org.apache.commons.lang.builder
Class HashCodeBuilder

java.lang.Object
  |
  +--org.apache.commons.lang.builder.HashCodeBuilder

public class HashCodeBuilder

extends Object

HashCode generation routines.

This class enables a good hashcode to be built for any class. It follows the rules laid out in the book Effective Java, by Joshua Bloch. Writing a good hashCode is actually quite difficult. This class aims to simplify the process.

All relevant fields from the object should be included in the hashCode. Derived fields may be excluded. In general, any field used in the equals method must be used in the hashCode method.

To use this class write code as follows:

 public class Person {
   String name;
   int age;
   boolean isSmoker;
   ...
 
   public int hashCode() {
     // you pick a hard-coded, randomly chosen, non-zero, odd number
     // ideally different for each class
     return new HashCodeBuilder(17, 37).   
       append(name).
       append(age).
       append(smoker).
       toHashCode();
   }
 }
 

Alternatively, there is a method that uses reflection to determine the fields to test. Because these fields are usually private, the method, reflectionHashCode, uses Field.setAccessible to change the visibility of the fields. This will fail under a security manager, unless the appropriate permissions are set. It is also slower than testing explicitly.

A typical invocation for this method would look like:

 public boolean hashCode(Object o) {
   return HashCodeBuilder.reflectionHashCode(this);
 }
 

Version:

$Id: HashCodeBuilder.java,v 1.4 2002/10/01 20:02:08 stevencaswell Exp $

Author:

Stephen Colebourne

Field Summary

private int	iConstant Constant to use in building the hashCode
private int	iTotal Running total of the hashCode

Constructor Summary

HashCodeBuilder()
Constructor for HashCodeBuilder.

HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber)
Constructor for HashCodeBuilder.

Method Summary

HashCodeBuilder	append(boolean value) Append a hashCode for a long.
HashCodeBuilder	append(boolean[] array) Append a hashCode for a boolean array.
HashCodeBuilder	append(byte value) Append a hashCode for a byte.
HashCodeBuilder	append(byte[] array) Append a hashCode for a byte array.
HashCodeBuilder	append(char value) Append a hashCode for a char.
HashCodeBuilder	append(char[] array) Append a hashCode for a char array.
HashCodeBuilder	append(double value) Append a hashCode for a double.
HashCodeBuilder	append(double[] array) Append a hashCode for a double array.
HashCodeBuilder	append(float value) Append a hashCode for a float.
HashCodeBuilder	append(float[] array) Append a hashCode for a float array.
HashCodeBuilder	append(int value) Append a hashCode for an int.
HashCodeBuilder	append(int[] array) Append a hashCode for an int array.
HashCodeBuilder	append(long value) Append a hashCode for a long.
HashCodeBuilder	append(long[] array) Append a hashCode for a long array.
HashCodeBuilder	append(Object object) Append a hashCode for an Object.
HashCodeBuilder	append(Object[] array) Append a hashCode for an Object array.
HashCodeBuilder	append(short value) Append a hashCode for a short.
HashCodeBuilder	append(short[] array) Append a hashCode for a short array.
static int	reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object) This method uses reflection to build a valid hash code.
static int	reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients) This method uses reflection to build a valid hash code.
static int	reflectionHashCode(Object object) This method uses reflection to build a valid hash code.
static int	reflectionHashCode(Object object, boolean testTransients) This method uses reflection to build a valid hash code.
int	toHashCode() Return the computed hashCode

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

iConstant

private final int iConstant

Constant to use in building the hashCode

iTotal

private int iTotal

Running total of the hashCode

Constructor Detail

HashCodeBuilder

public HashCodeBuilder()

Constructor for HashCodeBuilder. This constructor uses two hard coded choices for the constants needed to build a hashCode.

HashCodeBuilder

public HashCodeBuilder(int initialNonZeroOddNumber,
                       int multiplierNonZeroOddNumber)

Constructor for HashCodeBuilder. Two randomly chosen, non-zero, odd numbers must be passed in. Ideally these should be different for each class, however this is not vital. Prime numbers are preferred, especially for the multiplier.

Parameters:

initialNonZeroOddNumber - a non-zero, odd number used as the initial value

multiplierNonZeroOddNumber - a non-zero, odd number used as the multiplier

Throws:

IllegalArgumentException - if the number is zero or even

Method Detail

reflectionHashCode

public static int reflectionHashCode(Object object)

This method uses reflection to build a valid hash code.

