Java8使用stream對List列表分組

1. 定義實體類

public class Person {
    public Integer id;
    public String name;
    public Integer age;

    public Person(){}

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

    @Override
    public String toString(){
        return id + "，" + age + "，" + name;
    }
}

2. 構造數據

private static List<Person> getList(){
    List<Person> personList = new ArrayList<>();
    personList.add(new Person(1, 10, "王1"));
    personList.add(new Person(2, 10, "王2"));
    personList.add(new Person(3, 12, "王3"));
    personList.add(new Person(4, 14, "張1"));
    personList.add(new Person(5, 8, "張2"));
    personList.add(new Person(6, 6, "張3"));
    personList.add(new Person(7, 8, "李1"));
    personList.add(new Person(8, 15, "李2"));
    personList.add(new Person(9, 20, "何何何"));
    personList.add(new Person(10, 16, "李3"));
    return personList;
}

3. 分組查詢（按姓氏進行分組）

public static void main( String[] args )
{
    List<Person> list = getList();
    //按姓進行分組
    Map<String, List<Person>> collect = list.stream().collect(Collectors.groupingBy(p -> p.name.substring(0, 1), Collectors.toList()));
    System.out.println(collect);
}

4. 結果如下：

{張=[4，14，張1, 5，8，張2, 6，6，張3], 何=[9，20，何何何], 王=[1，10，王1, 2，10，王2, 3，12，王3], 李=[7，8，李1, 8，15，李2, 10，16，李3]}