java構建器模式

     原文出自:http://www.cnblogs.com/parryyang/p/5696916.html

  靜態工廠和構造器有個共同的侷限性：它們都不能很好地擴展到大量的可選參數。假如我們的一個實體類有很多的屬性值，但是這些屬性值又是可選的。如果我們遇到這樣的是類，如何設計出方便的實體類呢？

通常解決辦法一： 重疊構造器

public class User {

    private String id; // id(必填)

    private String name; // 用戶名(必填)

    private String email; // 郵箱(可選)

    private int age; // 年齡(可選)

    private String phoneNumber; // 電話(可選)

    private String address; // 地址(可選)

    public User(String id, String name) {
        this(id, name, "qq.com", 0, "120", "廣州");
    }

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

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

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

    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：許多你不想設置的參數，但是還不得不爲他們傳遞值

通常解決辦法二： JavaBean模式（調用一個無參構造器來創造對象，然後調用setter方法來設置每個必須的參數，以及每個相關的可選參數）

public class User {

    private String id; // id(必填)

    private String name; // 用戶名(必填)

    private String email; // 郵箱(可選)

    private int age; // 年齡(可選)

    private String phoneNumber; // 電話(可選)

    private String address; // 地址(可選)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模式自身有着很嚴重的缺點。因爲構造過程被分到幾個調用中，在構造過程中JavaBean可能處於非一致的狀態。JavaBeans模式阻止了把類做成不可變的可能，這就需要確保他的線程安全。

解決辦法三： 構建器

public class User {

    private String id;                 // id(必填)

    private String name;             // 用戶名(必填)

    private String email;             // 郵箱(可選)

    private int age;                 // 年齡(可選)

    private String phoneNumber;     // 電話(可選)

    private String address;         // 地址(可選)


    public static class Builder{
        
        private String id;                 // id(必填)

        private String name;             // 用戶名(必填)

        private String email;             // 郵箱(可選)

        private int age;                 // 年齡(可選)

        private String phoneNumber;     // 電話(可選)

        private String address;         // 地址(可選)

        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：不直接生成想要的對象，而是讓客戶端利用所有必要的參數調用構造器（或者靜態工廠），得到一個builder對象。然後客戶端在builder對象上調用類似於setter的方法，來設置每個相關的可選參數。最後，客戶端調用無參的build方法來生成不可變的對象。

測試代碼

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