Alternative for Collections.newSetFromMap to be used in JDK 1.5?

Question

I want to use such "Collections.newSetFromMap()" method in JDK 1.5 which doesnt support it. Also ConcurrentHashSet Class is not supported in java 5. Have to compile following line in JDK 1.5.How do i do it?

protected Set<String> knownLCWords = Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>());

Please guide me.

Answer 1

Below is the implementation of that method from the Java Collections source. Essentially, the method returns a subclass of AbstractSet that contains the Map, as well as a transient Set.

/**
 * Returns a set backed by the specified map.  The resulting set displays
 * the same ordering, concurrency, and performance characteristics as the
 * backing map.  In essence, this factory method provides a {@link Set}
 * implementation corresponding to any {@link Map} implementation.  There
 * is no need to use this method on a {@link Map} implementation that
 * already has a corresponding {@link Set} implementation (such as {@link
 * HashMap} or {@link TreeMap}).
 *
 * <p>Each method invocation on the set returned by this method results in
 * exactly one method invocation on the backing map or its <tt>keySet</tt>
 * view, with one exception.  The <tt>addAll</tt> method is implemented
 * as a sequence of <tt>put</tt> invocations on the backing map.
 *
 * <p>The specified map must be empty at the time this method is invoked,
 * and should not be accessed directly after this method returns.  These
 * conditions are ensured if the map is created empty, passed directly
 * to this method, and no reference to the map is retained, as illustrated
 * in the following code fragment:
 * <pre>
 *    Set&lt;Object&gt; weakHashSet = Collections.newSetFromMap(
 *        new WeakHashMap&lt;Object, Boolean&gt;());
 * </pre>
 *
 * @param map the backing map
 * @return the set backed by the map
 * @throws IllegalArgumentException if <tt>map</tt> is not empty
 * @since 1.6
 */
public static <E> Set<E> newSetFromMap(Map<E, Boolean> map) {
    return new SetFromMap<E>(map);
}

private static class SetFromMap<E> extends AbstractSet<E>
    implements Set<E>, Serializable
{
    private final Map<E, Boolean> m;  // The backing map
    private transient Set<E> s;       // Its keySet

    SetFromMap(Map<E, Boolean> map) {
        if (!map.isEmpty())
            throw new IllegalArgumentException("Map is non-empty");
        m = map;
        s = map.keySet();
    }

    public void clear()               {        m.clear(); }
    public int size()                 { return m.size(); }
    public boolean isEmpty()          { return m.isEmpty(); }
    public boolean contains(Object o) { return m.containsKey(o); }
    public boolean remove(Object o)   { return m.remove(o) != null; }
    public boolean add(E e) { return m.put(e, Boolean.TRUE) == null; }
    public Iterator<E> iterator()     { return s.iterator(); }
    public Object[] toArray()         { return s.toArray(); }
    public <T> T[] toArray(T[] a)     { return s.toArray(a); }
    public String toString()          { return s.toString(); }
    public int hashCode()             { return s.hashCode(); }
    public boolean equals(Object o)   { return o == this || s.equals(o); }
    public boolean containsAll(Collection<?> c) {return s.containsAll(c);}
    public boolean removeAll(Collection<?> c)   {return s.removeAll(c);}
    public boolean retainAll(Collection<?> c)   {return s.retainAll(c);}
// addAll is the only inherited implementation

    private static final long serialVersionUID = 2454657854757543876L;

    private void readObject(java.io.ObjectInputStream stream)
        throws IOException, ClassNotFoundException
    {
        stream.defaultReadObject();
        s = m.keySet();
    }
}