Java 實現 hashCode 方法

原文 [url]http://www.javapractices.com/topic/TopicAction.do;jsessionid=8D0BB2D9061334756DC17A10A4FB5F71?Id=28[/url]

Implementing hashCode :

* if a class overrides equals, it must override hashCode
* when they are both overridden, equals and hashCode must use the same set of fields
* if two objects are equal, then their hashCode values must be equal as well
* if the object is immutable, then hashCode is a candidate for caching and lazy initialization

It is a popular misconception that hashCode provides a unique identifier for an object. It does not.

[b]Example [/b]

The following utility class allows simple construction of an effective hashCode method. It is based on the recommendations of Effective Java, by Joshua Bloch.

import java.lang.reflect.Array;

/**
* Collected methods which allow easy implementation of <code>hashCode</code>.
*
* Example use case:
* <pre>
*  public int hashCode(){
*    int result = HashCodeUtil.SEED;
*    //collect the contributions of various fields
*    result = HashCodeUtil.hash(result, fPrimitive);
*    result = HashCodeUtil.hash(result, fObject);
*    result = HashCodeUtil.hash(result, fArray);
*    return result;
*  }
* </pre>
*/
public final class HashCodeUtil {

  /**
  * An initial value for a <code>hashCode</code>, to which is added contributions
  * from fields. Using a non-zero value decreases collisons of <code>hashCode</code>
  * values.
  */
  public static final int SEED = 23;

  /**
  * booleans.
  */
  public static int hash( int aSeed, boolean aBoolean ) {
    System.out.println("boolean...");
    return firstTerm( aSeed ) + ( aBoolean ? 1 : 0 );
  }

  /**
  * chars.
  */
  public static int hash( int aSeed, char aChar ) {
    System.out.println("char...");
    return firstTerm( aSeed ) + (int)aChar;
  }

  /**
  * ints.
  */
  public static int hash( int aSeed , int aInt ) {
    /*
    * Implementation Note
    * Note that byte and short are handled by this method, through
    * implicit conversion.
    */
    System.out.println("int...");
    return firstTerm( aSeed ) + aInt;
  }

  /**
  * longs.
  */
  public static int hash( int aSeed , long aLong ) {
    System.out.println("long...");
    return firstTerm(aSeed)  + (int)( aLong ^ (aLong >>> 32) );
  }

  /**
  * floats.
  */
  public static int hash( int aSeed , float aFloat ) {
    return hash( aSeed, Float.floatToIntBits(aFloat) );
  }

  /**
  * doubles.
  */
  public static int hash( int aSeed , double aDouble ) {
    return hash( aSeed, Double.doubleToLongBits(aDouble) );
  }

  /**
  * <code>aObject</code> is a possibly-null object field, and possibly an array.
  *
  * If <code>aObject</code> is an array, then each element may be a primitive
  * or a possibly-null object.
  */
  public static int hash( int aSeed , Object aObject ) {
    int result = aSeed;
    if ( aObject == null) {
      result = hash(result, 0);
    }
    else if ( ! isArray(aObject) ) {
      result = hash(result, aObject.hashCode());
    }
    else {
      int length = Array.getLength(aObject);
      for ( int idx = 0; idx < length; ++idx ) {
        Object item = Array.get(aObject, idx);
        //recursive call!
        result = hash(result, item);
      }
    }
    return result;
  }


  /// PRIVATE ///
  private static final int fODD_PRIME_NUMBER = 37;

  private static int firstTerm( int aSeed ){
    return fODD_PRIME_NUMBER * aSeed;
  }

  private static boolean isArray(Object aObject){
    return aObject.getClass().isArray();
  }
} 

