BAEL-1792 overview of visitor design pattern (#4352)

patterns/design-patterns/src/main/java/com/baeldung/visitor/Document.java

@@ -0,0 +1,20 @@
+package com.baeldung.visitor;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Document extends Element {
+
+    List<Element> elements = new ArrayList<>();
+
+    public Document(String uuid) {
+        super(uuid);
+    }
+
+    @Override
+    public void accept(Visitor v) {
+        for (Element e : this.elements) {
+            e.accept(v);
+        }
+    }
+}

patterns/design-patterns/src/main/java/com/baeldung/visitor/Element.java

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+package com.baeldung.visitor;
+
+public abstract class Element {
+
+    public String uuid;
+
+    public Element(String uuid) {
+        this.uuid = uuid;
+    }
+
+    public abstract void accept(Visitor v);
+}

patterns/design-patterns/src/main/java/com/baeldung/visitor/ElementVisitor.java

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+package com.baeldung.visitor;
+
+public class ElementVisitor implements Visitor {
+
+    @Override
+    public void visit(XmlElement xe) {
+        System.out.println("processing xml element with uuid: " + xe.uuid);
+    }
+
+    @Override
+    public void visit(JsonElement je) {
+        System.out.println("processing json element with uuid: " + je.uuid);
+    }
+}

patterns/design-patterns/src/main/java/com/baeldung/visitor/JsonElement.java

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+package com.baeldung.visitor;
+
+public class JsonElement extends Element {
+
+    public JsonElement(String uuid) {
+        super(uuid);
+    }
+
+    public void accept(Visitor v) {
+        v.visit(this);
+    }
+}

patterns/design-patterns/src/main/java/com/baeldung/visitor/Visitor.java

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+package com.baeldung.visitor;
+
+public interface Visitor {
+
+    void visit(XmlElement xe);
+
+    void visit(JsonElement je);
+}

patterns/design-patterns/src/main/java/com/baeldung/visitor/VisitorDemo.java

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+package com.baeldung.visitor;
+
+import java.util.UUID;
+
+public class VisitorDemo {
+
+    public static void main(String[] args) {
+
+        Visitor v = new ElementVisitor();
+
+        Document d = new Document(generateUuid());
+        d.elements.add(new JsonElement(generateUuid()));
+        d.elements.add(new JsonElement(generateUuid()));
+        d.elements.add(new XmlElement(generateUuid()));
+
+        d.accept(v);
+    }
+
+    private static String generateUuid() {
+        return UUID.randomUUID()
+            .toString();
+    }
+}

patterns/design-patterns/src/main/java/com/baeldung/visitor/XmlElement.java

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+package com.baeldung.visitor;
+
+public class XmlElement extends Element {
+
+    public XmlElement(String uuid) {
+        super(uuid);
+    }
+
+    public void accept(Visitor v) {
+        v.visit(this);
+    }
+}