Stream流的常用方法大全

目录

一、遍历

1.属性遍历

2.对象遍历

3.遍历设值

二、过滤

1.简单过滤

2.多条件过滤 

三、去重

四、统计

五、截取

六、跳过

七、排序

1.普通排序

2.指定排序

八、最值

1.集合确定不为空

2.集合可能为空

九、统计

十、List结构转换Map结构

 1.map值是对象

2.map值是字段

十一、判断

1.anyMatch(只要有一个匹配则返回true)​编辑

2.allMatch(需要全部匹配条件则返回true)

3.noneMatch(需要全部不匹配条件则返回true)

十二、转换

1.对象转成字段

2.对象转成对象

3.对象转成字符串

十三、分组

1.list结构分组成map结构（值是对象）

2.list结构分组成map结构（值是集合）

3.list结构分组成map结构（值是字段）

4.list结构分组成map结构（多条件分组）

5.list结构分组成二级list结构

---

一、遍历

1.属性遍历

package com.saas.demo;


import java.util.ArrayList;
import java.util.List;

/**
 * Stream流遍历
 */
public class StreamDemo {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add("小明");
        list.add("小红");
        list.add("小兰");
        list.stream().forEach(s -> System.out.println("value:"+s));
    }
}

2.对象遍历

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

/**
 * Stream流遍历
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000")));
        peopleList.add(new People("小红",24,new BigDecimal("2800")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200")));
        //遍历对象
        peopleList.stream().forEach(s -> System.out.println("value:"+s));
        System.out.println("------------------------------------------------");
        //遍历属性
        peopleList.stream().forEach(s -> System.out.println("value:"+s.getName()));
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
}

3.遍历设值

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

/**
 * Stream设值
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000")));
        peopleList.add(new People("小红",24,new BigDecimal("2800")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200")));
        //遍历对象
        peopleList.stream().forEach(s -> System.out.println("value:"+s));
        System.out.println("------------------------------------------------");
        //统一更改年龄成25
        peopleList.stream().forEach(s -> s.setAge(25));
        //判断如果姓名叫小明的,则工资加100
        peopleList.stream().forEach(s ->{
            if("小明".equals(s.getName())){
                s.setSalary(s.getSalary().add(new BigDecimal("100")));
            }
        });
        //遍历对象
        peopleList.stream().forEach(s -> System.out.println("value:"+s));
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
}

二、过滤

1.简单过滤

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

/**
 * Stream 过滤
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000")));
        peopleList.add(new People("小红",24,new BigDecimal("2800")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200")));
        //查找年龄大于等于23岁的人
        List newPeopleList = peopleList.stream().filter(s ->s.getAge()>=23).collect(Collectors.toList());
        System.out.println("value:"+newPeopleList);

    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
}

2.多条件过滤 

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

/**
 * Stream 多条件过滤
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000")));
        peopleList.add(new People("小红",24,new BigDecimal("2800")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200")));
        //查找工资低于3000,年龄大于23岁的人
        List newPeopleList1 = peopleList.stream().filter(s ->
                s.getSalary().compareTo(new BigDecimal("3000"))<0 && s.getAge()>23
        ).collect(Collectors.toList());
        System.out.println("value:"+newPeopleList1);
        //查找年龄大于等于23岁并且工资不低于3000的人
        List newPeopleList2 = peopleList.stream().filter(s ->{
            if(s.getAge()>=23 && s.getSalary().compareTo(new BigDecimal("3000"))>=0){
                return true;
            }
            return false;
        }).collect(Collectors.toList());
        System.out.println("value:"+newPeopleList2);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
}

三、去重

package com.saas.demo;

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

/**
 * Stream 去重
 */
public class StreamDemo {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add("小明");
        list.add("小红");
        list.add("小兰");
        list.add("小红");
        list = list.stream().distinct().collect(Collectors.toList());
        System.out.println("value:"+list);
    }
}

四、统计

package com.saas.demo;

import java.util.ArrayList;
import java.util.List;

/**
 * Stream 统计
 */
public class StreamDemo {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add("小明");
        list.add("小红");
        list.add("小兰");
        long count = list.stream().count();
        System.out.println("count:"+count);
    }
}

五、截取

package com.saas.demo;

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

/**
 * Stream 截取
 */
public class StreamDemo {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add("小明");
        list.add("小红");
        list.add("小兰");
        list = list.stream().limit(2).collect(Collectors.toList());
        System.out.println("value:"+list);
    }
}

六、跳过

 

package com.saas.demo;

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

/**
 * Stream 跳过
 */
public class StreamDemo {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add("小明");
        list.add("小红");
        list.add("小兰");
        //跳过前面两个
        list = list.stream().skip(2).collect(Collectors.toList());
        System.out.println("value:"+list);
    }
}

