如何确保构建器模式完成？

// 构建器类
public class UserBuilder {
    private String name;
    private int age;
    private String email;

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

    public UserBuilder setAge(int age) {
        this.age = age;
        return this;
    }

    public UserBuilder setEmail(String email) {
        this.email = email;
        return this;
    }

    public User build() {
        if (name == null || age == 0 || email == null) {
            throw new IllegalArgumentException("必须设置姓名、年龄和邮箱");
        }
        return new User(name, age, email);
    }
}

// 被构建的对象
public class User {
    private String name;
    private int age;
    private String email;

    public User(String name, int age, String email) {
        this.name = name;
        this.age = age;
        this.email = email;
    }

    // getters and setters
}

// 使用构建器模式创建对象
User user = new UserBuilder()
    .setName("张三")
    .setAge(25)
    .setEmail("zhangsan@example.com")
    .build();