Re: Force implementation of equals() and hashCode()?

From:
Tom Anderson <twic@urchin.earth.li>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 2 Oct 2008 13:32:05 +0100
Message-ID:
<Pine.LNX.4.64.0810021322190.31397@urchin.earth.li>
On Wed, 1 Oct 2008, Chris wrote:

Is there any way to force a class that implements an interface to
implement both equals() and hashCode()?


No.

I want all classes that implement a certain interface to work properly
in a HashMap. This means they need to implement their own version of
equals() and hashCode(). Trouble is, if I add those methods to the
interface, the system doesn't squawk if the class doesn't implement
them, because it just inherits them from Object.


interface ProperlyHashable {
  public int properHashCode() ;
  public boolean properEquals(Object obj) ;
}

class ProperWrapper<T extends ProperlyHashable> {
  public final T proper ;
  public ProperWrapper(T proper) {
  this.proper = proper ;
  }
  public int hashCode() {
  return proper.properHashCode() ;
  }
  public boolean equals(Object that) {
  if (!that instanceof ProperWrapper) return false ;
  return this.proper.properEquals(((ProperWrapper)that).proper) ;
  }
}

class ProperHashMap<K extends ProperlyHashable, V> extends AbstractMap<K, V> {
  private HashMap<ProperWrapper<K>, V> items = new HashMap<ProperWrapper<K>, V>() ;
  public V put(K key, V value) {
  return items.put(new ProperWrapper(key), value) ;
  }
  public V get(K key) {
  return items.get(new ProperWrapper(key)) ;
  }
  // etc
}

tom

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