Decorator pattern is a structural design pattern that allows for the dynamic addition of new behavior to an object without the need for inheritance. This pattern can wrap and decorate existing objects without affecting them, thereby expanding their functionality. Applicable scenarios: When it is necessary to dynamically add some additional functionality to an object without modifying its original code. When multiple functions need to be added to an object transparently and dynamically in sequence. 3. When special operations need to be performed on an object, but the source code of the object does not want to be modified. The benefits of this design pattern include: 1. The decorator mode makes it easy to extend the functionality of an object, and allows for flexible addition or removal of decorators to dynamically adjust functionality as needed. 2. Using the decorator pattern can avoid using too many subclasses to extend the functionality of objects, thereby reducing the number of classes. The following is a complete sample code of Java that demonstrates how to dynamically extend the functionality of objects using the decorator pattern: ```java //Define an interface to represent a component public interface Component { void operation(); } //Specific component implementation classes public class ConcreteComponent implements Component { @Override public void operation() { System. out. println ("Perform basic operations"); } } //Abstract Decorator Class public abstract class Decorator implements Component { protected Component component; public Decorator(Component component) { this.component = component; } @Override public void operation() { component.operation(); } } //Specific Decorative Class A public class ConcreteDecoratorA extends Decorator { public ConcreteDecoratorA(Component component) { super(component); } @Override public void operation() { super.operation(); System. out. println ("Extended Function A"); } } //Specific Decorative Class B public class ConcreteDecoratorB extends Decorator { public ConcreteDecoratorB(Component component) { super(component); } @Override public void operation() { super.operation(); System. out. println ("Extended Function B"); } } //Using the Decorator pattern to extend the functionality of objects public class Main { public static void main(String[] args) { Component component = new ConcreteComponent(); component = new ConcreteDecoratorA(component); component = new ConcreteDecoratorB(component); component.operation(); } } ``` In this example, the Component interface represents a component, and ConcreteComponent is the specific component implementation class. Decorator is an abstract decorator class that holds a Component object and implements the Component interface. ConcreteDecoratorA and ConcreteDecoratorB are specific decorator classes that inherit decorators and add specific functions. In the Main class, we first create a ConcreteComponent object, and then decorate it one by one through ConcreteDecoratorA and ConcreteDecoratorB, thereby extending the functionality of the object. Finally, when the component. operation() method is called, the original and new functions will be executed in the order defined in the Decorator class.

Composite pattern is a structural design pattern that aims to represent the overall/partial hierarchical relationship by combining objects into a tree structure, so that users have consistency in their use of individual and composite objects. This pattern is used to organize a group of objects into a Tree structure and treat them in a unified way. Applicable scenario: When you need to organize objects into a tree structure and want to unify the processing of individual and composite objects, you can use the composite mode. When you want the client to ignore the differences between composite objects and individual objects, you can use composite mode. Benefits: 1. Make the client code more concise, without the need to distinguish between handling individual objects and composite objects. 2. New components can be flexibly added, as the composite pattern unifies objects into composite objects and individual objects. The following is an example code for implementing composite patterns using the Java language: ```java //Abstract components in composite patterns interface Component { void operation(); } //Leaf component class Leaf implements Component { private String name; public Leaf(String name) { this.name = name; } public void operation() { System.out.println("Leaf [" + name + "] is performing operation."); } } //Container components class Composite implements Component { private List<Component> children = new ArrayList<>(); public void add(Component component) { children.add(component); } public void remove(Component component) { children.remove(component); } public void operation() { System.out.println("Composite is performing operation."); for (Component component : children) { component.operation(); } } } //Client code public class Client { public static void main(String[] args) { //Creating Components Leaf leaf1 = new Leaf("leaf1"); Leaf leaf2 = new Leaf("leaf2"); Leaf leaf3 = new Leaf("leaf3"); Composite composite1 = new Composite(); Composite composite2 = new Composite(); //Combine into a tree structure composite1.add(leaf1); composite1.add(leaf2); composite2.add(leaf3); composite1.add(composite2); //Calling the operation of a composite object composite1.operation(); } } ``` In the above example code, the Component interface defines a unified operation method, which is implemented by the Leaf class and the Composite class respectively, representing leaf components and container components. Sub components can be added or removed in container components, and the operation methods of sub components can be recursively called. Finally, the client code uses composite patterns to create components with a tree structure and calls the operation methods of the root component.

