Design Patterns & Refactoring Visitor Oliver Haase HTWG - - PowerPoint PPT Presentation

Design Patterns & Refactoring

Visitor Oliver Haase

HTWG Konstanz

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 1 / 24

Description

Classification: Object-based behavioral pattern Purpose: Separate algorithm from the object structure upon which it

• perates.

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Motivation

interface Lis t + sum(): int + chars(): String Client Nil + sum(): int + chars(): String IntElement

• head: int
• tail: List

+ sum(): int + chars(): String Element

• tail: List

+ sum(): int + chars(): String CharElement

• head: char
• tail: List

+ sum(): int + chars(): String Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 3 / 24

Motivation

public i n t e r f a c e L i s t { i n t sum ( ) ; S t r i n g chars ( ) ; } public c l a s s N i l implements L i s t { public S t r i n g chars () { return ”” ; } public i n t sum () { return 0; } } public abstract c l a s s Element implements L i s t { protected L i s t t a i l ; protected Element ( L i s t t a i l ) { t h i s . t a i l = t a i l ; } }

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 4 / 24

Motivation

public c l a s s IntElement extends Element { private i n t head ; public IntElement ( i n t number , L i s t t a i l ) { super ( t a i l ) ; head = number ; } public S t r i n g chars () { return t a i l . chars ( ) ; } public i n t sum () { return head + t a i l . sum ( ) ; } } public c l a s s CharElement extends Element { private char head ; public CharElement ( char character , L i s t t a i l ) { super ( t a i l ) ; head = c h a r a c t e r ; } public S t r i n g chars () { return head + t a i l . chars ( ) ; } public i n t sum () { return t a i l . sum ( ) ; } }

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 5 / 24

Motivation

public c l a s s C l i e n t { public s t a t i c void main ( S t r i n g [ ] args ) { L i s t l = new IntElement (4 , new CharElement ( ’ b ’ , new CharElement ( ’ a ’ , new IntElement (3 , new N i l ( ) ) ) ) ) ; System . out . p r i n t l n ( ”Summe: ” + l . sum ( ) ) ; System . out . p r i n t l n ( ”Summe: ” + l . chars ( ) ) ; } }

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Motivation

Problem: Introduction of a new operation, e.g. charCount(), requires

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Motivation

Problem: Introduction of a new operation, e.g. charCount(), requires modification of List interface IntElement class CharElement class Generally, of all classes of the object structure.

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 7 / 24

Key Idea

Idea: Separate operations (sum(), chars, . . . ) into visitor classes that traverse the object structure. ⇓ For each specific visitor, provide one visit operation per element type. To visit an element, call its accept operation which in turn calls the appropriate visit operation.

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Sample Structure

interface Lis t + accept(Visitor): void Client Nil + accept(Visitor): void IntElement

• head: int
• tail: List

+ accept(Visitor): void Element

• tail: List

+ accept(Visitor): void CharElement

• head: char
• tail: List

+ accept(Visitor): void interface Vis itor + visit(Nil): void + visit(IntElement): void + visit(CharElement): void S umVis itor + visit(Nil): void + visit(IntElement): void + visit(CharElement): void Chars Vis itor + visit(Nil): void + visit(IntElement): void + visit(CharElement): void Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 9 / 24

Sample Implementation

public i n t e r f a c e L i s t { void accept ( V i s i t o r v i s i t o r ) ; } public c l a s s N i l implements L i s t { public void accept ( V i s i t o r v i s i t o r ) { v i s i t o r . v i s i t ( t h i s ) ; } } public abstract c l a s s Element implements L i s t { protected L i s t t a i l ; protected Element ( L i s t t a i l ) { t h i s . t a i l = t a i l ; } public L i s t g e t T a i l () { return t a i l ; } }

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 10 / 24

Sample Implementation

public c l a s s IntElement extends Element { private i n t head ; public IntElement ( i n t number , L i s t t a i l ) { super ( t a i l ) ; head = number ; } public i n t getHead () { return head ; } public void accept ( V i s i t o r v i s i t o r ) { v i s i t o r . v i s i t ( t h i s ) ; } } public i n t e r f a c e V i s i t o r { void v i s i t ( N i l l i s t ) ; void v i s i t ( IntElement l i s t ) ; void v i s i t ( CharElement l i s t ) ; }

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Sample Implementation

public c l a s s SumVisitor implements V i s i t o r { private i n t sum = 0; public void v i s i t ( N i l l i s t ) {} public void v i s i t ( IntElement l i s t ) { sum += l i s t . getHead ( ) ; l i s t . g e t T a i l ( ) . accept ( t h i s ) ; } public void v i s i t ( CharElement l i s t ) { l i s t . g e t T a i l ( ) . accept ( t h i s ) ; } public i n t getSum () { return sum ; } }

Please note: Visitor can (and sometimes must) store state information.

