Java 建造者模式：鏈式調用構建複雜對象、不可變對象、驗證邏輯

模式定義

將複雜對象的建構與其表示分離，使相同建構過程可創建不同表示。適用於：參數多、可選參數多、需不可變對象、建構邏輯複雜 (驗證、預設值)。

經典實現：鏈式 Builder

public class Race {
    private final String name;
    private final String skinColor;
    private final String sex;

    private Race(Builder builder) {
        this.name = builder.name;
        this.skinColor = builder.skinColor;
        this.sex = builder.sex;
    }

    public static class Builder {
        private String name;
        private String skinColor;
        private String sex;

        public Builder name(String name) { this.name = name; return this; }
        public Builder skinColor(String skinColor) { this.skinColor = skinColor; return this; }
        public Builder sex(String sex) { this.sex = sex; return this; }

        public Race build() {
            // 驗證邏輯
            if (name == null || name.isEmpty()) throw new IllegalStateException("名稱必填");
            if (skinColor == null) skinColor = "黃色"; // 預設值
            if (sex == null) sex = "男";
            return new Race(this);
        }
    }

    // Getter...
    public String getName() { return name; }
    // ...
}

// 客戶端
Race race = new Race.Builder()
        .name("張三")
        .sex("男")
        .skinColor("白色")
        .build();

關鍵設計要點

進階：階段式 Builder (強制順序)

public class HttpRequest {
    private final String method, url, body;
    private final Map headers;

    private HttpRequest(Stage3 builder) { ... }

    // 階段 1: 必須設定 method
    public static class Stage1 { public Stage2 method(String m) { return new Stage2(m); } }
    // 階段 2: 必須設定 url
    public static class Stage2 { private final String method; public Stage3 url(String u) { return new Stage3(method, u); } }
    // 階段 3: 可選 headers/body
    public static class Stage3 { ... public HttpRequest build() { ... } }
}

// 強制順序調用
HttpRequest req = new HttpRequest.Stage1().method("POST").url("/api").build();

Lombok `@Builder` 自動生成

@Builder
@Value // 不可變
public class Race {
    String name;
    @Builder.Default String skinColor = "黃色";
    @Builder.Default String sex = "男";
}

編譯期生成完整 Builder，支援 `@Builder.Default`、 `@Singular` (集合添加)、`@Builder.ObtainVia` (工廠方法)。

對比：工廠模式 vs 建造者模式