java - 制作纯粹的不可变类

Question

我有以下类(class):

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;
    }
}

部门.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);

    }
}

这里 Emp 类并不是纯粹的不可变类，因为我能够以某种方式更改 Department 的值(如示例所示)。

哪些可能的最佳更改将使 Emp 类成为纯 Immutable 类？

Answer 1

在非原始字段的getters中，使用此结构

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);
   }
}  

因此，只需创建与之前的部门相同的新部门实例

附注在我的示例中，您可以看到纯不可变类
编辑:
您还可以添加到 Department 类复制构造函数

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

致雇主

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