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

03-behavioral/command/Command.java

@@ -0,0 +1,8 @@
+package command;
+
+/** Command interface: every action is an object */
+public interface Command {
+    void execute();
+    void undo();
+    String getDescription();
+}

03-behavioral/command/CommandHistory.java

@@ -0,0 +1,28 @@
+package command;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+/**
+ * Invoker — holds command history and triggers execute/undo.
+ * It doesn't know what the commands do; it just calls execute() and undo().
+ */
+public class CommandHistory {
+    private final Deque<Command> history = new ArrayDeque<>();
+
+    public void execute(Command cmd) {
+        cmd.execute();
+        history.push(cmd);
+        System.out.println("  Executed: " + cmd.getDescription());
+    }
+
+    public void undo() {
+        if (history.isEmpty()) {
+            System.out.println("  Nothing to undo.");
+            return;
+        }
+        Command cmd = history.pop();
+        cmd.undo();
+        System.out.println("  Undone: " + cmd.getDescription());
+    }
+}

03-behavioral/command/DeleteCommand.java

@@ -0,0 +1,23 @@
+package command;
+
+public class DeleteCommand implements Command {
+    private final TextEditor editor;
+    private final int start;
+    private final int length;
+    private String deletedText;  // saved for undo
+
+    public DeleteCommand(TextEditor editor, int start, int length) {
+        this.editor = editor;
+        this.start  = start;
+        this.length = length;
+    }
+
+    @Override
+    public void execute() {
+        deletedText = editor.getText().substring(start, start + length);
+        editor.deleteText(start, length);
+    }
+
+    @Override public void undo() { editor.insertText(deletedText, start); }
+    @Override public String getDescription() { return "Delete " + length + " chars at " + start; }
+}

03-behavioral/command/InsertCommand.java

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+package command;
+
+public class InsertCommand implements Command {
+    private final TextEditor editor;
+    private final String text;
+    private final int position;
+
+    public InsertCommand(TextEditor editor, String text, int position) {
+        this.editor   = editor;
+        this.text     = text;
+        this.position = position;
+    }
+
+    @Override public void execute() { editor.insertText(text, position); }
+    @Override public void undo()    { editor.deleteText(position, text.length()); }
+    @Override public String getDescription() { return "Insert \"" + text + "\" at " + position; }
+}

03-behavioral/command/Main.java

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+package command;
+
+/**
+ * Command Design Pattern — Runnable Demo
+ * Run: javac command/*.java -d out/command && java -cp out/command command.Main
+ * Article: https://ankurm.com/command-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Command Design Pattern Demo ===\n");
+
+        TextEditor editor = new TextEditor();
+        CommandHistory history = new CommandHistory();
+
+        System.out.println("Initial: " + editor);
+
+        history.execute(new InsertCommand(editor, "Hello", 0));
+        System.out.println("After:   " + editor);
+
+        history.execute(new InsertCommand(editor, " World", 5));
+        System.out.println("After:   " + editor);
+
+        history.execute(new DeleteCommand(editor, 5, 6));
+        System.out.println("After:   " + editor);
+
+        System.out.println("\n-- Undo sequence --");
+        history.undo();
+        System.out.println("After undo: " + editor);
+
+        history.undo();
+        System.out.println("After undo: " + editor);
+
+        history.undo();
+        System.out.println("After undo: " + editor);
+
+        history.undo();  // nothing left
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}

03-behavioral/command/TextEditor.java

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+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 + "\"]"; }
+}