Java List常用方法Stream()简要总结

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        Bean bean1 = new Bean(1,"a","za");
        Bean bean2 = new Bean(2,"b","zb");
        Bean bean3 = new Bean(3,"c","zc");

        ArrayList<Bean> list1 = new ArrayList();
        list1.add(bean1);
        list1.add(bean2);
        list1.add(bean3);

        

        //获取list1中的所有Bean对象的id，放入另一个list数组
        //注意，这个.map()方法返回的还是流对象，而不是返回map的意思

        //.stream().map().collect()

        List<Long> ids = list1.stream().map(bean -> bean.getId()).collect(Collectors.toList());
        System.out.println(ids);

        

        //使用filter方法，筛选list1中的所有Bean对象，留下id属于ids中的Bean对象，并把它们的name字段转成一个list
        //删除一个，便于看出差别来
        ids.remove(0);

        //.stream().filter().map().collect()

        List<String> names = list1.stream().filter(bean -> ids.contains(bean.getId())).map(Bean::getName).collect(Collectors.toList());
        System.out.println(names);

        

        //把list1中的所有Bean对象转成map，其中，id是map的key，Bean对象本身是map的value

        //.stream().collect()

        Map<Long, Bean> maps = list1.stream().collect(java.util.stream.Collectors.toMap(Bean::getId, e->e));
        System.out.println(maps);
        Set<Long> longs = maps.keySet();
        for(Long l : longs){
            System.out.print(l+" " + maps.get(l).getName() + " | ");
        }
        System.out.println();

        

        //按条件过滤后生成map，与上方的filter类似，只保留ids中有的id的bean对象

        //.stream().filter().collect()

        Map<Long, Bean> maps2 = list1.stream().filter(bean -> ids.contains(bean.getId())).collect(java.util.stream.Collectors.toMap(Bean::getId, e->e));
        System.out.println(maps2);
        Set<Long> longs2 = maps2.keySet();
        for(Long l : longs2){
            System.out.print(l+" " + maps2.get(l).getName() + " | ");
        }
        System.out.println();


    }
    
    //根据某个字段去重
     list1 = list1.stream().collect(
                Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(
                        Comparator.comparing(Bean::getName)
                )), ArrayList::new)
        );
      //抽取几个字段组成新的集合
       // 查询出所有的设备
       List<DeviceVo> deviceList = objectVo.getFaceObjectList().getFaceObject().stream().
                map(e -> new DeviceVo(e.getDeviceID(), e.getFaceAppearTime()))
                .filter(e -> e != null)
                .collect(Collectors.toList());
                
    //给字段赋值
     findClockList.stream().forEach(item-> {
                    try {
                        item.setPunchTypeStr(RfcClockNodeEnum.byCode(item.getPunchType()).getDesc());
                    } catch (NoSuchEnumApplicationException e) {
                        e.printStackTrace();
                    }
                });// 对查询出来的list统一赋值
    //根据字段分组
        maps = safeguardList.stream().collect(Collectors.groupingBy(
                    (Map m) -> (String) m.get("USERNAME")));
                    
   		 list.forEach((entity) ->{
                         
                        entity.setCreateTime(new Date());
                        entity.setCreateUser(ShiroUtils.getUserId());                         
                        entity.setContractId(hTity.getId());
                    }
                    );

    static class Bean {
        private long id;
        private String name;
        private String address;

        public Bean(long id, String name, String address) {
            this.id = id;
            this.name = name;
            this.address = address;
        }

        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;
        }

        public String getAddress() {
            return address;
        }

        public void setAddress(String address) {
            this.address = address;
        }
    }
}