Question

I have following classes :

Emp.java

final public class Emp {

    private Integer id;
    private String name;
    private Department department;

    public Emp(Integer id, String name, Department department) {
        this.id = id;
        this.name = name;
        this.department = department;
    }

    public Department getDepartment() {
        return department;
    }

    public Integer getId() {
        return id;
    }

    public String getName() {
        return name;
    }
}

Department.java

public class Department {

    private Integer id;
    private String name;

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

    public Integer getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }
}

EmployeeTest.java

public class EmployeeTest {

    public static void main(String args[]) {
        Department dept1 = new Department(1, "dept1");
        Emp emp = new Emp(1, "emp1", dept1);
        emp.getDepartment().setName("dept2");
        System.out.println("emp = "+emp);

    }
}

Here Emp class is not purely an immutable class because somehow I am able to change the values of Department (as shown in the example).

What are the best possible changes which will make Emp class a pure Immutable class ?

Answer 1

In getters for non-primitive field, use this structure

public class Line {

   private final Point start;
   private final Point end;

   public Line(final Point start, final Point end) {
       this.start = new Point(start);
       this.end = new Point(end);
   }

   public Point getStart() {
       return new Point(start);
   }

   public Point getEnd() {
       return new Point(end);
   }
}  

So, simply create new instance of department that is equals to previous

P.S. In my example you can see pure immutable class
EDIT:
Also you can add to Department class copy-contructor

public Department(final Department dep)  
{ ... } 

And to Employer

getDepartment()  
{  
   return new Department(department);  
}