final 关键字的使用方法及应用场景解析

final double PI = 3.14159;
// PI = 3.14;  // 编译错误，不能修改final变量

final List<String> NAMES = new ArrayList<>();
NAMES.add("Alice");  // 合法，修改对象内容
// NAMES = new ArrayList<>();  // 非法，不能修改引用

public class BaseClass {
    public final void printInfo() {
        System.out.println("This is a final method.");
    }
}

public class SubClass extends BaseClass {
    // 以下代码会导致编译错误
    // @Override
    // public void printInfo() { ... }
}

public final class UtilityClass {
    public static int add(int a, int b) {
        return a + b;
    }
}

// 编译错误：无法继承final类
// public class ExtendedUtility extends UtilityClass { ... }

public final class AppConstants {
    private AppConstants() {
        // 私有构造函数，防止实例化
    }
    
    public static final int MAX_UPLOAD_SIZE = 1024 * 1024;
    public static final String DEFAULT_ENCODING = "UTF-8";
}

int size = AppConstants.MAX_UPLOAD_SIZE;

public final class StringUtils {
    private StringUtils() {
        // 私有构造函数
    }
    
    public static boolean isEmpty(String str) {
        return str == null || str.length() == 0;
    }
    
    public static String capitalize(String str) {
        if (isEmpty(str)) return str;
        return Character.toUpperCase(str.charAt(0)) + str.substring(1);
    }
}

boolean empty = StringUtils.isEmpty("test");
String capitalized = StringUtils.capitalize("hello");

public final class UserDTO {
    private final String username;
    private final int age;
    private final List<String> roles;

    public UserDTO(String username, int age, List<String> roles) {
        this.username = username;
        this.age = age;
        // 防御性复制，确保不可变性
        this.roles = new ArrayList<>(roles);
    }

    public String getUsername() {
        return username;
    }

    public int getAge() {
        return age;
    }

    public List<String> getRoles() {
        // 返回副本，防止外部修改
        return new ArrayList<>(roles);
    }
}

List<String> roles = Arrays.asList("admin", "user");
UserDTO user = new UserDTO("john", 30, roles);
// user.getRoles().add("guest");  // 不会影响内部列表

public final class PaymentProcessor {
    private final TaxCalculator calculator;

    public PaymentProcessor(TaxCalculator calculator) {
        this.calculator = calculator;
    }

    public final double processPayment(double amount) {
        // 核心业务逻辑，不允许子类修改
        double tax = calculator.calculateTax(amount);
        return amount + tax;
    }
}

PaymentProcessor processor = new PaymentProcessor(new DefaultTaxCalculator());
double total = processor.processPayment(100.0);