Immutable objects

Immutable objects are simply objects whose state (the object's data) cannot change after construction. Examples of immutable objects from the JDK include String and Integer.
Immutable objects greatly simplify your program, since they :

• are simple to construct, test, and use
• are automatically thread-safe and have no synchronization issues
• do not need a copy constructor
• do not need an implementation of clone
• allow hashCode to use lazy initialization, and to cache its return value
• do not need to be copied defensively when used as a field
• make good Map keys and Set elements (these objects must not change state while in the collection)
• have their class invariant established once upon construction, and it never needs to be checked again
• always have "failure atomicity" (a term used by Joshua Bloch) : if an immutable object throws an exception, it's never left in an undesirable or indeterminate state

Immutable objects have a very compelling list of positive qualities. Without question, they are among the simplest and most robust kinds of classes you can possibly build. When you create immutable classes, entire categories of problems simply disappear.Make a class immutable by following these guidelines :

• ensure the class cannot be overridden - make the class final, or use static factories and keep constructors private
• make fields private and final
• force callers to construct an object completely in a single step, instead of using a no-argument constructor combined with subsequent calls to setXXX methods (that is, avoid the Java Beans convention)
• do not provide any methods which can change the state of the object in any way - not just setXXX methods, but any method which can change state
• if the class has any mutable object fields, then they must be defensively copied when passed between the class and its caller

In Effective Java, Joshua Bloch makes this compelling recommendation :"Classes should be immutable unless there's a very good reason to make them mutable....If a class cannot be made immutable, limit its mutability as much as possible."
It's interesting to note that BigDecimal is technically not immutable, since it's not final.
Example

import java.util.Date;

/**
* Planet is an immutable class, since there is no way to change
* its state after construction.
*/
public final class Planet {

  public Planet (double aMass, String aName, Date aDateOfDiscovery) {
     fMass = aMass;
     fName = aName;
     //make a private copy of aDateOfDiscovery
     //this is the only way to keep the fDateOfDiscovery
     //field private, and shields this class from any changes that 
     //the caller may make to the original aDateOfDiscovery object
     fDateOfDiscovery = new Date(aDateOfDiscovery.getTime());
  }

  /**
  * Returns a primitive value.
  *
  * The caller can do whatever they want with the return value, without 
  * affecting the internals of this class. Why? Because this is a primitive 
  * value. The caller sees its "own" double that simply has the
  * same value as fMass.
  */
  public double getMass() {
    return fMass;
  }

  /**
  * Returns an immutable object.
  *
  * The caller gets a direct reference to the internal field. But this is not 
  * dangerous, since String is immutable and cannot be changed.
  */
  public String getName() {
    return fName;
  }

//  /**
//  * Returns a mutable object - likely bad style.
//  *
//  * The caller gets a direct reference to the internal field. This is usually dangerous, 
//  * since the Date object state can be changed both by this class and its caller.
//  * That is, this class is no longer in complete control of fDate.
//  */
//  public Date getDateOfDiscovery() {
//    return fDateOfDiscovery;
//  }

  /**
  * Returns a mutable object - good style.
  * 
  * Returns a defensive copy of the field.
  * The caller of this method can do anything they want with the
  * returned Date object, without affecting the internals of this
  * class in any way. Why? Because they do not have a reference to 
  * fDate. Rather, they are playing with a second Date that initially has the 
  * same data as fDate.
  */
  public Date getDateOfDiscovery() {
    return new Date(fDateOfDiscovery.getTime());
  }

  // PRIVATE //

  /**
  * Final primitive data is always immutable.
  */
  private final double fMass;

  /**
  * An immutable object field. (String objects never change state.)
  */
  private final String fName;

  /**
  * A mutable object field. In this case, the state of this mutable field
  * is to be changed only by this class. (In other cases, it makes perfect
  * sense to allow the state of a field to be changed outside the native
  * class; this is the case when a field acts as a "pointer" to an object
  * created elsewhere.)
  */
  private final Date fDateOfDiscovery;
}
 

Note that javadoc 1.4 includes the -tag option, whereby simple custom tags may be defined. One might define an@is.Immutable tag, for example, to document a class as being immutable.You might also consider defining your own tag interface for immutable objects.