When you are using Java 8 streams and trying to collect the results into a map, this example helps you explain how to resolve the key collisions

ResolveConflictsWithMapCollector

class StudentEntity {
    String city;
    String name;

    public StudentEntity(String city, String name) {
        this.city=city;
        this.name=name;
    }

    public String getCity() { return city; }
    public String getName() { return name; }

    public String toString() { return "CITY:"+city+" NAME: "+name; }
}

public class TestMain {

    public static void main(String ar[]){


        List<StudentEntity> allStudents = new ArrayList<>();
        allStudents.add(new StudentEntity("Sunnyvale","John"));
        allStudents.add(new StudentEntity("Santa Clara","Jessi"));
        allStudents.add(new StudentEntity("Sunnyvale","Kate"));
        Map<String, StudentEntity> perCityStudents = allStudents.stream()
                .collect(Collectors.toMap(StudentEntity::getCity, studentEntity -> studentEntity,
                        new BinaryOperator<StudentEntity>() {
                            @Override
                            public StudentEntity apply(StudentEntity studentEntity,
                                    StudentEntity studentEntity2) {
                                return new StudentEntity(studentEntity.getCity(),studentEntity.getName() + " " + studentEntity2.getName());
                            }
                        }));
        System.out.print(perCityStudents);


    }

}