The Iterator pattern is a behavioral design pattern that allows clients to traverse the elements of a collection object one by one through Iterator without exposing the internal representation of the collection. This pattern encapsulates the iteration process in an independent Iterator object, so that the client can independently operate the collection object and Iterator object. Applicable scenarios: 1. When you need to traverse the elements of an aggregate object without exposing its internal representation, you can use the Iterator pattern. 2. When you need to share the logic of element traversal on different aggregate objects, you can use the Iterator pattern. 3. When you need to provide different traversal methods, you can use the Iterator pattern. The main benefits of this design pattern are: 1. Separation of collection objects and traversal logic makes the code easier to maintain and expand. 2. Provides a unified access interface for different aggregated objects, enhancing the flexibility and reusability of the code. 3. Simplify the use of the client, so that the client does not need to care about the internal structure of the collection object. The following is the complete Java sample code: ```java //Iterator interface interface Iterator<T> { boolean hasNext(); T next(); } //Aggregation interface interface Aggregate { Iterator<String> createIterator(); } //Concrete Iterator implementation class ConcreteIterator implements Iterator<String> { private String[] names; private int position; public ConcreteIterator(String[] names) { this.names = names; this.position = 0; } public boolean hasNext() { return position < names.length; } public String next() { if (!hasNext()) { throw new NoSuchElementException(); } String name = names[position]; position++; return name; } } //Specific aggregation implementation class ConcreteAggregate implements Aggregate { private String[] names; public ConcreteAggregate(String[] names) { this.names = names; } public Iterator<String> createIterator() { return new ConcreteIterator(names); } } public class IteratorPatternDemo { public static void main(String[] args) { String[] names = {"Alice", "Bob", "Charlie", "David"}; Aggregate aggregate = new ConcreteAggregate(names); Iterator<String> iterator = aggregate.createIterator(); while (iterator.hasNext()) { String name = iterator.next(); System.out.println(name); } } } ``` In the above code, we have defined a Iterator interface and an aggregate interface. The concrete Iterator and aggregate implement the corresponding interface. In the main method of 'IteratorPatternDemo', we obtain a Iterator object by creating a concrete aggregate object. Then use the Iterator to traverse the elements in the aggregation object, and finally output the name of each element.

The Chain of Responsibility Pattern is a behavioral design pattern that decouples the sender and receiver of a request, giving multiple objects the opportunity to process the request. String these objects into a chain and pass the request along this chain until an object processes it. 2. Applicable scenarios: -When multiple objects can handle the same request and the specific object that handles the request is determined at runtime, the chain of responsibilities pattern can be used. -When it is desired to have multiple objects have the opportunity to process the request, rather than having only one determined processor, the responsibility chain pattern can be used. -When it is necessary to submit a request to one of multiple objects without specifying the recipient, the responsibility chain pattern can be used. 3. Benefits: -The responsibility chain pattern decouples the sender and receiver of requests, and the sender does not need to know who processed the request or the structure of the chain. -Can dynamically add or modify objects for processing requests, with high flexibility. -You can control the order in which requests are processed. The following is the complete sample code for implementing the responsibility chain pattern in Java: //Defining Abstract Processors abstract class Handler { protected Handler successor; public void setSuccessor(Handler successor) { this.successor = successor; } public abstract void handleRequest(int request); } //Specific processor A class ConcreteHandlerA extends Handler { public void handleRequest(int request) { if (request >= 0 && request < 10) { System. out. println ("ConcreteHandlerA processing request"+request); } else if (successor != null) { successor.handleRequest(request); } } } //Specific processor B class ConcreteHandlerB extends Handler { public void handleRequest(int request) { if (request >= 10 && request < 20) { System. out. println ("ConcreteHandlerB processing request"+request); } else if (successor != null) { successor.handleRequest(request); } } } //Specific processor C class ConcreteHandlerC extends Handler { public void handleRequest(int request) { if (request >= 20 && request < 