软件设计¶

// 单例：静态内部类实现，兼顾懒加载与线程安全
public final class Config {
    private Config() {}

    private static class Holder {
        static final Config INSTANCE = new Config();
    }

    public static Config getInstance() {
        return Holder.INSTANCE;
    }
}

// 建造者：链式构造，避免超长参数列表
public final class HttpRequest {
    private final String url;
    private final Map<String, String> headers;

    private HttpRequest(Builder b) {
        this.url = b.url;
        this.headers = Map.copyOf(b.headers);
    }

    public static class Builder {
        private String url;
        private final Map<String, String> headers = new HashMap<>();

        public Builder url(String url) { this.url = url; return this; }
        public Builder header(String k, String v) { headers.put(k, v); return this; }
        public HttpRequest build() {
            Objects.requireNonNull(url, "url");
            return new HttpRequest(this);
        }
    }
}

// 装饰器：在不改动原有类的前提下叠加职责
public interface Reader {
    String read();
}

public class PlainReader implements Reader {
    public String read() { return "data"; }
}

public class CachedReader implements Reader {
    private final Reader delegate;
    public CachedReader(Reader delegate) { this.delegate = delegate; }
    public String read() { /* 先查缓存，未命中再 delegate.read() */ return delegate.read(); }
}

// 策略：把"做什么"从"怎么选"中抽离
public interface PricingStrategy {
    BigDecimal price(BigDecimal base);
}

public class VipPricing implements PricingStrategy {
    public BigDecimal price(BigDecimal base) { return base.multiply(new BigDecimal("0.8")); }
}

public class OrderService {
    private final PricingStrategy strategy;
    public OrderService(PricingStrategy strategy) { this.strategy = strategy; }
    public BigDecimal total(BigDecimal base) { return strategy.price(base); }
}