Add all 23 GoF design pattern implementations (2026-06-13)

03-behavioral/memento/Editor.java

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+package memento;
+
+/**
+ * Originator — the text editor whose state we're saving and restoring.
+ * Creates mementos and restores from them; no history management here.
+ */
+public class Editor {
+    private String content       = "";
+    private int    cursorPosition = 0;
+    private String selectedText  = "";
+
+    public void type(String text) {
+        content = content.substring(0, cursorPosition) + text + content.substring(cursorPosition);
+        cursorPosition += text.length();
+    }
+
+    public void selectText(int start, int end) {
+        this.selectedText = content.substring(start, end);
+        System.out.println("  Selected: \"" + selectedText + "\"");
+    }
+
+    public void deleteSelection() {
+        content = content.replace(selectedText, "");
+        selectedText = "";
+    }
+
+    /** Create a snapshot of current state */
+    public EditorMemento save() {
+        return new EditorMemento(content, cursorPosition, selectedText);
+    }
+
+    /** Restore state from a snapshot */
+    public void restore(EditorMemento memento) {
+        this.content        = memento.getContent();
+        this.cursorPosition = memento.getCursorPosition();
+        this.selectedText   = memento.getSelectedText();
+    }
+
+    @Override
+    public String toString() {
+        return "Editor[\"" + content + "\", cursor=" + cursorPosition + "]";
+    }
+}

03-behavioral/memento/EditorMemento.java

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+package memento;
+
+/**
+ * Memento — a snapshot of the editor's state.
+ * Immutable: once created, the state cannot be changed.
+ * The Caretaker holds these; the Originator creates and restores from them.
+ */
+public final class EditorMemento {
+    private final String content;
+    private final int cursorPosition;
+    private final String selectedText;
+
+    EditorMemento(String content, int cursorPosition, String selectedText) {
+        this.content        = content;
+        this.cursorPosition = cursorPosition;
+        this.selectedText   = selectedText;
+    }
+
+    // Package-private: only the Editor (Originator) should read the state back
+    String getContent()      { return content; }
+    int getCursorPosition()  { return cursorPosition; }
+    String getSelectedText() { return selectedText; }
+
+    @Override
+    public String toString() {
+        return "Snapshot[content=\"" + content + "\", cursor=" + cursorPosition + "]";
+    }
+}

03-behavioral/memento/History.java

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+package memento;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+/**
+ * Caretaker — manages the stack of mementos.
+ * It does NOT open or inspect the mementos; it just stores and returns them.
+ */
+public class History {
+    private final Deque<EditorMemento> snapshots = new ArrayDeque<>();
+
+    public void save(EditorMemento memento) {
+        snapshots.push(memento);
+        System.out.println("  [History] Saved: " + memento);
+    }
+
+    public EditorMemento undo() {
+        if (snapshots.isEmpty()) return null;
+        return snapshots.pop();
+    }
+
+    public int size() { return snapshots.size(); }
+}

03-behavioral/memento/Main.java

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+package memento;
+
+/**
+ * Memento Design Pattern — Runnable Demo
+ * Run: javac memento/*.java -d out/memento && java -cp out/memento memento.Main
+ * Article: https://ankurm.com/memento-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Memento Design Pattern Demo ===\n");
+
+        Editor editor = new Editor();
+        History history = new History();
+
+        System.out.println("Initial: " + editor);
+
+        editor.type("Hello");
+        history.save(editor.save());
+        System.out.println("After type: " + editor);
+
+        editor.type(", World");
+        history.save(editor.save());
+        System.out.println("After type: " + editor);
+
+        editor.type("! How are you?");
+        System.out.println("After type: " + editor);
+
+        System.out.println("\n-- Undo --");
+        editor.restore(history.undo());
+        System.out.println("After undo: " + editor);
+
+        editor.restore(history.undo());
+        System.out.println("After undo: " + editor);
+
+        System.out.println("\nHistory empty: " + (history.size() == 0));
+        System.out.println("\n=== Demo complete ===");
+    }
+}