add visitor DP

Author: Liu Yuning

src/designpattern/visitor/Element.java

@@ -0,0 +1,29 @@
+package designpattern.visitor;
+
+/**
+ * 定义一个accept操作，它以一个访问者为参数
+ * 
+ * @author liu yuning
+ *
+ */
+public abstract class Element {
+    public abstract void accept(Visitor visitor);
+}
+
+class ConcreteElementA extends Element {
+
+    @Override
+    public void accept(Visitor visitor) {
+	visitor.visitConcreteElementA(this);
+    }
+
+}
+
+class ConcreteElementB extends Element {
+
+    @Override
+    public void accept(Visitor visitor) {
+	visitor.visitConcreteElementB(this);
+    }
+
+}

src/designpattern/visitor/ObjectStructure.java

@@ -0,0 +1,29 @@
+package designpattern.visitor;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * 提供一个高层的接口以允许访问者访问它的元素
+ * 
+ * @author liu yuning
+ *
+ */
+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) {
+	for (Element element : elements) {
+	    element.accept(visitor);
+	}
+    }
+}

src/designpattern/visitor/Visitor.java

@@ -0,0 +1,47 @@
+package designpattern.visitor;
+
+/**
+ * 为该对象结构中ConcreteElement的每一个类声明一个Visit操作
+ * 
+ * @author liu yuning
+ *
+ */
+public abstract class Visitor {
+
+    public abstract void visitConcreteElementA(ConcreteElementA concreteElementA);
+
+    public abstract void visitConcreteElementB(ConcreteElementB concreteElementB);
+
+}
+
+class ConcreteVisitor1 extends Visitor {
+
+    @Override
+    public void visitConcreteElementA(ConcreteElementA concreteElementA) {
+	System.out.println(concreteElementA.getClass().getSimpleName() + "被"
+		+ this.getClass().getSimpleName() + "访问");
+    }
+
+    @Override
+    public void visitConcreteElementB(ConcreteElementB concreteElementB) {
+	System.out.println(concreteElementB.getClass().getSimpleName() + "被"
+		+ this.getClass().getSimpleName() + "访问");
+    }
+
+}
+
+class ConcreteVisitor2 extends Visitor {
+
+    @Override
+    public void visitConcreteElementA(ConcreteElementA concreteElementA) {
+	System.out.println(concreteElementA.getClass().getSimpleName() + "被"
+		+ this.getClass().getSimpleName() + "访问");
+    }
+
+    @Override
+    public void visitConcreteElementB(ConcreteElementB concreteElementB) {
+	System.out.println(concreteElementB.getClass().getSimpleName() + "被"
+		+ this.getClass().getSimpleName() + "访问");
+    }
+
+}

src/designpattern/visitor/VisitorClient.java

@@ -0,0 +1,23 @@
+package designpattern.visitor;
+
+/**
+ * 客户端
+ * 
+ * @author liu yuning
+ *
+ */
+public class VisitorClient {
+
+    public static void main(String[] args) {
+	ObjectStructure o = new ObjectStructure();
+
+	o.attach(new ConcreteElementA());
+	o.attach(new ConcreteElementB());
+
+	ConcreteVisitor1 visitor1 = new ConcreteVisitor1();
+	ConcreteVisitor2 visitor2 = new ConcreteVisitor2();
+
+	o.accept(visitor1);
+	o.accept(visitor2);
+    }
+}