ImmutableClass.java

import java.util.Date;

/**

* Always remember that your instance variables will be either mutable or immutable.

* Identify them and return new objects with copied content for all mutable objects.

* Immutable variables can be returned safely without extra effort.

* */

public final class ImmutableClass

{

  /**

  * Integer class is immutable as it does not provide any setter to change its content

  * */

  private final Integer immutableField1;

  /**

  * String class is immutable as it also does not provide setter to change its content

  * */

  private final String immutableField2;

  /**

  * Date class is mutable as it provide setters to change various date/time parts

  * */

  private final Date mutableField;

  //Default private constructor will ensure no unplanned construction of class

  private ImmutableClass(Integer fld1, String fld2, Date date)

  {

    this.immutableField1 = fld1;

    this.immutableField2 = fld2;

    this.mutableField = new Date(date.getTime());

  }

  //Factory method to store object creation logic in single place

  public static ImmutableClass createNewInstance(Integer fld1, String fld2, Date date)

  {

    return new ImmutableClass(fld1, fld2, date);

  }

  //Provide no setter methods

  /**

  * Integer class is immutable so we can return the instance variable as it is

  * */

  public Integer getImmutableField1() {

    return immutableField1;

  }

  /**

  * String class is also immutable so we can return the instance variable as it is

  * */

  public String getImmutableField2() {

    return immutableField2;

  }

  /**

  * Date class is mutable so we need a little care here.

  * We should not return the reference of original instance variable.

  * Instead a new Date object, with content copied to it, should be returned.

  * */

  public Date getMutableField() {

    return new Date(mutableField.getTime());

  }

  @Override

  public String toString() {

    return immutableField1 +" - "+ immutableField2 +" - "+ mutableField;

  }

}