/**
 * Product class: Complex configuration object 
 * Contains required and optional parameters, created via builder pattern
 */
public class Config {

    /**
     * Builder class for Config
     */
    public static class ConfigBuilder {

        /**
         * Default encoding UTF-8
         */
        private String encoding = "UTF-8";
        /**
         * Default max connections 10
         */
        private int maxConnections = 10;
        /**
         * Default timeout 3000ms
         */
        private int timeout = 3000;
        /**
         * Required parameter (passed in builder constructor)
         */
        private final String url;

        /**
         * Builder constructor: requires mandatory parameter
         * 
         * @param url    Target service URL (required)
         */
        public ConfigBuilder(String url){
            this.url = url;
        }

        /**
         * Build Config object and validate parameter legality
         * @return Immutable Config instance
         */
        public Config build(){
            // Validate parameters
            if (url == null || url.trim().isEmpty()) {
                throw new IllegalArgumentException("URL cannot be null or empty");
            }
            if (maxConnections <= 0) {
                throw new IllegalArgumentException("Max connections must be positive");
            }
            if (timeout <= 0) {
                throw new IllegalArgumentException("Timeout must be positive");
            }
            return new Config(this);
        }

        /**
         * Set encoding format (optional parameter)
         * @return Builder itself (for method chaining)
         * 
         * @param encoding    Encoding format (e.g. "GBK")
         */
        public ConfigBuilder setEncoding(String encoding){
            this.encoding = encoding;
            return this;
        }

        /**
         * Set max connections (optional parameter)
         * @return Builder itself (for method chaining)
         * 
         * @param maxConnections    Max connections
         */
        public ConfigBuilder setMaxConnections(int maxConnections){
            this.maxConnections = maxConnections;
            return this;
        }

        /**
         * Set timeout (optional parameter)
         * @return Builder itself (for method chaining)
         * 
         * @param timeout    Timeout in milliseconds
         */
        public ConfigBuilder setTimeout(int timeout){
            this.timeout = timeout;
            return this;
        }
    }

    /**
     * Optional parameter, demonstrating extensibility
     */
    private final String encoding;
    private final int maxConnections;
    /**
     * Optional parameter (has default value, can be modified via builder)
     */
    private final int timeout;
    /**
     * Required parameter (must be set, no default value)
     */
    private final String url;

    /**
     * Private constructor: only allowed to be called by internal builder
     * 
     * @param builder    Builder object providing parameters
     */
    private Config(ConfigBuilder builder){
        this.url = builder.url;
        this.encoding = builder.encoding;
        this.maxConnections = builder.maxConnections;
        this.timeout = builder.timeout;
    }

    public String getEncoding(){
        return encoding;
    }

    public int getMaxConnections(){
        return maxConnections;
    }

    public int getTimeout(){
        return timeout;
    }

    /**
     * Getter methods for all parameters (no setters to ensure immutability)
     */
    public String getUrl(){
        return url;
    }
}