建造者模式

意图

将复杂对象的构造步骤与表示分离，使同样的构建过程可以创建不同表示。适合参数多、可选参数多、构造易出错的场景。

类图

Java 示例（流式建造者）

public class HttpRequest {
    private final String method;
    private final String url;
    private final String body;

    private HttpRequest(Builder b) {
        this.method = b.method;
        this.url = b.url;
        this.body = b.body;
    }

    public static class Builder {
        private String method = "GET";
        private String url;
        private String body = "";

        public Builder method(String m) { this.method = m; return this; }
        public Builder url(String u) { this.url = u; return this; }
        public Builder body(String b) { this.body = b; return this; }
        public HttpRequest build() {
            if (url == null || url.isEmpty()) throw new IllegalStateException("url");
            return new HttpRequest(this);
        }
    }
}

// 使用
HttpRequest req = new HttpRequest.Builder()
    .method("POST")
    .url("https://api.example.com")
    .body("{}")
    .build();

JDK 中 StringBuilder、Stream.Builder 也体现建造思想。

下一节：原型模式