(toMap(Student::getName,Student::getStuId,(s,a)->s+","+a)); } public static List<Student> createStudentList() { List<Student> list = new ArrayList<Student>(); Student lily = new Student(); lily.setStuId("1"); lily.setName("lily"); lily.setAge("14"); lily.setSex("女"); Student xiaoming = new Student(); xiaoming.setStuId("2"); xiaoming.setName("xiaoming"); xiaoming.setAge("15"); xiaoming.setSex("男"); list.add(lily); list.add(xiaoming); return list; } public static Map<Object, Object> getStudentObjectMap(List<Student> list) { Map<Object, Object> map = list.stream().collect(Collectors.toMap(Student::getStuId, student -> student)); map.forEach((key, value) -> { System.out.println("key:" + key + ",value:" + value); }); return map; } public static Map<String, String> getStudentNameMap(List<Student> list) { Map<String, String> map = list.stream().collect(Collectors.toMap(Student::getStuId, Student::getName)); map.forEach((key, value) -> { System.out.println("key:" + key + ",value:" + value); }); return map; } public static List<String> getStudentNameList(List<Student> list) { List<String> result = list.stream().map(student -> student.getName()).collect(Collectors.toList()); for (String name : result) { System.out.println("name:" + name); } return result; }} |
以上为个人经验，希望能给大家一个参考，也希望大家多多支持服务器之家。如有错误或未考虑完全的地方，望不吝赐教。
原文链接：https://blog.csdn.net/weixin_44905182/article/details/105079266