七、排序

1.普通排序

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

/**
 * Stream 排序
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000")));
        peopleList.add(new People("小红",24,new BigDecimal("2800")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200")));
        //根据年龄从小到大排序
        peopleList =peopleList.stream()
                .sorted(Comparator.comparing(People::getAge))
                .collect(Collectors.toList());
        System.out.println("value:"+peopleList);
        System.out.println("------------------------------------------------");
        //根据年龄从大到小排序
        peopleList =peopleList.stream()
                .sorted(Comparator.comparing(People::getAge).reversed())
                .collect(Collectors.toList());
        System.out.println("value:"+peopleList);

    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
}

2.指定排序

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 排序
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000")));
        peopleList.add(new People("小红",24,new BigDecimal("2800")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200")));
        System.out.println("value:"+peopleList);
        System.out.println("------------------------------------------------");
        //根据名称指定排序
        List nameList = Arrays.asList("小红","小明","小兰");
        peopleList = peopleList.stream().sorted(Comparator.comparing(People::getName,(x,y)->{
            for (String name : nameList) {
                if (name.equals(x) || name.equals(y)){
                    if (x.equals(y)){
                        return 0;
                    }else if (name.equals(x)){
                        return -1;
                    }else {
                        return 1;
                    }
                }
            }
            return 0;
        })).collect(Collectors.toList());
        System.out.println("value:"+peopleList);
        //根据年龄指定排序
        List ageList = Arrays.asList(24,22,23);
        Collections.sort(peopleList, new Comparator() {
            public int compare(People p1, People p2) {
                int x = ageList.indexOf(p1.getAge());
                int y = ageList.indexOf(p2.getAge());
                return x - y;
            }
        });
        System.out.println("value:"+peopleList);

    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
}

八、最值

1.集合确定不为空

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

/**
 * Stream 获取最大值和最小值
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000")));
        peopleList.add(new People("小红",24,new BigDecimal("2800")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200")));
        //获取年龄最大的人
        People maxPeople =peopleList.stream().max(Comparator.comparing(People::getAge)).get();
        System.out.println("value:"+maxPeople);
        System.out.println("------------------------------------------------");
        //获取年龄最小的人
        People minPeople =peopleList.stream().min(Comparator.comparing(People::getAge)).get();
        System.out.println("value:"+minPeople);


    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
}

2.集合可能为空

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Optional;

/**
 * Stream 获取最大值和最小值
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000")));
        peopleList.add(new People("小红",24,new BigDecimal("2800")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200")));
        //获取年龄最大的人
        Optional max =peopleList.stream().max(Comparator.comparing(People::getAge));
        if(max.isPresent()){
            System.out.println("value:"+max.get());
        }
        System.out.println("------------------------------------------------");
        //获取年龄最小的人
        Optional min =peopleList.stream().min(Comparator.comparing(People::getAge));
        if(min.isPresent()) {
            System.out.println("value:" + min.get());
        }
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
}

九、统计

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Optional;

/**
 * Stream 统计
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //统计年龄总和
        int totalAge =peopleList.stream().mapToInt(People::getAge).sum();
        System.out.println("value:"+totalAge);
        System.out.println("------------------------------------------------");
        //统计工资总和
        BigDecimal totalSalary = peopleList.stream()
                .map(People::getSalary)
                .reduce(BigDecimal.ZERO,BigDecimal::add);
        System.out.println("value:"+totalSalary);
        //统计工资乘以各自系数的总和(向上保留两位)
        BigDecimal totalRatioSalary = peopleList.stream()
                .map(s->s.getSalary().multiply(s.getRatio()).setScale(2,BigDecimal.ROUND_HALF_UP))
                .reduce(BigDecimal.ZERO,BigDecimal::add);
        System.out.println("value:"+totalRatioSalary);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
}

十、List结构转换Map结构

 1.map值是对象

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream list转map
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //根据姓名转map
        Map map= peopleList.stream()
                .collect(Collectors.toMap(s-> s.getName(), object -> object,(oldValue, newValue) -> newValue));
        System.out.println("value:"+map);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
}

2.map值是字段

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream list转map
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //根据姓名转map
        Map map = peopleList.stream().collect(Collectors.toMap(People::getName
                , People::getAge));
        System.out.println("value:"+map);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
}

十一、判断

1.anyMatch(只要有一个匹配则返回true)

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 判断
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //判断集合里面是否超过23岁的人
        boolean flag= peopleList.stream().anyMatch(s -> s.getAge() > 23);
        System.out.println("value:"+flag);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
}

2.allMatch(需要全部匹配条件则返回true)

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 判断
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //判断集合里面的人是否全部超过23岁
        boolean flag= peopleList.stream().allMatch(s -> s.getAge() > 23);
        System.out.println("value:"+flag);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
}

3.noneMatch(需要全部不匹配条件则返回true)

 

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 判断
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //判断集合里面的人是否全部不超过23岁
        boolean flag= peopleList.stream().noneMatch(s -> s.getAge() > 23);
        System.out.println("value:"+flag);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
}

