Favor composition over inheritance

Favor Composition over
Inheritance
Kochih Wu
Based on Item 16 from Effective Java, 2nd ed.

Use case
Count how many elements have been added to a HashSet

InstrumentedHashSet implementation
public class InstrumentedHashSet<E> extends HashSet<E> {
// The number of attempted element insertions
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 getAddCount() {
return addCount;
}
}

Let’s run it
InstrumentedHashSet<String> s = new InstrumentedHashSet<String>();
s.addAll(Arrays.asList("Snap", "Crackle", "Pop"));
assertEquals(3, s.getAddCount());

Output
java.lang.AssertionError:
Expected :3
Actual :6

Issues with inheritance
● Inheritance violates encapsulation
○ Internally HashSet.addAll() calls add()
● Reimplement addAll() using add(), instead of calling super.addAll()?
○ Not reusing code
○ Not always possible without access to private fields
○ New methods can be added to the superclass

Take 2 using composition
// 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;
}
}

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(); }
}

Let’s run it again
Set<String> s = new InstrumentedHashSet<String>(new HashSet<String>(capacity));
s.addAll(Arrays.asList("Snap", "Crackle", "Pop"));
assertEquals(3, s.getAddCount());

Composition pros & cons
● The wrapper class can be used to instrument any Set implementation
○ Also known as the Decorator pattern
● Not suitable for callback frameworks

Takeaway
● Inheritance should only be used to represent an “is-a” relationship
● Inheritance propagates flaws in the API
● Law of Demeter (or principle of least knowledge)

Favor composition over inheritance

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