The extraction method of Java Code refactoring is to extract part of the code logic as an independent method to improve the readability, maintainability and reusability of the code. Here is an example to illustrate when method extraction and refactoring are required, as well as the code example after refactoring. Assuming we have a program for calculating employee annual salary, which includes the following code snippet: ```java double calculateYearlySalary(String name, double monthlySalary) { double yearlySalary; if (monthlySalary < 0) { throw new IllegalArgumentException("Monthly salary cannot be negative."); } If (name==null | | name. isEmpty()){ throw new IllegalArgumentException("Name cannot be null or empty."); } //Assuming there is other logic yearlySalary = monthlySalary * 12; return yearlySalary; } ``` The above code checks the validity of the parameters before calculating the annual salary, and there are also some other logic, such as calculating taxes. However, we found that the logic of parameter checking (if statement) occupies a large number of lines of code, and it repeats in many places, resulting in Duplicate code duplication and verbosity. At this point, method extraction can be considered to optimize the code. We can extract the logic of parameter checking as an independent method, as follows: ```java void validateInputs(String name, double monthlySalary) { if (monthlySalary < 0) { throw new IllegalArgumentException("Monthly salary cannot be negative."); } If (name==null | | name. isEmpty()){ throw new IllegalArgumentException("Name cannot be null or empty."); } } ``` Then, call the newly extracted method before calling the original method, as shown in the following example: ```java double calculateYearlySalary(String name, double monthlySalary) { validateInputs(name, monthlySalary); double yearlySalary; //Assuming there is other logic yearlySalary = monthlySalary * 12; return yearlySalary; } ``` By using the extraction method, we make the original method more concise and clear, and by extracting the logic of parameter checking into an independent method, we can improve the readability and maintainability of the code. At the same time, the extracted methods can also be reused elsewhere, avoiding duplicate code writing. Summary: Method extraction and refactoring can be used when reusing a segment of the same or similar code logic. By extracting an independent method to achieve code reuse and logical organization, the readability, maintainability, and reusability of the code can be improved.

In Java, abstract classes and interfaces are powerful tools for Code refactoring. Below is a simple example to illustrate when such refactoring is necessary and the code after refactoring. Imagine a car manufacturing company that produces various types of cars. The initial code is as follows: ```java public class Car { private String brand; private String model; private int year; public Car(String brand, String model, int year) { this.brand = brand; this.model = model; this.year = year; } public void drive() { System.out.println("Driving " + brand + " " + model); } } public class Suv extends Car { private boolean offRoad; public Suv(String brand, String model, int year, boolean offRoad) { super(brand, model, year); this.offRoad = offRoad; } public void drive() { System.out.println("Driving " + brand + " " + model + " off-road"); } } public class Sedan extends Car { public Sedan(String brand, String model, int year) { super(brand, model, year); } } public class Main { public static void main(String[] args) { Car car1 = new Car("Toyota", "Camry", 2022); car1.drive(); Suv suv1 = new Suv("Ford", "Explorer", 2022, true); suv1.drive(); Sedan sedan1 = new Sedan("Honda", "Accord", 2022); sedan1.drive(); } } ``` In the above code, the features and behaviors of the car type are placed in the 'Car' class, while the 'SUV' and 'Sedan' classes only inherit the 'Car' class without adding additional features or behaviors. In this case, abstract classes and interfaces can be used to refactor the code. Firstly, create an abstract class' Vehicle 'that contains features and behaviors common to all vehicles. Abstract classes can define abstract methods (without concrete implementation) and concrete methods (with concrete implementation). Abstract classes cannot be directly instantiated and can only be inherited. ```java public abstract class Vehicle { protected String brand; protected String model; protected int year; public Vehicle(String brand, String model, int year) { this.brand = brand; this.model = model; this.year = year; } public abstract void drive(); } ``` Next, change the 'Car' class to inherit from the 'Vehicle' class, and also change the 'SUV' and 'Sedan' classes to inherit from the 'Vehicle' class. ```java public class Car extends Vehicle { public Car(String brand, String model, int year) { super(brand, model, year); } @Override public void drive() { System.out.println("Driving " + brand + " " + model); } } public class Suv extends Vehicle { private boolean offRoad; public Suv(String brand, String model, int year, boolean offRoad) { super(brand, model, year); this.offRoad = offRoad; } @Override public void drive() { System.out.println("Driving " + brand + " " + model + " off-road"); } } public class Sedan extends Vehicle { public Sedan(String brand, String model, int year) { super(brand, model, year); } @Override public void drive() { System.out.println("Driving " + brand + " " + model); } } ``` Finally, modify the code in the 'Main' class to use the refactored class. ```java public class Main { public static void main(String[] args) { Vehicle car1 = new Car("Toyota", "Camry", 2022); car1.drive(); Vehicle suv1 = new Suv("Ford", "Explorer", 2022, true); suv1.drive(); Vehicle sedan1 = new Sedan("Honda", "Accord", 2022); sedan1.drive(); } } ``` The refactored code extracts common features and behaviors into the abstract class' Vehicle 'and implements specific behaviors for different vehicle types through inheritance. The advantage of doing this is that the code is clearer and more maintainable. When a new vehicle type needs to be added, simply create a new subclass to inherit from the 'Vehicle' class and implement specific behavior.

