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, "qq.com", 0, "120", "广州");
    }

    public User(String id, String name, String email) {
        this(id, name, email, 0, "120", "广州");
    }

    public User(String id, String name, String email, int age) {
        this(id, name, email, age, "120", "广州");
    }

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

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

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getEmail() {
        return email;
    }

    public int getAge() {
        return age;
    }

    public String getPhoneNumber() {
        return phoneNumber;
    }

    public String getAddress() {
        return address;
    }
}

注：许多你不想设置的参数，但是还不得不为他们传递值

通常解决办法一： JavaBean模式（调用一个无参构造器来创造对象，然后调用setter方法来设置每个必须的参数，以及每个相关的可选参数）

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() {
        super();
    }

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

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

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

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

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

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

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getEmail() {
        return email;
    }

    public int getAge() {
        return age;
    }

    public String getPhoneNumber() {
        return phoneNumber;
    }

    public String getAddress() {
        return address;
    }

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

注：JavaBeans模式自身有着很严重的缺点。因为构造过程被分到几个调用中，在构造过程中JavaBean可能处于非一致的状态。JavaBeans模式阻止了把类做成不可变的可能，这就需要确保他的线程安全。

解决办法三： 构建器

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 static class Builder{

        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 Builder(String id, String name) {
            super();
            this.id = id;
            this.name = name;
        }

        public Builder email(String email){
            this.email = email;
            return this;
        }
        public Builder age(int age){
            this.age = age;
            return this;
        }
        public Builder phoneNumber(String phoneNumber){
            this.phoneNumber = phoneNumber;
            return this;
        }

        public Builder address(String address){
            this.address = address;
            return this;
        }
        public User builder(){
            return new User(this);
        }
    }
    private User(Builder builder){
        this.id = builder.id;
        this.name = builder.name;
        this.email = builder.email;
        this.age = builder.age;
        this.phoneNumber = builder.phoneNumber;
        this.address = builder.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 String getName() {
        return name;
    }

    public String getEmail() {
        return email;
    }

    public int getAge() {
        return age;
    }

    public String getPhoneNumber() {
        return phoneNumber;
    }

    public String getAddress() {
        return address;
    }
}

注：不直接生成想要的对象，而是让客户端利用所有必要的参数调用构造器（或者静态工厂），得到一个builder对象。然后客户端在builder对象上调用类似于setter的方法，来设置每个相关的可选参数。最后，客户端调用无参的build方法来生成不可变的对象。

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());
    }