Java 建造者模式詳解：分步構建複雜對象，導演者與建造者分離

模式動機

當對象構建過程**復雜**、**步驟固定但細節可變**、或需**多種表示**時，將構建邏輯從業務代碼中分離。

角色結構

• Product：最終產品 (如 Computer)
• Builder (抽象建造者)：聲明構建步驟 buildCpu(), buildRam()... 及 getResult()
• ConcreteBuilder：具體實現步驟，維護半成品狀態
• Director (導演者)：**掌握構建算法/順序**，調用 Builder 步驟，屏蔽客戶端細節

代碼實現

// 產品
class Computer {
    String cpu, ram, storage, gpu;
    @Override public String toString() {
        return "Computer{cpu=" + cpu + ", ram=" + ram + ", storage=" + storage + ", gpu=" + gpu + "}";
    }
}

// 抽象建造者
interface Builder {
    void buildCpu();
    void buildRam();
    void buildStorage();
    void buildGpu();
    Computer getResult();
}

// 具體建造者：高端電腦
class HighEndBuilder implements Builder {
    private Computer c = new Computer();
    public void buildCpu() { c.cpu = "i9-13900K"; }
    public void buildRam() { c.ram = "64GB DDR5"; }
    public void buildStorage() { c.storage = "2TB NVMe"; }
    public void buildGpu() { c.gpu = "RTX 4090"; }
    public Computer getResult() { return c; }
}

// 導演者
class Director {
    private Builder builder;
    public Director(Builder builder) { this.builder = builder; }
    public void construct() {
        builder.buildCpu();
        builder.buildRam();
        builder.buildStorage();
        builder.buildGpu();
    }
}

// 客戶端
public class BuilderDemo {
    public static void main(String[] args) {
        Builder builder = new HighEndBuilder();
        Director director = new Director(builder);
        director.construct();
        Computer pc = builder.getResult();
        System.out.println(pc);
    }
}

與工廠模式區別

實戰應用

• StringBuilder / StringBuffer (內部建造者)
• JavaMail MimeMessage 構建
• HTTP 請求構建器 (OkHttp Request.Builder)
• MyBatis SqlSessionFactoryBuilder

現代寫法：鏈式調用 + Lambda (無導演者)

Computer pc = new Computer.Builder()
    .cpu("i7-13700K")
    .ram("32GB")
    .storage("1TB")
    .gpu("RTX 4070")
    .build();

此為 **Builder 模式變體**，省去 Director，適合步驟固定、可選參數多的場景。