Java8 实现stream将对象集合list中抽取属性集合转化为map或list

首先新建一个实体类person

@data
public 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

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

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

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

private static list<string> getnamelist(list<person> personlist) {
 return personlist.stream().map(person::getname).collect(collectors.tolist());
}

调用这个方法

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

import lombok.data; 
@data
public class student{ 
 private string stuid; 
 private string name; 
 private string age; 
 private string sex; 
}

2.测试类

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;
 }
}

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