集合分组：Collectors.groupingBy()

数据库中有groupby 分组，Java里面其实也有的，感觉很好用。对List里面的数据进行分组，一行代码解决问题，太爽了

话不多说，上代码。

people实体：按照人员的地址分组，使用map>接收。

public class People {
    private Integer id;
    private String name;
    private String address;
    private Integer age;

    public Integer getId() {
        return id;
    }

    public void setId(Integer 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;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public People() {
    }

    public People(Integer id, String name, String address, Integer age) {
        this.id = id;
        this.name = name;
        this.address = address;
        this.age = age;
    }

    @Override
    public String toString() {
        return "People{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", address='" + address + '\'' +
                ", age=" + age +
                '}';
    }
}

实例方法：

public class Test {
    public static void main(String[] args) {
        People people1=new People(1,"only-qi1","11",11);
        People people2=new People(2,"only-qi2","12",12);
        People people3=new People(3,"only-qi3","14",13);
        People people4=new People(1,"only-qi4","14",14);
        People people5=new People(2,"only-qi5","13",15);
        People people6=new People(1,"only-qi6","11",16);
        ArrayListarrayList=new ArrayList<>();
        arrayList.add(people1);
        arrayList.add(people2);
        arrayList.add(people3);
        arrayList.add(people4);
        arrayList.add(people5);
        arrayList.add(people6);
        Map>map=arrayList.stream().collect(Collectors.groupingBy(People::getAddress));
        for (String s : map.keySet()) {
            System.out.println("key值是："+s+"========="+"value的值是"+map.get(s));
        }
    }
}

执行结果：

按照多个字段分组：

 Map> map = schoolYearWarnStatisticVOS.stream().collect(Collectors.groupingBy(p -> p.getSchoolName()+"_"+p.getGradeType()));

===另外看到两个好用的方法，分享给大家。开发中肯定用的到：====

1.String.join()方法的方法和使用：将数组或集合以某拼接符拼接到一起形成新的字符串。

    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add("Mxy");
        list.add("StringUtils");
        list.add("join");
        String join = String.join("-",list);//传入String类型的List集合，使用"-"号拼接
        System.out.println("=========only-qi========"+join);
        String[] s = new String[]{"Yuan","Mxy"};//传入String类型的数组，使用"-"号拼接
        String join2 = String.join("-",s);
        System.out.println("=========only-qi========"+join2);
    }

执行的结果：

2.StringUtils.join()方法的方法和使用：将数组或集合以某拼接符拼接到一起形成新的字符串。 

首先引入common-lang3依赖

        
            org.apache.commons
            commons-lang3
            3.8.1
        

代码：

package com.gq.upup01.day01;

import org.apache.commons.lang3.StringUtils;

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

public class Up02 {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add("Mxy");
        list.add("StringUtils");
        list.add("join");
        String join = StringUtils.join(list,"-");//传入String类型的List集合，使用"-"号拼接
        System.out.println("********************common-lang3"+join);

        String[] s = new String[]{"Yuan","Mxy"};//传入String类型的数组，使用"-"号拼接
        String join2 = StringUtils.join(s,"-");
        System.out.println("********************common-lang3"+join2);
    }
}

执行结果：

=======java 8 根据对象的某个字段排序（升序，降序）=====

//根据Dict对象的sort字段降序排序
dictList.sort(Comparator.comparing(Dict::getSort).reversed());
//根据Dict对象的sort字段升序排序
dictList.sort(Comparator.comparing(Dict::getSort));

=====================Collectors.toMap()==============================

 public static void main(String[] args) {
        List userList = Lists.newArrayList(
                new Teacher().setId(1).setName("张三"),
                new Teacher().setId(2).setName("李四"),
                new Teacher().setId(3).setName("王五")
        );
        Map map = userList.stream().collect(Collectors.toMap(Teacher::getId, Teacher::getName));
        System.out.println("**********************1"+map);
        Map map1 = userList.stream().collect(Collectors.toMap(Teacher::getId, t -> t));
        System.out.println("**********************2"+map1);


    }

@Data
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true) // 链式方法
public class Teacher {
    private Integer id;
    private String name;
}

运行结果：