public Histogram getHistogram() {
        if (linkHistogram == null) {
            linkHistogram = createHistogram0();
        }
        return linkHistogram;
    }
A class that describes a relationship between apps in application map

@author netspider
@author emeroad
@author HyunGil Jeong
web/src/main/java/com/navercorp/pinpoint/web/applicationmap/link/Link.java