java - Collectors.groupingBy 基于嵌套静态类属性

Question

我有一个College具有嵌套静态类的类 Dept

学院

class College {
private String collegeName;

private Dept dept;

public Dept getDept() {
    return dept;
}

public void setDept(Dept dept) {
    this.dept = dept;
}

public String getCollegeName() {
    return CollegeName;
}

public void setCollegeName(String collegeName) {
    CollegeName = collegeName;
}

public static class Dept {

    private String deptName;

    public String getDeptName() {
        return deptName;
    }

    public void setDeptName(String deptName) {
        this.deptName = deptName;
        }

    }

 }

我有list的College对象并试图 groupingBy基于deptName i,e (Map<String>, List<College>) ，但到目前为止还没有运气，它给出了编译错误消息

List<College> list = new ArrayList<College>();
list.stream().collect(Collectors.groupingBy(College.Dept::getDeptName));

编译错误

The method collect(Collector<? super College,A,R>) in the type Stream<College> is not applicable for the arguments (Collector<College.Dept,capture#1-of ?,Map<String,List<College.Dept>>>)

Answer 1

College.Dept::getName

是Function<College.Dept, String> 。它不接受 College作为输入。

使用 lambda 构造 Function<College, String> :

groupingBy(c -> c.getDept().getName())