Java Refactoring: Techniques and Best Practices with Examples

What Is Java Code Refactoring?

Java code refactoring is the process of restructuring or reorganizing existing Java code without changing its external behavior. The primary goal of refactoring is to improve the code's design, structure, readability, maintainability, and efficiency, making it easier for developers to understand and work with the codebase.

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Refactoring involves a series of small, incremental changes to the code, such as renaming variables or methods, extracting code blocks into separate methods, simplifying conditional statements, or replacing inheritance with composition. These changes can help reduce code duplication, eliminate dead or unused code, and make the code more modular and extensible.

Java Refactoring Benefits 

Refactoring Java code offers numerous benefits to developers and the overall development process. Some of the key benefits include:

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• Improved code readability: Refactoring makes the code easier to read and understand by using meaningful names, breaking down complex methods into smaller, focused ones, and organizing the code structure.
• Easier maintenance: A well-structured and organized codebase is easier to maintain, debug, and modify. Refactoring helps to identify and eliminate code smells, making it easier for developers to find and fix issues.
• Enhanced code quality: Refactoring can lead to better overall code quality by removing dead or unused code, reducing code duplication, and improving the code's design and structure.
• Better performance: Refactoring can help optimize the code, leading to improved performance and efficiency. This is achieved by eliminating unnecessary operations, simplifying algorithms, and using more efficient data structures.
• Increased reusability: Refactoring promotes code reusability by extracting common functionality into separate methods or classes that can be used across different parts of the application.
• Facilitates extensibility: Refactoring makes it easier to extend or modify the codebase as requirements change. By using design patterns and principles like SOLID, the code becomes more modular, flexible, and adaptable to changes.
• Encourages collaboration: A well-organized and structured codebase is easier for team members to collaborate on, as they can more quickly understand and navigate the code.
• Reduces technical debt: Regular refactoring helps reduce technical debt by addressing code smells and design issues early on, preventing them from accumulating and becoming more challenging to resolve later.
• Shortens development time: In the long run, refactoring can shorten the development time by making it easier for developers to understand and work with the code, resulting in fewer bugs and faster feature development.
• Ensures consistency: Refactoring helps maintain consistency across the codebase by adhering to coding standards and best practices, making it easier for developers to switch between different parts of the project.

Java Refactoring Techniques

Refactoring Java code can be approached in various ways, with two prominent paradigms being the object-oriented (OO) approach and the Functional approach. Each approach has its own set of principles and techniques to improve code quality and maintainability.

Refactoring Code Using an OO Approach

The OO approach is based on the principles of object-oriented programming (OOP), where code is organized around objects and their interactions. This approach focuses on encapsulating data and behavior within classes and using inheritance, abstraction, and polymorphism to promote code reusability and modularity. 

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Key techniques for refactoring Java code using the OO approach include:

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• Encapsulate fields: Restrict direct access to class fields by making them private and providing public getter and setter methods.
• Extract class: Divide a large class into smaller, more focused classes, each handling a specific responsibility.
• Extract method: Break down long or complex methods into smaller, more manageable methods, each handling a single responsibility.
• Pull up method / push down method: Move a method to a superclass or a subclass, respectively, to better represent the inheritance hierarchy and minimize code duplication.
• Replace inheritance with delegation: Replace a subclass with a separate class that delegates responsibilities to the original class, promoting composition over inheritance.

Refactoring Code Using Functional Approach

The functional approach is based on the principles of functional programming (FP), where code is organized around functions and immutable data. This approach promotes the use of pure functions (functions without side effects) and higher-order functions to create modular and expressive code. 

