Effective Java 16章「継承よりコンポジションを選ぶ」 - Qiita

public class InstrumentedHashSet<E> extends HashSet<E> { private int addCount = 0; public InstrumentedHashSet() { } public InstrumentedHashSet(int initCap, float loadFactor) { super(initCap, loadFactor); } @Override public boolean add(E e) { addCount++; return super.add(e); } @Override public boolean addAll(Collection<? extends <E> c) { addCount += c.size(); return super.addAll(c); } public int ge