Java模式-三種工廠模式

簡單工廠

簡單工廠模式又叫靜態(tài)工廠方法模式（Static FactoryMethod Pattern）,是通過專門定義一個(gè)類來負(fù)責(zé)創(chuàng)建其他類的實(shí)例筛欢，被創(chuàng)建的實(shí)例通常都具有共同的父類田藐。
我們從一個(gè)實(shí)例展開
現(xiàn)在有一道面試題：使用java實(shí)現(xiàn)一個(gè)計(jì)算機(jī)控制臺(tái)程序，要求輸入數(shù)的運(yùn)算呕诉，得到結(jié)果。
這道題目最原始的寫法：

public class Computer {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println("請(qǐng)輸入第一個(gè)數(shù)字：");
        float firstNum  = in.nextFloat();
        System.out.println("請(qǐng)輸入第二個(gè)數(shù)字：");
        float secondNum  = in.nextFloat();
        System.out.println("請(qǐng)輸入運(yùn)算符號(hào)：");
        String countQuato = in.next();
        if("+".equals(countQuato)){
            System.out.println("result : "+(firstNum+secondNum));
        }else if("-".equals(countQuato)){
            System.out.println("result : "+(firstNum-secondNum));
        }else if("*".equals(countQuato)){
            System.out.println("result : "+(firstNum*secondNum));
        }else if("/".equals(countQuato)){
            System.out.println("result : "+(firstNum/secondNum));
        }
    }

上面的寫法實(shí)現(xiàn)雖然簡單，但是卻沒有面向?qū)ο蟮奶匦云季ǎa拓展性差味咳，顯然不是出題者想要考察的意圖庇勃。
那么面向?qū)ο缶幊桃绾卧陬}中體現(xiàn)呢？
在面向?qū)ο缶幊陶Z言中莺葫，一切都是對(duì)象匪凉，所以上面運(yùn)算符號(hào)也應(yīng)當(dāng)作對(duì)象來處理。
我們首先建立一個(gè)接口

public abstract class Operation {

    public abstract float getResult(float firstNumber, float secondNumber);

}
//把符號(hào)都當(dāng)做對(duì)象處理捺檬，實(shí)現(xiàn)此接口
public class AddOperation extends Operation {
    @Override
    public float getResult(float firstNumber, float secondNumber) {
        return firstNumber+secondNumber;
    }

}
public class SubOperation extends Operation {
    @Override
    public float getResult(float firstNumber, float secondNumber) {
        return firstNumber-secondNumber;
    }
}
public class MulOperation extends Operation {
    @Override
    public float getResult(float firstNumber, float secondNumber) {
        return firstNumber*secondNumber;
    }
}
public class DivOperation extends Operation {
    @Override
    public float getResult(float firstNumber, float secondNumber) {
        return firstNumber/secondNumber;
    }
}

//接下來需要解決的就是對(duì)象的創(chuàng)建問題了再层，既如何根據(jù)不同的情況創(chuàng)建不同的對(duì)象：我們正好可以通過簡單工廠模式實(shí)現(xiàn)
public class OperationFactory {

    public static Operation getOperation(String quotaFlag){
        Operation o = null;
        switch (quotaFlag){
            case "+" :  o = new AddOperation();
            case "-" :  o = new SubOperation();
            case "*" :  o = new MulOperation();
            case "/" :  o = new DivOperation();
            default:break;
        }
        return o;
    }
}
//調(diào)用：
public class Computer {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println("請(qǐng)輸入第一個(gè)數(shù)字：");
        float firstNum  = in.nextFloat();
        System.out.println("請(qǐng)輸入第二個(gè)數(shù)字：");
        float secondNum  = in.nextFloat();
        System.out.println("請(qǐng)輸入運(yùn)算符號(hào)：");
        String countQuato = in.next();
        System.out.println(count(firstNum,secondNum,countQuato));
    }
    private static float count(float firstNum,float secondNum , String countQuota){
    //通過工廠類獲取對(duì)象
        Operation operation = OperationFactory.getOperation(countQuota);
        return operation.getResult(firstNum,secondNum);
    }
}

簡單工廠將對(duì)象的創(chuàng)建過程進(jìn)行了封裝，用戶不需要知道具體的創(chuàng)建過程堡纬，只需要調(diào)用工廠類獲取對(duì)象即可聂受。

這種簡單工廠的寫法是通過switch-case來判斷對(duì)象創(chuàng)建過程的。在實(shí)際使用過程中烤镐，違背了開放-關(guān)閉原則蛋济，當(dāng)然有些情況下可以通過反射調(diào)用來彌補(bǔ)這種不足。

工廠方法

工廠方法定義一個(gè)用于創(chuàng)建對(duì)象的接口炮叶，讓子類決定實(shí)例化哪一個(gè)類碗旅，工廠方法使得一個(gè)類的實(shí)例化延遲到了子類
工廠方法在簡單工廠的基礎(chǔ)上再包了一層工廠，所有的工廠都是此工廠的子類镜悉。而產(chǎn)生對(duì)象的類型由子類工廠決定祟辟。使用工廠方法來實(shí)現(xiàn)上面的加減乘除對(duì)象的創(chuàng)建