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Sample Implementation

public c l a s s C l i e n t { public s t a t i c void main ( S t r i n g [ ] args ) { L i s t l = new IntElement (4 , new CharElement ( ’ b ’ , new CharElement ( ’ a ’ , new IntElement (3 , new N i l ( ) ) ) ) ) ; SumVisitor sv = new SumVisitor ( ) ; l . accept ( sv ) ; System . out . p r i n t l n ( ”Summe: ” + sv . getSum ( ) ) ; C h a r s V i s i t o r cv = new C h a r s V i s i t o r ( ) ; l . accept ( cv ) ; System . out . p r i n t l n ( ”Summe: ” + cv . getChars ( ) ) ; } }

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General Structure

interface Element + accept(Visitor): void Client ConcreteElementA + accept(Visitor): void interface Vis itor + visit(ConcreteElementA): void + visit(ConcreteElementB): void ConcreteElementB + accept(Visitor): void ConcreteVis itor + visit(ConcreteElementA): void + visit(ConcreteElementB): void

Please note: Concrete elements need not have a common supertype.

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Participants

Visitor: declares an overloaded visit operation for each concrete element type ConreteVisitor:

provide implementation for each overloaded visit operation stores state information provides operation to retrieve final state

Element: defines an accept operation with a Visitor as argument ConcreteElement: implements the accept operation, usually by calling the visitor’s appropriate visit operation.

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Consequences of Modifications

modify operation modify object structure w/o visitor pattern adapt all structural classes adapt

• nly

affected structural class with visitor pattern adapt only affected visitor adapt all visitor classes Visitor pattern is beneficial if object structure is more stable than

• perations on it.

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Traversal Logic

There are two main variants of the visitor pattern, depending on which entities contain the logic for the traversal of the object structure:

1 Visitors (see example)

+ know the desired traversal order − structural information is duplicated (object structure + all visitors)

2 Elements

+ natural place to keep structural information confined − don’t know the desired traversal order

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Advanced Considerations

The interaction sequence

ConcreteVis itor ConcreteElement accept(this) visit(this)

seems a little awkward. It is only necessary, because most OO languages (including Java and C#) support only single dispatch rather than double dispatch.

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Advanced Considerations

If the visitor inspects the runtime type of an element, it can directly call its

• wn appropriate visit operations without the indirection via the

element’s accept operation:

public abstract c l a s s V i s i t o r { f i n a l public void v i s i t ( L i s t l i s t ) { i f ( l i s t instanceof N i l ) { v i s i t (( N i l ) l i s t ) ; } i f ( l i s t instanceof IntElement ) { v i s i t (( IntElement ) l i s t ) ; } i f ( l i s t instanceof CharElement ) { v i s i t (( CharElement ) l i s t ) ; } } abstract public void v i s i t ( N i l l i s t ) ; abstract public void v i s i t ( IntElement l i s t ) ; abstract public void v i s i t ( CharElement l i s t ) ; }

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Advanced Considerations

public c l a s s SumVisitor extends V i s i t o r { private i n t sum = 0; public void v i s i t ( N i l l i s t ) {} public void v i s i t ( IntElement l i s t ) { sum += l i s t . getHead ( ) ; v i s i t ( l i s t . g e t T a i l ( ) ) ; } public void v i s i t ( CharElement l i s t ) { v i s i t ( l i s t . g e t T a i l ( ) ) ; } public i n t getSum () { return sum ; } }

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Advanced Considerations

public i n t e r f a c e L i s t {} public c l a s s N i l implements L i s t {} public abstract c l a s s Element implements L i s t { protected L i s t t a i l ; protected Element ( L i s t t a i l ) { t h i s . t a i l = t a i l ; } public L i s t g e t T a i l () { return t a i l ; } }

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Advanced Considerations

public c l a s s IntElement extends Element { private i n t head ; public IntElement ( i n t number , L i s t t a i l ) { super ( t a i l ) ; head = number ; } public i n t getHead () { return head ; } }

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Advanced Considerations

public c l a s s C l i e n t { public s t a t i c void main ( S t r i n g [ ] args ) { L i s t l = new IntElement (4 , new CharElement ( ’ b ’ , new CharElement ( ’ a ’ , new IntElement (3 , new N i l ( ) ) ) ) ) ; SumVisitor sv = new SumVisitor ( ) ; sv . v i s i t ( l ) ; System . out . p r i n t l n ( ”Summe: ” + sv . getSum ( ) ) ; C h a r s V i s i t o r cv = new C h a r s V i s i t o r ( ) ; cv . v i s i t ( l ) ; System . out . p r i n t l n ( ”Summe: ” + cv . getChars ( ) ) ; } }

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Even More Advanced Consideration

With introspection, the Visitor base class can do without the switch:

public abstract c l a s s V i s i t o r { f i n a l public void v i s i t ( L i s t l i s t ) { Object [ ]

• s = { l i s t };

Class <?>[] cs = { l i s t . g e t C l a s s ( ) } ; try { t h i s . g e t C l a s s ( ) . getMethod ( ” v i s i t ” , cs ) . invoke ( this ,

• s ) ;

} catch ( Exception e ) { . . . } } abstract public void v i s i t ( N i l l i s t ) ; abstract public void v i s i t ( IntElement l i s t ) ; abstract public void v i s i t ( CharElement l i s t ) ; }

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