Transcript Mutability Slides

Mutability
SWE 332
Fall 2015
Paul Ammann
Data Abstraction Operation
Categories
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Creators
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Producers
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Create objects of their type based on existing objects
Typically used in immutable data types
Mutators
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Create objects of a data abstraction
Modify objects of their type
Typically used in mutable data types
Observers
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Take objects of their type as inputs and return results of
other types
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Adequacy of a API
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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
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Possible to obtain every possible abstract
object state
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Some Examples
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Polynomials – We’ve already seen this
Poly.java
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Complex numbers (pp 74-75 in Bloch)
Complex.java
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Note use of producers instead of
mutators
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Mutable/Immutable Transform
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Mutable Stack example in Bloch (page 56)
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Stack.java
Goal: Transform to an immutable version
ImmutableStack s = new ImmutableStack();
s = s.push(“cat”);
s = s.push(“dog”);
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What do these objects look like?
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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
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Typical Transformation
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Typical method in mutable class Foo:
public void foo(T1 t1, T2, t2, …) {modify “this”}
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Immutable version of Foo:
public Foo foo(T1 t1, T2, t2, …) {
Foo f = …
…
return f;
}
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Functional programming vs. procedural
programming.
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Disadvantage: Performance
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Typical approach:
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Clients choose on performance needs
Example in Java Library:
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Provide immutable class
Provide mutable companion class for situations
where performance is an issue
String (Immutable)
StringBuilder (Companion Mutable Class)
Static factories can cache frequently used
items
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Caching Frequently Used
Items
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Goals:
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Control creation of immutable objects
Limit occurrence of given object to 0 or 1
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Return existing object if possible,
Else construct new object
Flyweight Pattern
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Requires a table of existing items
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Table design is a key decision
Requires hiding constructors
Enables “fast” equals(), hashCode()
Memory leakage may be an issue
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