Visitor mode of design mode (note)

Visitor pattern: Represents an element action that acts on an object structure. It allows you to define new operations that act on these elements without changing the class of each element.

First, define a visitor abstract class, declaring a visit operation for each specific class

publicabstractclass Visitor {    publicabstractvoidvisitConcreteElementA(ConcreteElementA elementA);    publicabstractvoidvisitConcreteElementB(ConcreteElementB elementB);}

Then define the specific classes that inherit the visitor

 Public  class ConcreteVisitor1 extends Visitor{    @Override     Public void Visitconcreteelementa(Concreteelementa Elementa) {System.out.println (Elementa.getclass (). GetName () +"Access"+ This. GetClass (). GetName ()); }@Override     Public void VISITCONCRETEELEMENTB(Concreteelementb ELEMENTB) {System.out.println (Elementb.getclass (). GetName () +"Access"+ This. GetClass (). GetName ()); }} Public  class ConcreteVisitor2 extends Visitor{    @Override     Public void Visitconcreteelementa(Concreteelementa Elementa) {System.out.println (Elementa.getclass (). GetName () +"Access"+ This. GetClass (). GetName ()); }@Override     Public void VISITCONCRETEELEMENTB(Concreteelementb ELEMENTB) {System.out.println (Elementb.getclass (). GetName () +"Access"+ This. GetClass (). GetName ()); }}

Then define an element abstract class that defines an accept method

publicabstractclass Element {    publicabstractvoidaccept(Visitor visitor);}

Then define the specific class that inherits the element

 Public  class concreteelementa extends Element{    @Override     Public void Accept(Visitor Visitor) {Visitor.visitconcreteelementa ( This); } Public void Operationa(){    }} Public  class concreteelementb extends Element{    @Override     Public void Accept(Visitor Visitor) {VISITOR.VISITCONCRETEELEMENTB ( This); } Public void operationb(){    }}

Then define a Objectstructure class to enumerate its elements.

public  class  objectstructure {private  list<element> elements= new  Arraylist<element> (); public  void  attach  (element Element) {Elements.add (element); } public  void  detach  (element Element) {Elements.remove (element); } public  void  accept  (Visitor Visitor) {iterator<element> iterator=elements.iterator (); while  (Iterator.hasnext ()) {Element element=iterator.next (); Element.accept (visitor); } }}

Client code

publicstaticvoidmain(String[] args) {        //访问者模式        ObjectStructure oStructure=new ObjectStructure();        oStructure.attach(new ConcreteElementA());        oStructure.attach(new ConcreteElementB());        ConcreteVisitor1 visitor1=new ConcreteVisitor1();        ConcreteVisitor2 visitor2=new ConcreteVisitor2();        oStructure.accept(visitor1);        oStructure.accept(visitor2);    }

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Visitor mode of design mode (note)