Here is an example of its use. When debugging statements are uncommented in HashCodeUtil, the reuse of the boolean, char, int and long versions of hash is demonstrated :


boolean...
char...
int...
long...
long...
int...
int...
int...
int...
int...
hashCode value: -608077094

import java.util.*;

public final class ApartmentBuilding {

  public ApartmentBuilding (
    boolean aIsDecrepit,
    char aRating,
    int aNumApartments,
    long aNumTenants,
    double aPowerUsage,
    float aWaterUsage,
    byte aNumFloors,
    String aName,
    List aOptions,
    Date[] aMaintenanceChecks
  ){
    fIsDecrepit = aIsDecrepit;
    fRating = aRating;
    fNumApartments = aNumApartments;
    fNumTenants = aNumTenants;
    fPowerUsage = aPowerUsage;
    fWaterUsage = aWaterUsage;
    fNumFloors = aNumFloors;
    fName = aName;
    fOptions = aOptions;
    fMaintenanceChecks = aMaintenanceChecks;
  }

  @Override public boolean equals(Object that) {
    if ( this == that ) return true;
    if ( !(that instanceof ApartmentBuilding) ) return false;
    ApartmentBuilding thatBuilding = (ApartmentBuilding)that;
    return hasEqualState(thatBuilding);
  }

  @Override public int hashCode() {
    //this style of lazy initialization is 
    //suitable only if the object is immutable
    if ( fHashCode == 0) {
      int result = HashCodeUtil.SEED;
      result = HashCodeUtil.hash( result, fIsDecrepit );
      result = HashCodeUtil.hash( result, fRating );
      result = HashCodeUtil.hash( result, fNumApartments );
      result = HashCodeUtil.hash( result, fNumTenants );
      result = HashCodeUtil.hash( result, fPowerUsage );
      result = HashCodeUtil.hash( result, fWaterUsage );
      result = HashCodeUtil.hash( result, fNumFloors );
      result = HashCodeUtil.hash( result, fName );
      result = HashCodeUtil.hash( result, fOptions );
      result = HashCodeUtil.hash( result, fMaintenanceChecks );
      fHashCode = result;
    }
    return fHashCode;
  }

  //..other methods elided

  // PRIVATE ////

  /**
  * The following fields are chosen to exercise most of the different
  * cases.
  */
  private boolean fIsDecrepit;
  private char fRating;
  private int fNumApartments;
  private long fNumTenants;
  private double fPowerUsage;
  private float fWaterUsage;
  private byte fNumFloors;
  private String fName; //possibly null, say
  private List fOptions; //never null
  private Date[] fMaintenanceChecks; //never null
  private int fHashCode;

  /**
  * Here, for two ApartmentBuildings to be equal, all fields must be equal.
  */
  private boolean hasEqualState( ApartmentBuilding that ) {
    //note the different treatment for possibly-null fields
    return
      ( this.fName==null ? that.fName==null : this.fName.equals(that.fName) ) &&
      ( this.fIsDecrepit == that.fIsDecrepit )&&
      ( this.fRating == that.fRating )&&
      ( this.fNumApartments == that.fNumApartments ) &&
      ( this.fNumTenants == that.fNumTenants ) &&
      ( this.fPowerUsage == that.fPowerUsage ) &&
      ( this.fWaterUsage ==  that.fWaterUsage ) &&
      ( this.fNumFloors == that.fNumFloors ) &&
      ( this.fOptions.equals(that.fOptions) )&&
      ( Arrays.equals(this.fMaintenanceChecks, that.fMaintenanceChecks) );
  }

  /**
  * Exercise hashcode.
  */
  public static void main (String [] aArguments) {
    List options = new ArrayList();
    options.add("pool");
    Date[] maintenanceDates = new Date[1];
    maintenanceDates[0] = new Date();
    byte numFloors = 8;

    ApartmentBuilding building = new ApartmentBuilding (
      false,
      'B',
      12,
      396L,
      5.2,
      6.3f,
      numFloors,
      "Palisades",
      options,
      maintenanceDates
    );

    System.out.println("hashCode value: " + building.hashCode());
  }
}