用java写一个建造者模式代码

好的，下面是一个用 Java 实现建造者模式的示例代码：

public class User {
    private final String firstName; // 必填项
    private final String lastName; // 必填项
    private final int age; // 可选项
    private final String phone; // 可选项

    private User(UserBuilder builder) {
        this.firstName = builder.firstName;
        this.lastName = builder.lastName;
        this.age = builder.age;
        this.phone = builder.phone;
    }

    // 省略getter方法

    // 建造者类
    public static class UserBuilder {
        private final String firstName; // 必填项
        private final String lastName; // 必填项
        private int age; // 可选项
        private String phone; // 可选项

        public UserBuilder(String firstName, String lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
        }

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

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

        public User build() {
            return new User(this);
        }
    }
}

// 调用示例
User user = new User.UserBuilder("San", "Zhang")
        .age(18)
        .phone("139xxxxxx")
        .build();

以上代码实现了一个包含必填项和可选项的实体类 `User` 和与之对应的建造者类 `UserBuilder`。在构建实体对象时，先使用建造者类设置必填项和可选项的值，最后调用 `build()` 方法构建实体对象。