Sunday, May 10, 2015

Sorting a collection by more than one field

Let's say we have an Employee Object with the fields as given below.

class Employee implements Comparable {

    private int empid;
    private String deptname;
    private String name;
    private float salary;

    public int getEmpid() {
        return empid;
    }

    public String getDeptname() {
        return deptname;
    }

    public String getName() {
        return name;
    }

    public float getSalary() {
        return salary;
    }

    public Employee(int empid, String deptname, String name, float salary) {
        this.empid = empid;
        this.deptname = deptname;
        this.name = name;
        this.salary = salary;
    }

    @Override
    public String toString() {
        return "Employee{" + "empid=" + empid + ", deptname=" + deptname + ", name=" + name + ", salary=" + salary + '}' + "\n";
    }

    @Override
    public int hashCode() {
        int hash = 3;
        hash = 23 * hash + this.empid;
        return hash;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Employee other = (Employee) obj;
        if (this.empid != other.empid) {
            return false;
        }
        return true;
    }

    @Override
    public int compareTo(Object o) {
        if (o instanceof Employee) {
            Employee other = (Employee) o;
            return this.name.compareTo(other.name);
        } else {
            throw new RuntimeException("Comparison object is not matching");
        }
    }
}

Requirement: We want to sort Employee objects first by deptname and then followed by employee name within each department.
public class MultipleSort {

    public static void main(String[] args) {
        List emplist = new ArrayList();
        emplist.add(new Employee(353744, "MCA", "Athiruban", 47000));
        emplist.add(new Employee(311344, "CSE", "Nandhini", 41000));
        emplist.add(new Employee(351144, "EEE", "Kishore", 44400));
        emplist.add(new Employee(353321, "MCA", "Karthik", 37000));
        emplist.add(new Employee(353711, "CSE", "Gopal", 37000));
        emplist.add(new Employee(353743, "IT", "AthiNivas", 67000));

        defaultSortandDisplay(emplist);
        sortByDeptAndThenByName(emplist);
    }

    private static void defaultSortandDisplay(List emplist) {
        Collections.sort(emplist);
        System.out.println("DefaultSortandDisplay->\n" + emplist);
    }

    private static void sortByDeptAndThenByName(List emplist) {
        Map> mapSortedByDept = new TreeMap>();
        List finallist = new ArrayList();
        for (Employee e : emplist) {
            if (mapSortedByDept.containsKey(e.getDeptname()) == true) {
                mapSortedByDept.get(e.getDeptname()).add(e);
            } else {
                List templist = new ArrayList();
                templist.add(e);
                mapSortedByDept.put(e.getDeptname(), templist);
            }
        }
        System.out.println("Ordered by Department->\n" + mapSortedByDept);

        Collection> collectionSortedByDept = mapSortedByDept.values();
        for (List indvlist : collectionSortedByDept) {
            Collections.sort(indvlist);
            finallist.addAll(indvlist);
        }
        System.out.println("Ordered by name within Department->\n" + mapSortedByDept);
        System.out.println("Final list->\n" + finallist);
    }
}