十二、转换

1.对象转成字段

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 转换
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //获取姓名集合
        List nameList = peopleList.stream().map(s-> s.getName()).collect(Collectors.toList());
        System.out.println("value:"+nameList);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
}

2.对象转成对象

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 转换
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //对象转对象
        List studentList = peopleList.stream().map(s->{
            return new Student(s.getName(),s.getAge());
        }).collect(Collectors.toList());
        System.out.println("value:"+studentList);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
}

@Data
@AllArgsConstructor
class Student {
    //姓名
    private String name;
    //年龄
    private Integer age;
}

3.对象转成字符串

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 转换
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //获取姓名字符串,逗号隔开
        String name = peopleList.stream().map(s-> s.getName()).collect(Collectors.joining(","));
        System.out.println("value:"+name);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
}

十三、分组

1.list结构分组成map结构（值是对象）

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

/**
 * Stream 分组
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,new BigDecimal("3200"),new BigDecimal("1.1"),"开发"));
        peopleList.add(new People("小明",23,new BigDecimal("3100"),new BigDecimal("1.1"),"开发"));
        peopleList.add(new People("小明",23,new BigDecimal("3000"),new BigDecimal("1.1"),"开发"));
        peopleList.add(new People("小红",24,new BigDecimal("2800"),new BigDecimal("1.2"),"产品"));
        peopleList.add(new People("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3"),"开发"));
        //根据名称分组(前提是名称是唯一的,要不然获取的是重复名称的最后一条)
        Map studentMap = peopleList.stream().collect(Collectors.toMap(People::getName, Function.identity(), (key1, key2) -> key2));
        System.out.println("value:"+studentMap);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
    //职业
    private String occupation;
}

2.list结构分组成map结构（值是集合）

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 分组
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明","开发"));
        peopleList.add(new People("小红","产品"));
        peopleList.add(new People("小兰","开发"));
        //根据职业分组无序
        Map> studentMap = peopleList.stream().collect(Collectors.groupingBy(People::getOccupation));
        System.out.println("value:"+studentMap);
        //根据职业分组有序
        Map> studentSortMap = peopleList.stream().collect(Collectors.groupingBy(People::getOccupation, LinkedHashMap::new,Collectors.toList()));
        System.out.println("value:"+studentSortMap);

    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //职业
    private String occupation;
}

3.list结构分组成map结构（值是字段）

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 分组
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People(1,"小明",23,new BigDecimal("3000"),new BigDecimal("1.1"),"开发"));
        peopleList.add(new People(2,"小红",24,new BigDecimal("2800"),new BigDecimal("1.2"),"产品"));
        peopleList.add(new People(3,"小兰",22,new BigDecimal("3200"),new BigDecimal("1.3"),"开发"));
        //根据id分组
        Map map = peopleList.stream().collect(Collectors.toMap(People::getId, People::getName,(oldValue, newValue) -> newValue));
        System.out.println("value:"+map);
    }
}

@Data
@AllArgsConstructor
class People {
    //主键id
    private Integer id;
    //姓名
    private String name;
    //年龄
    private Integer age;
    //工资
    private BigDecimal salary;
    //比率
    private BigDecimal ratio;
    //职业
    private String occupation;
}

4.list结构分组成map结构（多条件分组）

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 分组
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明",23,"开发"));
        peopleList.add(new People("小红",24,"产品"));
        peopleList.add(new People("小兰",22,"开发"));
        peopleList.add(new People("小紫",22,"开发"));
        //根据年龄和职业分组
        Map> map = peopleList.stream().collect(Collectors.groupingBy(People::getAgeAndOccupation));
        System.out.println("value:"+map);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //年龄
    private Integer age;
    //职业
    private String occupation;

    public static String getAgeAndOccupation(People people){
        return people.getAge() + "#" + people.getOccupation();
    }
}

5.list结构分组成二级list结构

package com.saas.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Stream 分组
 */
public class StreamDemo {
    public static void main(String[] args) {
        List peopleList = new ArrayList();
        peopleList.add(new People("小明","开发"));
        peopleList.add(new People("小红","产品"));
        peopleList.add(new People("小兰","开发"));
        //根据职业分组无序
        List> newList = new ArrayList<>();
        peopleList.stream().collect(Collectors.groupingBy(People::getOccupation, Collectors.toList()))
                .forEach((occupation, listOccupation) -> {
                    newList.add(listOccupation);
                });
        System.out.println("value:"+newList);
        //根据职业分组有序
        List> newSortList = new ArrayList<>();
        peopleList.stream().collect(Collectors.groupingBy(People::getOccupation, LinkedHashMap::new, Collectors.toList()))
                .forEach((occupation, listOccupation) -> {
                    newSortList.add(listOccupation);
                });
        System.out.println("value:"+newSortList);
    }
}

@Data
@AllArgsConstructor
class People {
    //姓名
    private String name;
    //职业
    private String occupation;
}