There are several situations where variable and method names need to be refactored: 1. Variable names and method names do not comply with naming conventions: The naming convention can be determined based on the company's coding conventions, usually using the hump naming method. If variable and method names are all uppercase or lowercase, special characters or abbreviations are used, refactoring is necessary to comply with naming conventions. Code before refactoring: ```java String UFN// Incorrect naming convention, using all uppercase Public void setd() {//Incorrect naming convention, using abbreviations \t// code logic } ``` Refactored code: ```java String userName// Correct naming conventions Public void setDate() {//Correct naming convention \t// code logic } ``` 2. Variable and method names should not be descriptive: variable and method names should clearly express their purpose and meaning, making it easy to understand the function and meaning of the code. If variable and method names are too simple or too abstract, it will make it difficult for others to read and maintain the code. Code before refactoring: ```java Int a// Lack of descriptive variable names Public void func (String p) {//Lack of descriptive method names \t// code logic } ``` Refactored code: ```java Int userAge// Descriptive variable names Public void deleteUser (String userId) {//Descriptive method name \t// code logic } ``` 3. Variable names and method names may have the same name or are prone to confusion: If there are variables or methods with the same name within the same scope, or if the names are too similar and are prone to confusion, refactoring is needed to avoid naming conflicts. Code before refactoring: ```java int count; public void getCount() { \t// code logic } ``` Refactored code: ```java Int totalCount// Rename to avoid duplicate names with method names Public void fetchCount() {//Rename to avoid duplicate names with variable names \t// code logic } ``` By modifying variable and method names, the readability and maintainability of the code can be increased, making it easier to understand and use.

Decomposing classification is a refactoring technique that aims to split a large and complex class into multiple small and simple classes, each responsible for a specific responsibility or function. Doing so can improve the readability, maintainability, and scalability of the code. The following is an example code that requires the use of split class refactoring: ```java public class Order { private String orderId; private String customerName; private String customerEmail; private List<Product> products; private double total; // constructor, getters, setters, etc. public void processOrder() { //Processing Order Logic } public void sendConfirmationEmail() { //Send confirmation email logic } public void calculateTotal() { //Logic for calculating total order amount } } ``` In this example code, the Order class takes on too much responsibility, responsible for both the order processing logic and sending confirmation emails and calculating the total order amount. This leads to the Order class becoming large and complex, which does not comply with the Single Responsibility Principle (SRP). To improve this issue, the Order class can be divided into three subcategories: ```java public class Order { private String orderId; private String customerName; private String customerEmail; private List<Product> products; // constructor, getters, setters, etc. } public class OrderProcessor { public void processOrder(Order order) { //Processing Order Logic } } public class OrderMailer { public void sendConfirmationEmail(Order order) { //Send confirmation email logic } } public class OrderCalculator { public double calculateTotal(Order order) { //Logic for calculating total order amount } } ``` By splitting and categorizing, each class is only responsible for specific functions, making the code clearer and more maintainable. The Order class only retains the attributes of the order, while the specific processing, email sending, and total calculation logic have been moved to the corresponding split class. The code after splitting and refactoring can be more easily modified and extended for functionality. For example, if you need to change the order processing logic or send confirmation email logic, you only need to modify the corresponding sorting without affecting other functions. At the same time, it also improves the Testability of the code, because the function of each class is single, it is more convenient to conduct unit testing. In short, classification is a refactoring technique that helps improve code readability, maintainability, and scalability, suitable for situations where classes become large and complex, and take on too much responsibility.