30) { System. out. println ("ConcreteHandlerC processing request"+request); } else if (successor != null) { successor.handleRequest(request); } } } //Test Code public class Main { public static void main(String[] args) { //Building a Responsibility Chain Handler handlerA = new ConcreteHandlerA(); Handler handlerB = new ConcreteHandlerB(); Handler handlerC = new ConcreteHandlerC(); handlerA.setSuccessor(handlerB); handlerB.setSuccessor(handlerC); //Send Request int[] requests = {5, 15, 25}; for (int request : requests) { handlerA.handleRequest(request); } } } Output results: ConcreteHandlerA processing request 5 ConcreteHandlerB processing request 15 ConcreteHandlerC processing request 25 In the above code, an abstract handler and its three concrete handlers (ConcreteHandlerA, ConcreteHandlerB, and ConcreteHandlerC) are defined, with each concrete handler having a successor handler. In the test code, a chain of responsibilities was constructed and three different requests were sent. According to the value of the request, the processors in the responsibility chain process the request in sequence. The requested value is processed by ConcreteHandlerA if it is less than 10, ConcreteHandlerB if it is greater than or equal to 10 but less than 20, and ConcreteHandlerC if it is greater than or equal to 20 but less than 30. If a processor is unable to process the request, the request is passed on to the next processor until a processor processes the request or the end of the responsibility chain.

The Visitor pattern is a behavioral design pattern that allows you to define a new operation without changing the class of the element being operated. Using the Visitor pattern can separate the logic of the algorithm from the operation of traversing a data structure. Applicable scenarios include: 1. When each element in an object structure has different operation modes, and these operation modes may need to be changed frequently, the Visitor pattern can be used. For example, different products in a shopping mall may require different discount methods. 2. When the structure of the element class in an object structure rarely changes, but new operations are often needed in the element class, the Visitor pattern can be used. The benefits of using the Visitor pattern include: 1. Decouple the data structure and operations, making it convenient to add new operations without modifying the original code. 2. It is easy to add new operations without changing the class of the element, thereby improving the scalability of the code. The following is an example code of the Visitor pattern implemented in Java: ```java //Element Interface interface Element { void accept(Visitor visitor); } //Specific Element A class ConcreteElementA implements Element { @Override public void accept(Visitor visitor) { visitor.visitElementA(this); } public void operationA() { System. out. println ("Operation of specific element A"); } } //Specific Element B class ConcreteElementB implements Element { @Override public void accept(Visitor visitor) { visitor.visitElementB(this); } public void operationB() { System. out. println ("Operation of specific element B"); } } //Visitor Interface interface Visitor { void visitElementA(ConcreteElementA elementA); void visitElementB(ConcreteElementB elementB); } //Specific Visitor Class class ConcreteVisitor implements Visitor { @Override public void visitElementA(ConcreteElementA elementA) { System. out. println ("Specific visitor access element A"); elementA.operationA(); } @Override public void visitElementB(ConcreteElementB elementB) { System. out. println ("Specific visitor accessing element B"); elementB.operationB(); } } //Object Structure Class class ObjectStructure { private List<Element> elements = new ArrayList<>(); public void addElement(Element element) { elements.add(element); } public void removeElement(Element element) { elements.remove(element); } public void accept(Visitor visitor) { for (Element element : elements) { element.accept(visitor); } } } //Client code public class Main { public static void main(String[] args) { ObjectStructure objectStructure = new ObjectStructure(); objectStructure.addElement(new ConcreteElementA()); objectStructure.addElement(new ConcreteElementB()); Visitor visitor = new ConcreteVisitor(); objectStructure.accept(visitor); } } ``` In the above example, the Visitor pattern is used to implement specific operations through the Visitor interface, and the specific operations are encapsulated in the ConcreteVisitor class. The element interface defines the accept method, which is used to delegate specific operations to the visitor for execution. The concrete element class implements the Element interface, and calls the corresponding method of the visitor in the accept method to perform specific operations. By using the Visitor pattern, you can easily implement different operations on different elements, while improving the scalability and maintainability of the code.

