7.9. Immutable Objects and Classes

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If the contents of an object cannot be changed once the object is created, the object is called an immutable object and its class is called an immutable class . If you delete the set method in the Circle3 class in the preceding example, the class would be immutable because radius is private and cannot be changed without a set method.

A class with all private data fields and no mutators is not necessarily immutable. For example, the following class Student has all private data fields and no mutators, but it is mutable.

    public class   Student {    private int   id;    private   BirthDate birthDate;    public   Student(   int   ssn,   int   year,   int   month,   int   day) {     id = ssn;  birthDate = new BirthDate(year, month, day);  }   public int   getId() {   return   id;     }   public   BirthDate getBirthDate() {   return   birthDate;     } }    public class   BirthDate {    private int   year;   private int   month;   private int   day;    public   BirthDate(   int   newYear,   int   newMonth,   int   newDay) {  year = newYear;      month = newMonth;      day = newDay;    }   public void   setYear(   int   newYear) {      year = newYear;    } } 

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As shown in the following code, the data field birthDate is returned using the getBirthDate() method. This is a reference to a BirthDate object. Through this reference, the year of the birth date is changed, which effectively changes the contents of the Student object.

   public class   Test {   public static void   main(String[] args) {  Student student =   new  Student(   111223333   ,   1970   ,   5   ,   3   );    BirthDate date = student.getBirthDate();   date.setYear(   2010   );   //Now the student birth year is changed!  } } 

For a class to be immutable, it must mark all data fields private and provide no mutator methods and no accessor methods that would return a reference to a mutable data field object.