1. Basic Mutability
Paul Ammann

2. Data Abstraction Operation Categories
Creators
Create objects of a data abstraction
Producers
Create objects of their type based on existing objects
Typically used in immutable data types
Mutators
Modify objects of their type
Typically used in mutable data types
Observers
Take objects of their type as inputs and return results of other types

3. Adequacy of a API
Should provide enough operations so that users can manipulate objects conveniently and efficiently
Should have at least three of the four category operations
Should be fully populated
Possible to obtain every possible abstract object state

4. Some Immutable Examples
Polynomials – (from Liskov)
Poly.java
Sets of integer primitives (from Liskov)
IntSet.java
Complex numbers (from Bloch)
Complex.java
Note use of producers instead of mutators

5. Mutable/Immutable Transform
Mutable Stack example in Bloch
Stack.java
Goal: Transform to an immutable version
ImmutableStack s = new ImmutableStack();
s = s.push(“cat”);
s = s.push(“dog”);
What do these objects look like?

6. Mutator  Producer Consider a void mutator method in class C:
public void mutator1(…)
Corresponding producer method:
public C producer1(…)
Consider a non-void mutator method in class C:
public S mutator2(…)
Corresponding observer/producer methods are:
public S observerPart(…)
public C producerPart(…)
Note that non-void mutator needs to be split into two methods.
Example: pop() in Stack vs. pop(), top() in ImmutableStack

7. Typical Transformation
Typical method in mutable class Foo:
public void foo(T1 t1, T2, t2, …) {modify “this”}
Immutable version of Foo:
public Foo foo(T1 t1, T2, t2, …) {
Foo f = … …
return f; }
Functional programming vs. procedural programming.

8. Disadvantage: Performance
Typical approach:
Provide immutable class
Provide mutable companion
Clients choose on performance needs
Example in Java Library:
String (Immutable)
StringBuilder (Companion Mutable Class)
Static factories can cache frequently used items
Notes
Performance gain from mutable objects is often illusory (Bloch Item 88)
Immutability requires additional rules (Bloch Item 17)