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Key techniques for refactoring Java code using the functional approach include:

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• Replace loops with Stream API: The Stream API provides a functional approach to data processing, offering a more readable and compact syntax than traditional loops. It allows operations like filtering, mapping, or reducing on collections, and supports parallel processing.
• Use Lambda expressions: Lambda expressions are a succinct way to create anonymous functions, often used to replace anonymous inner classes. They simplify the syntax of your code and are often used as arguments for Stream API methods like filter, map, and reduce.
• Immutable data structures: Immutable data means it can't change once created, promoting predictability and simplicity in your code. In Java, you can create immutable data structures using the final keyword, the Collections.unmodifiableList() method, or libraries like Google's Guava.
• Extract function: This technique involves simplifying complex functions into smaller, more manageable ones, each handling a single responsibility. This improves readability, reusability, and testability of your code.
• Higher-order functions: These are functions that take other functions as arguments or return functions as results. They allow flexible and expressive function customization, and in Java, you can create them using the Function interface, or its specialized forms like UnaryOperator or BiFunction.

Java Refactoring Techniques with Examples 

Naming

When using names, it is important to ensure that each variable, function, or class’s name says something about its purpose. It is best to avoid mental maps and prefixes and keep things simple—one word for each concept, with nouns for classes and verbs for functions. Ideally, you should use computer science terminology. The main objective of naming is to convey meaning clearly and concisely while avoiding misinformation.

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For example, when naming a public method or class variables, you should use the following:

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public Date modifiedDate;

public List<Testcase> findAllTestcasesByUser(User user){};

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Sample code that users above implementation:

import java.util.Date;

import java.util.List;

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public class AdminUsers {

    private Date modifiedDate;

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    public List<TestCases> findAllTestcasesByUser(User user) {

        // Implementation to fetch and return test cases

        return null;

    }

    private class User {

private String name;

public User(String name){

this.name = name;

}

public String getName() { return this.name; }

public void setName(String name) { this.name = name; }

}

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    private class TestCases {

        // Properties of the TestCases class

        private int id;

        private String name;

        private Date dateCreated;

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        // Constructor for the TestCases class

        public TestCases(int id, String name, Date dateCreated) {

            this.id = id;

            this.name = name;

            this.dateCreated = dateCreated;

        }

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        // Getters and setters for the properties of the TestCases class

        public int getId() {

            return id;

        }

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        public void setId(int id) {

            this.id = id;

        }

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        public String getName() {

            return name;

        }

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        public void setName(String name) {

            this.name = name;

        }

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        public Date getDateCreated() {

            return dateCreated;

        }

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        public void setDateCreated(Date dateCreated) {

            this.dateCreated = dateCreated;

        }

    }

}

Don’t use something like this:

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public Date d; // modified date

public List<Testcase> find(User user){}; //find all the Testcases by named User

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You don’t need to add a prefix, such as pUser (parameters + user)—user by itself is fine.

Another important element is consistency. For example, use saveUser(){}; saveAccount(){} rather than createUser(){}; saveAccount(){}. 

Another tip is to use proper words and avoid abbreviations. For instance, DAYS_IN_YEAR and HOURS_IN_WEEK are better than daysInYear and hoursInWeek. Variables with all capital alphabets are normally used for constants in the code.

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Key takeaways: 

• Clear and concise naming is crucial in programming. 
• Variables, functions, or classes should be named based on their purpose. 
• Avoid mental maps and prefixes and use computer science terminology when possible. 
• Naming should convey meaning without misinformation. 
• Consistency in naming is also important. For instance, use 'save' consistently in method names like saveUser() and saveAccount() rather than mixing 'create' and 'save'. 
• Use proper words and avoid abbreviations. For constants, use all capital alphabets.

Methods 

Each method should have one clear purpose (do one thing and do it well). Avoid having multiple levels of abstraction in one method. A method should be easy to read and arranged from top to bottom—place local variables near where they will be used. 