Command pattern is a behavioral design pattern, which encapsulates a request into an object, thus decoupling the object sending the request from the object processing the request. By encapsulating the request as an object, the Command pattern allows request parameterization, and supports queuing, logging, revocation, and redo of requests. Applicable scenarios: 1. When the specific request needs to be decoupled from the object processing the request, the Command pattern can be used. 2. When you need to support the undo and redo functions of operations, you can use the Command pattern. 3. The Command pattern can be used when the request needs to be queued and logged. 4. When you need to combine a group of operations into a command, you can use the Command pattern. Benefits of Command pattern: 1. Reduce system coupling: the sender and receiver are Loose coupling, and the sender does not need to know the specific implementation of the receiver. 2. Easy to expand new commands: Adding a new command does not affect other existing commands, making it easy to add and maintain. 3. Support for undo and redo operations: By saving the history of commands, undo and redo functions can be achieved. The following is a simple example code of Java Command pattern: Firstly, define a command interface Command: ```java public interface Command { void execute(); } ``` Then implement the command interface and create specific command classes, such as opening and closing TV commands: ```java public class OpenTVCommand implements Command { private Television tv; public OpenTVCommand(Television tv) { this.tv = tv; } @Override public void execute() { tv.open(); } } public class CloseTVCommand implements Command { private Television tv; public CloseTVCommand(Television tv) { this.tv = tv; } @Override public void execute() { tv.close(); } } ``` Create the executor of the command again, such as for television: ```java public class Television { public void open() { System. out. println ("Turn on TV"); } public void close() { System. out. println ("Turn off TV"); } } ``` Finally, create a command caller who can receive commands and execute: ```java public class CommandInvoker { private Command command; public void setCommand(Command command) { this.command = command; } public void executeCommand() { command.execute(); } } ``` Now you can test the above code, create a command caller, set the command, and then execute the command: ```java public class Main { public static void main(String[] args) { Television tv = new Television(); Command openCommand = new OpenTVCommand(tv); Command closeCommand = new CloseTVCommand(tv); CommandInvoker invoker = new CommandInvoker(); invoker.setCommand(openCommand); invoker.executeCommand(); invoker.setCommand(closeCommand); invoker.executeCommand(); } } ``` The above code will print the following results: ``` Turn on TV Turn off TV ``` This indicates that the Command pattern successfully decouples the requester from the processor and executes the command through the command caller.

State mode is a behavioral design pattern that allows an object to change its behavior when its internal state changes. State patterns encapsulate the behavior of objects in different state classes, allowing states to transition to each other, thereby achieving the goal of dynamically changing object behavior. Applicable scenario: When the behavior of an object depends on its state and needs to be changed at runtime based on the state, state mode can be considered. When the behavior of an object is related to its state and state transitions are frequent, state patterns can be used to simplify the code. Benefits: 1. Through state patterns, state related code can be encapsulated in state classes, making the code clearer and easier to maintain. 2. It is easier to extend the state class and add new states, which conforms to the Open–closed principle. 3. Separate the state and behavior of objects to make state transitions more controllable. Below is a simple example code to demonstrate the implementation of state mode: ```java //State interface interface State { void doAction(Context context); } //Specific Status Class A class StateA implements State { public void doAction(Context context) { System. out. println ("The current state is A"); context.setState(new StateB()); } } //Specific Status Class B class StateB implements State { public void doAction(Context context) { System. out. println ("The current state is B"); context.setState(new StateA()); } } //Context class class Context { private State currentState; public Context() { currentState = new StateA(); } public void setState(State state) { currentState = state; } public void doAction() { currentState.doAction(this); } } //Test Code public class Main { public static void main(String[] args) { Context context = new Context(); Context. doAction()// Output: The current state is A Context. doAction()// Output: The current state is B Context. doAction()// Output: The current state is A } } ``` In the above example code, the state pattern defines the behavior of the state through the 'State' interface, and the specific state class implements the 'State' interface and changes the state of the context object in the 'doAction' method. The context class' Context 'holds a reference to the current state, executes the behavior of the current state by calling the' doAction 'method, and transitions the state to the next state. In the test code, the transition process of the state can be seen based on the output.