It uses Field.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manger, if the permissions are not set up. It is also not as efficient as testing explicitly. Transient members will be not be used, as they are likely derived fields, and not part of the value of the object. Static fields will not be tested. This constructor uses two hard coded choices for the constants needed to build a hash code.

Parameters:

object - the object to create a hash code for

Returns:

int hash code

Throws:

IllegalArgumentException - if the object is null

reflectionHashCode

public static int reflectionHashCode(Object object,
                                     boolean testTransients)

This method uses reflection to build a valid hash code.

It uses Field.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manger, if the permissions are not set up. It is also not as efficient as testing explicitly. If the TestTransients parameter is set to true, transient members will be tested, otherwise they are ignored, as they are likely derived fields, and not part of the value of the object. Static fields will not be tested. This constructor uses two hard coded choices for the constants needed to build a hash code.

Parameters:

object - the object to create a hash code for

testTransients - whether to include transient fields

Returns:

int hash code

Throws:

IllegalArgumentException - if the object is null

reflectionHashCode

public static int reflectionHashCode(int initialNonZeroOddNumber,
                                     int multiplierNonZeroOddNumber,
                                     Object object)

This method uses reflection to build a valid hash code.

It uses Field.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manger, if the permissions are not set up. It is also not as efficient as testing explicitly. Transient members will be not be used, as they are likely derived fields, and not part of the value of the object. Static fields will not be tested.

Two randomly chosen, non-zero, odd numbers must be passed in. Ideally these should be different for each class, however this is not vital. Prime numbers are preferred, especially for the multiplier.

Parameters:

initialNonZeroOddNumber - a non-zero, odd number used as the initial value

multiplierNonZeroOddNumber - a non-zero, odd number used as the multiplier

object - the object to create a hash code for

Returns:

int hash code

Throws:

IllegalArgumentException - if the object is null

IllegalArgumentException - if the number is zero or even

reflectionHashCode

public static int reflectionHashCode(int initialNonZeroOddNumber,
                                     int multiplierNonZeroOddNumber,
                                     Object object,
                                     boolean testTransients)

This method uses reflection to build a valid hash code.

It uses Field.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manger, if the permissions are not set up. It is also not as efficient as testing explicitly. If the TestTransients parameter is set to true, transient members will be tested, otherwise they are ignored, as they are likely derived fields, and not part of the value of the object. Static fields will not be tested.

Two randomly chosen, non-zero, odd numbers must be passed in. Ideally these should be different for each class, however this is not vital. Prime numbers are preferred, especially for the multiplier.

Parameters:

initialNonZeroOddNumber -

multiplierNonZeroOddNumber -

object - the object to create a hash code for

testTransients - whether to include transient fields

Returns:

int hash code

Throws:

IllegalArgumentException - if the object is null

IllegalArgumentException - if the number is zero or even

append

public HashCodeBuilder append(Object object)

Append a hashCode for an Object.

Parameters:

object - the object to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(long value)

Append a hashCode for a long.

Parameters:

value - the long to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(int value)

Append a hashCode for an int.

Parameters:

value - the int to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(short value)

Append a hashCode for a short.

Parameters:

value - the short to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(char value)

Append a hashCode for a char.

Parameters:

value - the char to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(byte value)

Append a hashCode for a byte.

Parameters:

value - the byte to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(double value)

Append a hashCode for a double.

Parameters:

value - the double to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(float value)

Append a hashCode for a float.

Parameters:

value - the float to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(boolean value)

Append a hashCode for a long.

Parameters:

value - the long to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(Object[] array)

Append a hashCode for an Object array.

Parameters:

array - the array to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(long[] array)

Append a hashCode for a long array.

Parameters:

array - the array to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(int[] array)

Append a hashCode for an int array.

Parameters:

array - the array to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(short[] array)

Append a hashCode for a short array.

Parameters:

array - the array to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(char[] array)

Append a hashCode for a char array.

Parameters:

array - the array to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(byte[] array)

Append a hashCode for a byte array.

Parameters:

array - the array to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(double[] array)

Append a hashCode for a double array.

Parameters:

array - the array to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(float[] array)

Append a hashCode for a float array.

Parameters:

array - the array to add to the hashCode

Returns:

this

append

public HashCodeBuilder append(boolean[] array)

Append a hashCode for a boolean array.

Parameters:

array - the array to add to the hashCode

Returns:

this

toHashCode

public int toHashCode()

Return the computed hashCode

Returns:

int hashCode based on the fields appended