Composite Pattern

The Composite Pattern lets a single object and a whole tree of objects be used through the same interface.

Some structures nest. A folder holds files and other folders. An invoice holds line items and bundles that hold their own items. A UI panel holds widgets and other panels.

Now compute the total size, or the total price, or draw the whole thing. Without a pattern, every one of those operations starts the same way:

if (node instanceof Folder) {
    int total = 0;
    for (Node child : ((Folder) node).children()) total += sizeOf(child);
    return total;