Interpreter Pattern is a behavior design pattern that defines the grammar of a language and defines an interpreter that interprets statements in that language. This design pattern is applicable to the following scenarios: When a language needs to be interpreted and executed with scalability, the interpreter mode can be used. For example, a calculator interpreter can parse and execute mathematical expressions. When the grammar of the language is relatively simple, the interpreter mode can be used. Complex syntax may lead to overly complex implementation of the interpreter. The benefits of using interpreter mode include: 1. Syntax rules can be expressed as Abstract syntax tree, which is easy to understand and modify. 2. New expressions can be added without modifying the interpreter code. 3. Complex interpreters can be constructed by combining different expressions. The following is a simple Java sample code that implements a simple calculator interpreter. ```java //Abstract expression interface interface Expression { int interpret(); } //Terminator expression implementation class class Number implements Expression { private int value; public Number(int value) { this.value = value; } public int interpret() { return value; } } //Non Terminator Expression Implementation Class class Add implements Expression { private Expression left; private Expression right; public Add(Expression left, Expression right) { this.left = left; this.right = right; } public int interpret() { return left.interpret() + right.interpret(); } } //Client code public class InterpreterPatternDemo { public static void main(String[] args) { //Building Abstract syntax tree of Interpreter Expression expression = new Add(new Number(10), new Add(new Number(5), new Number(3))); //Interpret Execution Grammar Tree int result = expression.interpret(); System.out.println("Result: " + result); } } ``` In the above sample code, we defined an abstract expression interface 'Expression' and implemented two concrete expression classes' Number 'and' Address'` The 'Number' class represents numbers, and the 'Add' class represents addition operations. The client code builds a Abstract syntax tree, then executes the syntax tree through the interpreter, and outputs the results. In this simple calculator interpreter, we only implemented addition operations, but we can extend the interpreter's functionality by implementing new expression classes, such as subtraction, multiplication, and so on. This is a major advantage of the interpreter mode.

The Mediator pattern is a behavioral design pattern, which is realized by transferring the interaction between objects to the mediator object. In this pattern, objects do not directly communicate with each other, but communicate and coordinate through intermediary objects. Applicable scenario: 1. When the change of one object needs to change other objects at the same time, the Mediator pattern can be used. By introducing intermediary objects, the coupling between objects can be reduced, reducing direct interaction between objects. 2. When an interaction process involves a large number of interactions between objects, resulting in complex interactions, the Mediator pattern can be used to simplify the interaction process. Benefits: 1. Reduced direct coupling between objects, making the code easier to maintain and expand. 2. Focus on the intermediary object to make the code clearer and easier to understand. 3. It can avoid circular dependencies between objects and improve code flexibility. The following is an example code of using Java to implement the Mediator pattern: ```java //Mediator Interface interface Mediator { void sendMessage(String message, Colleague colleague); } //Specific intermediaries class ConcreteMediator implements Mediator { private ColleagueA colleagueA; private ColleagueB colleagueB; public void setColleagueA(ColleagueA colleagueA) { this.colleagueA = colleagueA; } public void setColleagueB(ColleagueB colleagueB) { this.colleagueB = colleagueB; } @Override public void sendMessage(String message, Colleague colleague) { if (colleague == colleagueA) { colleagueB.receiveMessage(message); } else if (colleague == colleagueB) { colleagueA.receiveMessage(message); } } } //Abstract colleague class abstract class Colleague { protected Mediator mediator; public Colleague(Mediator mediator) { this.mediator = mediator; } public abstract void sendMessage(String message); public abstract void receiveMessage(String message); } //Specific colleague category A class ColleagueA extends Colleague { public ColleagueA(Mediator mediator) { super(mediator); } @Override public void sendMessage(String