Add all 23 GoF design pattern implementations

03-behavioral/command/TextEditor.java

@@ -0,0 +1,21 @@
+package command;
+
+/**
+ * Receiver — contains the actual text editing logic.
+ * The commands call methods on this object.
+ */
+public class TextEditor {
+    private final StringBuilder text = new StringBuilder();
+
+    public void insertText(String content, int position) {
+        text.insert(position, content);
+    }
+
+    public void deleteText(int start, int length) {
+        text.delete(start, start + length);
+    }
+
+    public String getText() { return text.toString(); }
+
+    @Override public String toString() { return "Editor[\"" + text + "\"]"; }
+}