// first version, might run out of memory
public class String {

  private Map<String,String> m = new HashMap<String,String>();

  public String intern() {
    String entry = m.get(this);
    if (entry == null) {
      m.put(this,this);
      entry = this;
    }
    return entry;
  }
  // ... more methods, etc.
}


// second version, safe against out of memory situations
// but is eager to clear cache, recommended when
// recomputation is cheap, uses WeakReference
public class String {

  private WeakHashMap<String,WeakReference<String>> m = new WeakHashMap<String,WeakReference<String>>();

  public String intern() {
    WeakReference<String> entry = m.get(this);
    if ((entry == null) || (entry.get() == null)) {
      entry = new WeakReference(this);
      m.put(this,entry);
    } 
    return entry.get();
  }
  // ... more methods, etc.
}


// recomputation is expensive, use SoftReference
public class String {

  private WeakHashMap<String,SoftReference<String>> m = new WeakHashMap<String,SoftReference<String>>();

  public String intern() {
    SoftReference<String> entry = m.get(this);
    if ((entry == null) || (entry.get() == null)) {
      entry = new SoftReference(this);
      m.put(this,entry);
    } 
    return entry.get();
  }
  // ... more methods, etc.
}