Use composition in place of inheritance

From Effective java, 2nd edition:

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// Wrapper class - uses composition in place of inheritance public class InstrumentedSet<E> extends ForwardingSet<E> {     private int addCount = 0;     public InstrumentedSet(Set<E> s) {         super(s);     }     @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 getAddCount() {         return addCount;     } }

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// Reusable forwarding class public class ForwardingSet<E> implements Set<E> {     private final Set<E> s;       public ForwardingSet(Set<E> s) { this.s = s; }     public void clear() { s.clear(); }     public boolean contains(Object o) { return s.contains(o); }     public boolean isEmpty() { return s.isEmpty(); }     public int size() { return s.size(); }     public Iterator<E> iterator() { return s.iterator(); }     public boolean add(E e) { return s.add(e); }     public boolean remove(Object o) { return s.remove(o); }     public boolean containsAll(Collection<?> c)     { return s.containsAll(c); }     public boolean addAll(Collection<? extends E> c)     { return s.addAll(c); }     public boolean removeAll(Collection<?> c)     { return s.removeAll(c); }     public boolean retainAll(Collection<?> c)     { return s.retainAll(c); }     public Object[] toArray() { return s.toArray(); }     public <T> T[] toArray(T[] a) { return s.toArray(a); }     @Override public boolean equals(Object o)     { return s.equals(o); }     @Override public int hashCode() { return s.hashCode(); }     @Override public String toString() { return s.toString(); } }

1 2	// Example use Set<Date> s = new InstrumentedSet<Date>(new TreeSet<Date>(comparator));