@Override
    public String toString() {
            final StringBuilder buf = new StringBuilder();
            buf.append("[");
            if (maxAge >= 0) {
                buf.append("max-age=").append(maxAge).append(",");
            }
            if (sharedMaxAge >= 0) {
                buf.append("shared-max-age=").append(sharedMaxAge).append(",");
            }
            if (noCache) {
                buf.append("no-cache").append(",");
            }
            if (noStore) {
                buf.append("no-store").append(",");
            }
            if (cachePrivate) {
                buf.append("private").append(",");
            }
            if (cachePublic) {
                buf.append("public").append(",");
            }
            if (mustRevalidate) {
                buf.append("must-revalidate").append(",");
            }
            if (proxyRevalidate) {
                buf.append("proxy-revalidate").append(",");
            }
            if (staleWhileRevalidate >= 0) {
                buf.append("state-while-revalidate=").append(staleWhileRevalidate).append(",");
            }
            if (staleIfError >= 0) {
                buf.append("stale-if-error").append(staleIfError).append(",");
            }
            if (mustUnderstand) {
                buf.append("must-understand").append(",");
            }
            if (immutable) {
                buf.append("immutable").append(",");
            }
            if (buf.charAt(buf.length() - 1) == ',') {
                buf.setLength(buf.length() - 1);
            }
            buf.append("]");
            return buf.toString();
    }
Returns the 'immutable' Cache-Control directive status.

@return true if the 'immutable' directive is present in the Cache-Control header.
httpclient5-cache/src/main/java/org/apache/hc/client5/http/cache/ResponseCacheControl.java