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For example, you should use separate functions for related but distinct tasks:

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  public void saveIncome(Income income){

    incomeReposiroty.save(income);

  }

   public void saveExpense(Expense expense){

    expenseRepository.save(expense);

  }

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The following sample Java class incorporates the above methods in the prescribed manner:

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public class UserFinance {

    // Properties to store user data

    private User user;

    // Method to save a user

    public void saveUser(User user) {

        this.user = user;

    }

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    // Method to save an income

    public void saveIncome(Income income) {

       System.out.println("Income Saved");

    }

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    // Method to save an expense

    public void saveExpense(Expense expense) {

        System.out.println("Expense Saved");

    }

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    // Internal class for storing information about an income

    public static class Income {

        private String source;

        private double amount;

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        public Income(String source, double amount) {

            this.source = source;

            this.amount = amount;

        }       

    }

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    // Internal class for storing information about an expense

    public static class Expense {

        private String category;

        private double amount;

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        public Expense(String category, double amount) {

            this.category = category;

            this.amount = amount;

        }        

    }

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    // Internal class for storing information about a user

    public static class User {

        private String name;

        private int age;

        private String email;

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        public User(String name, int age, String email) {

            this.name = name;

            this.age = age;

            this.email = email;

        }        

    }

}

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Don’t pack everything into one method:

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  public void saveCashFlow(Income income, Expense expense){

    incomeReposiroty.save(income);

    expenseRepository.save(expense);

  }

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Arrange functions in logical order: 

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  public void bulkUpdateUser(List<User> users){

    // get Users from repos

    // update Users one at a time

    // save User List in repo

  };

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Key takeaways:

• Each method should have a single clear purpose. 
• Avoid multiple levels of abstraction in one method. 
• A method should be easy to read and arranged logically. 
• Separate functions should be used for related but distinct tasks. 
• Do not pack everything into one method. 
• Functions should be arranged in a logical order.

Error Handling

When handling errors, it’s better to use exceptions than to return code (i.e., error code or flags). Error messages and exceptions should have adequate context. 

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Here is an example of the right way to handle errors:

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  public User saveUser(User user){

      try {

        loadUser(user.getId());

        throw new UserExistsException(user.getId())

      }

      catch (UserNotFoundException exception){

//Handle UserNotFoundException

      }

      resolveGroup(user);

      return userRepository.save(user);

  }

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The sample code below illustrates this point in more detail:

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public class User {

    private String name;

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    // Constructor for the User class

    public User(String name) {

        this.name = name;

    }

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    private User loadUser(User user) {

//Simulate failure by returning null

return null;

}

    // Method to save a user

    public void saveUser(User user) throws UserNotFoundException {

        // Check if the user exists

        if (loadUser(user) == null) {

            throw new UserNotFoundException("User not found");

        }

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        // Save the user

        // ...

    }

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    // Nested class for UserNotFoundException

    public static class UserNotFoundException extends Exception {

        public UserNotFoundException(String message) {

            super(message);

        }

    }

}

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Avoid using the following: 

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  public Object saveUser(User user){

      if(loadUser(user.getId()) instanceof User){

        // user exists

        return USER_EXISTS; // error flag

        return 101; // error code

      };

      resolveGroup(user);

      return userRepository.save(user);

  }

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Here is an example of how to provide context for an error:

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  public void validateOwner(String userId){

    if(userRepository.countTestcasesForUser(userId) == 0){

      throw new ValidationException("user: " + userId + " has no test cases. Test cases should be populated...")

    }

  }

This is important because it might be harder to understand what went wrong without the context:

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    public void validateUser(String userId){

    if(userRepository.countTestcasesForUser(userrId) == 0){

      throw new ValidationException("no valid user...")

    }

  }

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  public User saveUser(User user){

      if(loadUser(user.getId()) instanceof User){

        // the user exists

  return User

      };

      resolveGroup(user);

      return userRepository.save(user);

  }

  // pass null

    public User editUser(String userId, User user){

      if(userId == null){

        // new user

        throw new UserException("the user cannot be edited..."); 

      };

      return userRepository.update(user);

  }

}

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Key takeaways:

• Exceptions are preferable for error handling rather than return codes. 
• Error messages and exceptions should have enough context to understand the issue. 
• Provide detailed context when throwing exceptions rather than vague messages. 
• Avoid returning error flags or codes. 
• Validation should be meaningful and specific to the context.