In Java, merging classes is a refactoring technique that merges associated classes into a larger class, typically used to eliminate situations of over refinement or duplicate work. The following is an example code to illustrate when merging class refactoring is necessary and the refactored code: ```java //Code before merging class User { private String firstName; private String lastName; public User(String firstName, String lastName) { this.firstName = firstName; this.lastName = lastName; } public String getFirstName() { return firstName; } public String getLastName() { return lastName; } } class Order { private String orderNumber; private double amount; private User user; public Order(String orderNumber, double amount, User user) { this.orderNumber = orderNumber; this.amount = amount; this.user = user; } public String getOrderNumber() { return orderNumber; } public double getAmount() { return amount; } public User getUser() { return user; } } public class Main { public static void main(String[] args) { User user = new User("John", "Doe"); Order order = new Order("12345", 100.00, user); System.out.println("Order " + order.getOrderNumber() + " belongs to " + order.getUser().getFirstName() + " " + order.getUser().getLastName()); } } ``` In the above code, the 'User' and 'Order' classes have a close correlation, and the 'Order' class relies on the 'User' class to provide information related to the user. However, this correlation may lead to duplicate code and logical dispersion. By merging these two classes, we can add the 'getFirstName()' and 'getLastName()' methods to the 'Order' class and directly access user information without passing through the 'User' object. The following is the code after merging and refactoring the class: ```java //Merged Code class Order { private String orderNumber; private double amount; private String firstName; private String lastName; public Order(String orderNumber, double amount, String firstName, String lastName) { this.orderNumber = orderNumber; this.amount = amount; this.firstName = firstName; this.lastName = lastName; } public String getOrderNumber() { return orderNumber; } public double getAmount() { return amount; } public String getFirstName() { return firstName; } public String getLastName() { return lastName; } } public class Main { public static void main(String[] args) { Order order = new Order("12345", 100.00, "John", "Doe"); System.out.println("Order " + order.getOrderNumber() + " belongs to " + order.getFirstName() + " " + order.getLastName()); } } ``` In the refactored code, the 'Order' class contains all user related information, which can improve the readability and maintainability of the code and eliminate dependencies on the 'User' class. The situation where classes need to be merged usually involves having a close association between two classes, but this association does not need to remain independent, or one class needs to use members of another class and does not want to be accessed through objects, thereby increasing the complexity of the code. It should be noted that merging classes is not an unconditionally applicable refactoring technique and must be evaluated based on specific circumstances. In some scenarios, maintaining class independence and low coupling may be more important, while merging classes may cause code to become cluttered or difficult to maintain. Therefore, before conducting any refactoring, the advantages and disadvantages of various options should be carefully evaluated and weighed.

Refactoring logical expressions can make the code more concise and readable, with the following common situations: 1. Multiple conditional statements: When multiple conditional statements appear in the code, the readability of the code can be improved by merging or simplifying conditional expressions. For example, the original code is as follows: ``` if (a > 10 && b < 5 && c == 0 && d != 10) { // do something } ``` The reconstructed code is as follows: ``` if (a > 10 && b < 5 && c == 0 && d != 10) { // do something } ``` By refactoring the code, you can see all the conditions more clearly and the code is more concise. 2. Nested logical expression: When there is nesting in a logical expression, you can simplify the code by using the extraction method or Logical connective. For example, the original code is as follows: ```java If (a>5&&b<10) | | (c==0&&d!=5)){ // do something } ``` The reconstructed code is as follows: ```java boolean condition1 = a > 5 && b < 10; boolean condition2 = c == 0 && d != 5; If (condition1 | | condition2){ // do something } ``` By refactoring the code, the logical meaning can be better expressed through variable names, making the code easier to read. 3. Repeated logical judgment: When there are repeated logical judgments in the code, the code can be simplified by using the extraction method or Logical connective. For example, the original code is as follows: ```java If (a>10 | | b>5){ // do something } If (a>10 | | b>5){ // do something else } ``` The reconstructed code is as follows: ```java Boolean condition=a>10 | | b>5; if (condition) { // do something } if (condition) { // do something else } ``` By refactoring the code, repetitive logical judgments can be avoided and the maintainability of the code can be improved. Refactoring logical expressions can make code more concise, readable, and improve its maintainability and comprehensibility.