//定義上級(jí)工廠的接口
public interface IFractory {
    public Operation generateOper();
}
//為每一個(gè)類創(chuàng)建工廠
/**
 * 工廠方法  為每個(gè)對(duì)象生成一個(gè)工廠類
 */
public class AddOperationFactory implements IFractory{

    @Override
    public Operation generateOper() {
        return new AddOperation();
    }
}
public class SubOperationFactory implements IFractory {
    @Override
    public Operation generateOper() {
        return new SubOperation();
    }
}
public class MulOperationFactory implements IFractory {
    @Override
    public Operation generateOper() {
        return new MulOperation();
    }
}
public class DivOperationFactory implements IFractory {
    @Override
    public Operation generateOper() {
        return new DivOperation();
    }
}
//客戶端代碼
IFractory fractory = new AddOperationFactory();
Operation operation = fractory.generateOper();
operation.getResult(firstNum,secondNum);

工廠方法將類的實(shí)例化推遲到了其子類。所以使用工廠方法模式時(shí)侣肄，需要客戶端決定實(shí)例化哪一個(gè)工廠類旧困。選擇判斷問題還是存在的。也就是說，工廠方法把簡單的工廠內(nèi)部邏輯判斷轉(zhuǎn)移到了客戶端來運(yùn)行吼具。你想要加的功能僚纷，本來是要改工廠類的，而現(xiàn)在是修改客戶端拗盒。不過怖竭，我們?cè)谀承┣闆r下通過工廠方法，只需要修改一行實(shí)例化的代碼就可以實(shí)現(xiàn)系統(tǒng)元素的切換(比如切換數(shù)據(jù)源)锣咒。這也是很方便的侵状。

抽象工廠

提供一個(gè)創(chuàng)建一系列相關(guān)相互依賴對(duì)象的接口，而無需指定他們具體的類毅整。抽象工廠為不同產(chǎn)品族的對(duì)象創(chuàng)建提供接口趣兄。
使用場景：系統(tǒng)需要在不同產(chǎn)品族進(jìn)行切換
代碼實(shí)現(xiàn)：

public interface IFacfory {
    public IUser createUser();
    public IDepartment createDepartment();
}
public interface IUser {
    public void insert();
    public void getById();
}
public interface IDepartment {
    public void insert();
    public void getDepartmentById();
}
public class SqlServerUser implements IUser {
    @Override
    public void insert() {
        System.out.println("insert into sqlserver.");
    }

    @Override
    public void getById() {
        System.out.println("get user by id from sqlserver.");
    }
}
public class SqlServerDepartment implements IDepartment {
    @Override
    public void insert() {
        System.out.println("insert department into sqlserver.");
    }

    @Override
    public void getDepartmentById() {
        System.out.println("get department in sqlserver by id.");
    }
}

public class AccessUser implements IUser {
    @Override
    public void insert() {
        System.out.println("insert into access");
    }

    @Override
    public void getById() {
        System.out.println("get by id from access");
    }
}
public class AccessDepartment implements IDepartment {
    @Override
    public void insert() {
        System.out.println("insert department into sqlserver.");
    }

    @Override
    public void getDepartmentById() {
        System.out.println("get department in sqlserver by id.");
    }
}

//不同產(chǎn)品組使用一個(gè)工廠
public class SqlServerFactory implements IFacfory {
    @Override
    public IUser createUser() {
        return new SqlServerUser();
    }

    @Override
    public IDepartment createDepartment() {
        return new SqlServerDepartment();
    }
}
public class AccessFactory implements IFacfory {
    @Override
    public IUser createUser() {
        return new AccessUser();
    }

    @Override
    public IDepartment createDepartment() {
        return new AccessDepartment();
    }
}
客戶端：
IFacfory facfory = new AccessFactory();
IUser user = facfory.createUser();
IDepartment department = facfory.createDepartment();
user.insert();
user.getById();
department.insert();
department.getDepartmentById();

抽象工廠最大的好處就是便于交換產(chǎn)品系列，具體工廠在代碼中一般只出現(xiàn)一次悼嫉。這就使得改變應(yīng)用的具體工廠很容易艇潭。
第二個(gè)好處是他能讓具體的創(chuàng)建對(duì)象實(shí)例和客戶端分離，客戶端是通過他們的抽象接口操作實(shí)例
抽象工廠不太易于拓展戏蔑，如果需要自增功能蹋凝，或者自增產(chǎn)品，則需要至少修改三個(gè)類总棵，而且實(shí)例化的代碼是寫死在程序中的鳍寂，這樣無法避免違背開放-關(guān)閉原則。
對(duì)于上述問題情龄，可以通過配置文件迄汛，結(jié)合反射的方式來解決。在這里就不再啰嗦