Add all 23 GoF design pattern implementations

03-behavioral/command/InsertCommand.java

@@ -0,0 +1,17 @@
+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; }
+}