src/main/java/org/jfree/graphics2d/svg/SVGGraphics2D.java
private void appendOptionalElementIDFromHint(StringBuilder sb) {
        String elementID = (String) this.hints.get(SVGHints.KEY_ELEMENT_ID);
        if (elementID != null) {
            this.hints.put(SVGHints.KEY_ELEMENT_ID, null); // clear it
            if (this.elementIDs.contains(elementID)) {
                throw new IllegalStateException("The element id " 
                        + elementID + " is already used.");
            } else {
                this.elementIDs.add(elementID);
            }
            this.sb.append("id=\"").append(elementID).append("\" ");
        }
    }
private void appendOptionalElementIDFromHint(StringBuilder sb) {
        String elementID = (String) this.hints.get(SVGHints.KEY_ELEMENT_ID);
        if (elementID != null) {
            this.hints.put(SVGHints.KEY_ELEMENT_ID, null); // clear it
            if (this.elementIDs.contains(elementID)) {
                throw new IllegalStateException("The element id " 
                        + elementID + " is already used.");
            } else {
                this.elementIDs.add(elementID);
            }
            this.sb.append("id=\"").append(elementID).append("\" ");
        }
    }
A utility method that appends an optional element id if one is
specified via the rendering hints.

@param sb  the string builder ({@code null} not permitted).