message) { mediator.sendMessage(message, this); } @Override public void receiveMessage(String message) { System.out.println("ColleagueA received message: " + message); } } //Specific colleague category B class ColleagueB extends Colleague { public ColleagueB(Mediator mediator) { super(mediator); } @Override public void sendMessage(String message) { mediator.sendMessage(message, this); } @Override public void receiveMessage(String message) { System.out.println("ColleagueB received message: " + message); } } //Testing class public class MediatorPatternDemo { public static void main(String[] args) { ConcreteMediator mediator = new ConcreteMediator(); ColleagueA colleagueA = new ColleagueA(mediator); ColleagueB colleagueB = new ColleagueB(mediator); mediator.setColleagueA(colleagueA); mediator.setColleagueB(colleagueB); colleagueA.sendMessage("Hello, colleagueB!"); colleagueB.sendMessage("Hi, colleagueA!"); } } ``` In the above example code, the Mediator pattern realizes communication and coordination between objects through the implementation of the mediator interface and the specific mediator class. Abstract colleague classes define the common behavior of colleague classes and their dependence on intermediaries. Specific colleague classes inherit abstract colleague classes and implement their respective methods. The test class creates the mediator and colleague class instance, sets the corresponding colleague class instance through the mediator, and then calls the method of the colleague class to send the message. Through the intermediary's forwarding, the colleague class receives the message sent by the other party and processes it.

The memo pattern is a behavioral design pattern used to capture the internal state of an object and restore it when needed, without exposing the implementation details of the object. This pattern encapsulates the object state in a memo object, enabling the storage and recovery of the object state, while also preventing external code from directly accessing the object's state. Applicable scenario: When it is necessary to save and restore the internal state of an object, memo mode can be used. For example, in an application, a user can undo some actions, which requires saving the state of the object for recovery when needed. When you want to encapsulate the state of an object and prevent external code from directly accessing its state, you can use memo mode. Benefits: 1. It provides the ability to save and restore object states, making object state management more convenient and flexible. Encapsulating the object state in a memo object can prevent external code from directly accessing the object's state, increasing the encapsulation and security of the object. The following is a complete sample code for a Java based memo mode: Firstly, we need to create a Memonto class to store the state of the object: ```java public class Memento { private String state; public Memento(String state) { this.state = state; } public String getState() { return state; } } ``` Then, we need to create an original object (Originator) for creating a memo and restoring the state: ```java public class Originator { private String state; public void setState(String state) { this.state = state; } public Memento saveStateToMemento() { return new Memento(state); } public void restoreStateFromMemento(Memento memento) { this.state = memento.getState(); } } ``` Finally, we need to create a Caretaker to manage memo objects: ```java public class Caretaker { private List<Memento> mementoList = new ArrayList<>(); public void addMemento(Memento memento) { mementoList.add(memento); } public Memento getMemento(int index) { return mementoList.get(index); } } ``` An example of using memo mode is as follows: ```java public class Main { public static void main(String[] args) { Originator originator = new Originator(); Caretaker caretaker = new Caretaker(); originator.setState("State1"); originator.setState("State2"); caretaker.addMemento(originator.saveStateToMemento()); originator.setState("State3"); caretaker.addMemento(originator.saveStateToMemento()); originator.setState("State4"); System.out.println("Current state: " + originator.getState()); originator.restoreStateFromMemento(caretaker.getMemento(0)); System.out.println("Restored state: " + originator.getState()); } } ``` Run the above code and the output result is: ``` Current state: State4 Restored state: State2 ``` In this example, the Originator saves memo objects in different states, while the Caretaker is used to save and retrieve memo objects. By saving and restoring the internal state of an object, we can restore it to its previous state at any time.