Head first design pattern -- 11. Composite pattern

Thinking questions

We don't just have to support multiple menus , We also support the menu in the menu . How do you deal with this new design requirement ？ P355

• 【 Tips 】 In our new design , The following three points are really needed ： P354
  • We need some kind of attribute structure , You can hold a menu 、 Submenus and menu items
  • We need to be sure to be able to swim between items in each menu , And at least as easy as using an iterator now
  • We also need to be able to move more flexibly between menu items . For example , Maybe you just need to go through the dessert menu , Or you can go through the restaurant's entire menu （ Including dessert menu ）
• Provide an interface , Unify the superclass of menu and menu item
  • Interface contains common methods for menus and menu items , The execution method in the menu is to execute the same method in each subitem in turn
  • Interface contains menu specific methods of adding, deleting, modifying and checking subitems , The method of adding, deleting and checking subitems in menu items is to throw out directly UnsupportedOperationException
  • Can pass instanceof Determine whether the current item is a menu or a menu item

Portfolio model

Allows you to combine objects into a tree structure to represent ” whole / part “ hierarchy . Composition allows customers to handle individual objects and composition of objects in a consistent way .

characteristic

• Apply the same operations to composite and individual objects , That is, you can ignore the differences between object combinations and individual objects P357
• In exchange for transparency by violating the principle of Single Responsibility Design , It's not just about managing hierarchies , It also includes the operation of managing composition and leaf nodes , To treat composition and leaf nodes alike P367

Empty the iterator ： An empty object （ The command mode mentioned ） An example of . Empty the iterator , hasNext() Never return false , next() Never return null （ I think it can be thrown out NoSuchElementException）, remove() Always throw out UnsupportedOperationException . P372

Thinking questions

public class Waitress {
MenuComponent allMenus;
public Waitress(MenuComponent allMenus) {
this.allMenus = allMenus;
}
public void printMenu() {
allMenus.print();
}
public void printVegetarianMenu() {
Iterator iterator = allMenus.createIterator();
System.out.println("\nVEGETARIAN MENU\n----");
while (iterator.hasNext()) {
MenuComponent menuComponent = (MenuComponent)iterator.next();
try {
if (menuComponent.isVegetarian()) {
menuComponent.print();
}
} catch (UnsupportedOperationException e) {}
}
}
}

printVegetarianMenu() Method has only menu items print() Methods can be called , Never call the menu （ Combine ） Of print() Method . Can you tell me why ？ P373

• When using iterator traversal, all nodes will be traversed （ Including composite nodes and leaf nodes ）, And the composite node will print all the sub node information , If you call the print() , Some leaf nodes will be printed repeatedly .

Thinking questions

Match the following patterns and descriptions ： P379
The strategy pattern ： Encapsulating interchangeable behavior , And use delegation to decide which one to use
Adapter pattern ： Change the interface of one or more classes
Iterator pattern ： Provides a way to traverse the collection , Without exposing the implementation of the collection
Appearance mode ： Simplify the interface of a group of classes
Portfolio model ： Customers can treat a collection of objects as well as individual objects equally
Observer mode ： When a state changes , Allow a group of objects to be informed of

Thinking and thinking

• This idea has been used in many places before , Binary tree 、trie Tree structure and line tree structure allow nodes to form a tree structure to represent ” whole / part “ Hierarchical structure , And provide a consistent way to deal with non leaf nodes and leaf nodes （ Inside the method of a non leaf node, it processes and calls the same method of a child node according to specific logic ）

This article was first published on the official account ： Man Fu Zhu Ji （ Click to view the original ） Open source in GitHub ：reading-notes/head-first-design-patterns