Java面向对象三大特性深度解析

Java面向对象三大特性深度解析

一、封装 (Encapsulation)

1.1 核心实现

   public class BankAccount {// 私有化字段private String accountNumber;private double balance;private String owner;// 构造方法封装初始化逻辑public BankAccount(String accountNumber, String owner) {if(accountNumber == null || accountNumber.isEmpty()) {throw new IllegalArgumentException("账号不能为空");}this.accountNumber = accountNumber;this.owner = owner;this.balance = 0.0;}// 受控的访问方法public void deposit(double amount) {if(amount <= 0) {throw new IllegalArgumentException("存款金额必须为正数");}this.balance += amount;}public void withdraw(double amount) {if(amount <= 0) {throw new IllegalArgumentException("取款金额必须为正数");}if(amount > balance) {throw new IllegalStateException("余额不足");}this.balance -= amount;}// 只读访问public double getBalance() {return balance;}}

1.2 核心价值

• 安全性：防止非法数据修改（如余额不能直接设置）
• 可维护性：内部实现变更不影响调用方（如余额存储单位从元改为分）
• 数据验证：保证对象始终处于有效状态（如取款不能超过余额）

1.3 设计模式应用

• 工厂模式：封装对象创建过程
• 建造者模式：封装复杂对象的构建过程
• 代理模式：封装实际对象的访问

二、继承 (Inheritance)

2.1 完整示例

   // 基类abstract class Shape {protected String color;public Shape(String color) {this.color = color;}// 抽象方法强制子类实现public abstract double area();public void display() {System.out.println("这是" + color + "的图形");}}// 子类继承class Circle extends Shape {private double radius;public Circle(String color, double radius) {super(color);  // 调用父类构造this.radius = radius;}@Overridepublic double area() {return Math.PI * radius * radius;}// 子类特有方法public double perimeter() {return 2 * Math.PI * radius;}}// 多层继承class Cylinder extends Circle {private double height;public Cylinder(String color, double radius, double height) {super(color, radius);this.height = height;}@Overridepublic double area() {// 重写父类方法return 2 * super.area() + perimeter() * height;}public double volume() {return super.area() * height;}}

2.2 核心价值

• 代码复用：共用父类属性和方法
• 扩展性：通过继承扩展新功能
• 多态基础：为运行时多态提供条件

2.3 设计模式应用

• 模板方法模式：父类定义算法骨架
• 装饰器模式：通过继承扩展功能
• 组合模式：处理树形结构

三、多态 (Polymorphism)

3.1 完整示例

   interface Payment {void pay(double amount);}class CreditCard implements Payment {@Overridepublic void pay(double amount) {System.out.println("信用卡支付：" + amount + "元");}}class Alipay implements Payment {@Overridepublic void pay(double amount) {System.out.println("支付宝支付：" + amount + "元");}}class PaymentProcessor {public void processPayment(Payment payment, double amount) {// 多态调用payment.pay(amount);}}// 使用示例public class Main {public static void main(String[] args) {PaymentProcessor processor = new PaymentProcessor();// 同一接口不同表现processor.processPayment(new CreditCard(), 100.0);processor.processPayment(new Alipay(), 200.0);// 运行时多态Payment payment = Math.random() > 0.5 ? new CreditCard() : new Alipay();payment.pay(300.0);}}

3.2 核心价值

• 扩展性：新增支付方式无需修改处理器代码
• 解耦：调用方只依赖抽象接口
• 灵活性：运行时动态决定具体实现

3.3 设计模式应用

• 策略模式：算法可互换
• 命令模式：将请求封装为对象
• 观察者模式：事件处理多态

四、三大特性协同应用

4.1 典型协作示例

// 封装基础public abstract class Employee {private String name;private int id;public Employee(String name, int id) {this.name = name;this.id = id;}// 封装通用行为public void work() {System.out.println(name + "开始工作");}// 多态接口public abstract void calculateSalary();}// 继承扩展class FullTimeEmployee extends Employee {private double monthlySalary;public FullTimeEmployee(String name, int id, double monthlySalary) {super(name, id);this.monthlySalary = monthlySalary;}@Overridepublic void calculateSalary() {System.out.println("固定月薪：" + monthlySalary);}}class PartTimeEmployee extends Employee {private double hourlyRate;private int hours;public PartTimeEmployee(String name, int id, double hourlyRate, int hours) {super(name, id);this.hourlyRate = hourlyRate;this.hours = hours;}@Overridepublic void calculateSalary() {System.out.println("计时工资：" + (hourlyRate * hours));}}// 使用多态class PayrollSystem {private List<Employee> employees = new ArrayList<>();public void addEmployee(Employee emp) {employees.add(emp);}public void processPayroll() {for(Employee emp : employees) {emp.work();emp.calculateSalary();  // 多态调用}}}

4.2 协同价值

1. 封装保证对象内部状态安全
2. 继承实现代码复用和层次化设计
3. 多态提供统一接口处理不同实现

4.3 综合设计模式案例

工厂方法模式：

   // 产品接口（多态）interface Logger {void log(String message);}// 具体产品（继承）class FileLogger implements Logger {@Overridepublic void log(String message) {System.out.println("文件记录：" + message);}}class DatabaseLogger implements Logger {@Overridepublic void log(String message) {System.out.println("数据库记录：" + message);}}// 工厂基类（封装创建逻辑）abstract class LoggerFactory {// 工厂方法（多态）public abstract Logger createLogger();// 封装通用逻辑public void writeLog() {Logger logger = createLogger();logger.log("测试日志");}}// 具体工厂（继承）class FileLoggerFactory extends LoggerFactory {@Overridepublic Logger createLogger() {return new FileLogger();}}class DatabaseLoggerFactory extends LoggerFactory {@Overridepublic Logger createLogger() {return new DatabaseLogger();}}