Java 构建器

假如我们的一个实体类有很多的属性值，但是这些属性值又是可选的。如果我们遇到这样的是类，如何设计出方便的实体类呢？

通常解决办法一： 重叠构造器

public class User {

    private String id;                 // id(必填)

    private String name;             // 用户名(必填)

    private String email;             // 邮箱(可选)

    private int age;                 // 年龄(可选)

    private String phoneNumber;     // 电话(可选)

    private String address;         // 地址(可选)

    public User(String id, String name) {
        this(id, name, null, 0, null, null);
    }

    public User(String id, String name, String email) {
        this(id, name, email, 0, null, null);
    }

    public User(String id, String name, String email, int age) {
        this(id, name, email, age, null, null);
    }

    public User(String id, String name, String email, int age, String phoneNumber) {
        this(id, name, email, age, phoneNumber, null);
    }

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

    @Override
    public String toString() {
        return "User [id=" + id + ", name=" + name + ", email=" + email + ", age=" + age + ", phoneNumber="
                + phoneNumber + ", address=" + address + "]";
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

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

    public String getEmail() {
        return email;
    }

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

    public int getAge() {
        return age;
    }

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

    public String getPhoneNumber() {
        return phoneNumber;
    }

    public void setPhoneNumber(String phoneNumber) {
        this.phoneNumber = phoneNumber;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

}

注：重叠构造器模式可行，但是当有很多参数的时候，客户端代码会很难编写，并且难以阅读

通常解决办法二：JavaBean模式

public class User {

    private String id;                 // id(必填)

    private String name;             // 用户名(必填)

    private String email;             // 邮箱(可选)

    private int age;                 // 年龄(可选)

    private String phoneNumber;     // 电话(可选)

    private String address;         // 地址(可选)

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

    public User() {
        super();
    }

    @Override
    public String toString() {
        return "User [id=" + id + ", name=" + name + ", email=" + email + ", age=" + age + ", phoneNumber="
                + phoneNumber + ", address=" + address + "]";
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

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

    public String getEmail() {
        return email;
    }

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

    public int getAge() {
        return age;
    }

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

    public String getPhoneNumber() {
        return phoneNumber;
    }

    public void setPhoneNumber(String phoneNumber) {
        this.phoneNumber = phoneNumber;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

}

注：JavaBean存在很多缺点，构造过程被分到几个中调用，在多线程很难保证线程安全。

构建器解决

public class NutritionFacts {

    private int servingSize;

    private int servings;

    private int calorizs;

    private int fat;

    private int soduim;

    private int carbophydrate;

    public static class Builder {

        private int servingSize;

        private int servings;

        private int calorizs = 0;

        private int fat = 0;

        private int soduim = 0;

        private int carbophydrate = 0;

        public Builder(int servingSize, int servings) {
            super();
            this.servingSize = servingSize;
            this.servings = servings;
        }

        public Builder colorizes(int val) {
            calorizs = val;
            return this;
        }

        public Builder fat(int val) {
            fat = val;
            return this;
        }

        public Builder soduim(int val) {
            soduim = val;
            return this;
        }

        public Builder carbophydrate(int val) {
            carbophydrate = val;
            return this;
        }

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

    private NutritionFacts(Builder builder) {
        servingSize = builder.servingSize;
        servings = builder.servings;
        calorizs = builder.calorizs;
        fat = builder.fat;
        soduim = builder.soduim;
        carbophydrate = builder.carbophydrate;
    }

    public int getServingSize() {
        return servingSize;
    }

    public int getServings() {
        return servings;
    }

    public int getCalorizs() {
        return calorizs;
    }

    public int getFat() {
        return fat;
    }

    public int getSoduim() {
        return soduim;
    }

    public int getCarbophydrate() {
        return carbophydrate;
    }

    @Override
    public String toString() {
        return "NutritionFacts [servingSize=" + servingSize + ", servings=" + servings + ", calorizs=" + calorizs
                + ", fat=" + fat + ", soduim=" + soduim + ", carbophydrate=" + carbophydrate + "]";
    }

}

测试代码

    public static void main( String[] args )
    {
        User user = new User.Builder(UUID.randomUUID().toString(), "parry").address("广州").builder();
        System.out.println(user.toString());
    }