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

03-behavioral/memento/EditorMemento.java

@@ -0,0 +1,28 @@
+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 + "]";
+    }
+}