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

03-behavioral/memento/Editor.java

@@ -0,0 +1,43 @@
+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 + "]";
+    }
+}