The reconstruction of conditional statements can be carried out in the following situations: 1. Repetitive conditional logic: If the same conditional statement is used in multiple places in the code, it can be considered to be extracted as a method or function. This can reduce the repeatability of the code and improve its readability and maintainability. ```java //Before refactoring if (condition) { // do something // ... } //After refactoring if (hasConditionTrue()) { // do something // ... } //Move to another method or class public boolean hasConditionTrue() { return condition; } ``` 2. Complex conditional judgment: If the conditional statement is too complex to understand and test, you can consider splitting it into multiple independent conditions or methods. This can make the code clearer, more readable, and write unit tests that are easier to test under different conditions. ```java //Before refactoring If (condition1&&(condition2 | | condition3)&&! Condition4){ // do something // ... } //After refactoring if (isConditionTrue()) { // do something // ... } //Move to another method or class public boolean isConditionTrue() { Return condition1&&(condition2 | | condition3)&&! Condition4; } ``` 3. A large number of nested conditions: If too many nested conditional statements result in code structure confusion, poor readability, and an increased likelihood of errors, simplification can be considered. Split complex conditional logic into multiple independent conditional judgments, and use techniques such as early return and early judgment to make the logic of the code more intuitive and concise. ```java //Before refactoring if (condition1) { if (condition2) { if (condition3) { // do something // ... } } } //After refactoring if (!condition1) { return; } if (!condition2) { return; } if (!condition3) { return; } // do something // ... ``` The above examples are all for the reconstruction of conditional statements to improve the readability, maintainability and Testability of the code. By extracting repetitive logic into methods or functions, breaking down complex conditional judgments, and simplifying nested conditions, the code can be made simpler, easier to understand, and modify.

Reuse code refers to the process of abstracting existing code fragments into independent functional units that can be used in multiple places to achieve code reuse. The following is a sample code that explains when Code refactoring is required and the refactored code. Example code: ```java public class Calculator { public int add(int a, int b) { return a + b; } public int subtract(int a, int b) { return a - b; } public int multiply(int a, int b) { return a * b; } public int divide(int a, int b) { if(b == 0) { Throw new IllegalArgumentException ("divisor cannot be 0"); } return a / b; } } public class Main { public static void main(String[] args) { Calculator calculator = new Calculator(); int result1 = calculator.add(5, 3); System. out. println (result1)// Output: 8 int result2 = calculator.subtract(5, 3); System. out. println (result2)// Output: 2 int result3 = calculator.multiply(5, 3); System. out. println (result3)// Output: 15 int result4 = calculator.divide(10, 2); System. out. println (result4)// Output: 5 } } ``` In the above example code, the Calculator class implements basic mathematical operations, and then performs different mathematical operations by creating a Calculator object in the Main class and calling its different methods. In the current implementation, a new Calculator object needs to be created every time mathematical operations are required, which leads to code duplication. If we need to perform mathematical operations in multiple places, we need to repeat the task of creating objects and calling methods in each place. To solve this problem, we can put the code for creating and calling the Calculator object into a public method, and pass the Calculator object as a parameter, which can achieve code reuse. Refactored code: ```java public class Calculator { public int add(int a, int b) { return a + b; } public int subtract(int a, int b) { return a - b; } public int multiply(int a, int b) { return a * b; } public int divide(int a, int b) { if(b == 0) { Throw new IllegalArgumentException ("divisor cannot be 0"); } return a / b; } } public class MathUtil { public static int calculate(Calculator calculator, int a, int b) { return calculator.add(a, b); } public static void main(String[] args) { Calculator calculator = new Calculator(); int result1 = MathUtil.calculate(calculator, 5, 3); System. out. println (result1)// Output: 8 int result2 = MathUtil.calculate(calculator, 5, 3); System. out. println (result2)// Output: 2 int result3 = MathUtil.calculate(calculator, 5, 3); System. out. println (result3)// Output: 15 int result4 = MathUtil.calculate(calculator, 10, 2); System. out. println (result4)// Output: 5 } } ``` In the reconstructed code, we created a MathUtil class, where the calculate method receives a Calculator object and two parameters, and then calls the add method of the Calculator object for mathematical operations. In this way, we can directly call the calculate method of the MathUtil class in different places to perform mathematical operations, avoiding the work of repeatedly creating objects and calling methods.