HashMap from nested objects

Question

I am in need of help to create a hashmap.

public class Driver {
    
    private String id;
    private String name;
    private List<Student> students;

    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public List<Student> getStudents() {
        return students;
    }
    public void setStudents(List<Student> students) {
        this.students = students;
    }
}

public class Student {
    
    private String id;
    private String name;

    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

There will be a List of Driver objects. Every Driver has List of Student objects. Could someone help me how to create this Map using streams: <Student.id, Driver.id>

Thanks!

Eran · Accepted Answer · 2022-12-01 12:10:44Z

2

You can use flatMap to build a Stream of all the (Student ID, Driver ID) pairs and then collect them into a Map:

List<Driver> drivers = new ArrayList<> ();
Map<String,String>
    studentDrivers   =
        drivers.stream ()
               .flatMap (drv -> drv.getStudents ()
                                   .stream ()
                                   .map (st -> new SimpleEntry<String,String> (st.getId (), drv.getId ())))
               .collect (Collectors.toMap (Map.Entry::getKey, 
                                           Map.Entry::getValue));

answered Dec 1, 2022 at 12:10

Eran

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