code: public WaveformPreview requestWaveformPreviewFrom(final DataReference dataReference) {
          ensureRunning();
          for (WaveformPreview cached : previewHotCache.values()) {
              if (cached.dataReference.equals(dataReference)) {  // Found a hot cue hit, use it.
                  return cached;
              }
          }
          return requestPreviewInternal(dataReference, false);
      }
docs: Ask the specified player for the specified waveform preview from the specified media slot, first checking if we
  have a cached copy.
  
  @param dataReference uniquely identifies the desired waveform preview
  
  @return the preview, if it was found, or {@code null}
  
  @throws IllegalStateException if the WaveformFinder is not running