Hash Code Builder
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
/**
* <code>HashCode</code> generation routines.
* <p>
* 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.
* <p>
* 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.
* <p>
* To use this class write code as follows:
* <pre>
* 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();
* }
* }
* </pre>
* <p>
* Alternatively, there is a method that uses reflection to determine
* the fields to test. Because these fields are usually private, the method,
* <code>reflectionHashCode</code>, uses <code>Field.setAccessible</code> 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.
* <p>
* A typical invocation for this method would look like:
* <pre>
* public boolean hashCode(Object o) {
* return HashCodeBuilder.reflectionHashCode(this);
* }
* </pre>
*
* @author <a href="mailto:scolebourne@joda.org">Stephen Colebourne</a>
* @version $Id: HashCodeBuilder.java 517121 2007-03-12 07:45:49Z ate $
*/
public class HashCodeBuilder
{
/**
* Constant to use in building the hashCode
*/
private final int iConstant;
/**
* Running total of the hashCode
*/
private int iTotal = 0;
/**
* Constructor for HashCodeBuilder.
* This constructor uses two hard coded choices for the constants needed
* to build a hashCode.
*/
public HashCodeBuilder()
{
super();
iConstant = 37;
iTotal = 17;
}
/**
* 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.
*
* @param initialNonZeroOddNumber a non-zero, odd number used as the initial value
* @param multiplierNonZeroOddNumber a non-zero, odd number used as the multiplier
* @throws IllegalArgumentException if the number is zero or even
*/
public HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber)
{
super();
if (initialNonZeroOddNumber == 0)
{
throw new IllegalArgumentException("HashCodeBuilder requires a non zero initial value");
}
if (initialNonZeroOddNumber % 2 == 0)
{
throw new IllegalArgumentException("HashCodeBuilder requires an odd initial value");
}
if (multiplierNonZeroOddNumber == 0)
{
throw new IllegalArgumentException("HashCodeBuilder requires a non zero multiplier");
}
if (multiplierNonZeroOddNumber % 2 == 0)
{
throw new IllegalArgumentException("HashCodeBuilder requires an odd multiplier");
}
iConstant = multiplierNonZeroOddNumber;
iTotal = initialNonZeroOddNumber;
}
//-------------------------------------------------------------------------
/**
* This method uses reflection to build a valid hash code.
* <p>
* 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.
*
* @param object the object to create a hash code for
* @return int hash code
* @throws IllegalArgumentException if the object is null
*/
public static int reflectionHashCode(Object object)
{
return reflectionHashCode(object, false);
}
/**
* This method uses reflection to build a valid hash code.
* <p>
* 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.
*
* @param object the object to create a hash code for
* @param testTransients whether to include transient fields
* @return int hash code
* @throws IllegalArgumentException if the object is null
*/
public static int reflectionHashCode(Object object, boolean testTransients)
{
return reflectionHashCode(17, 37, object, testTransients);
}
/**
* This method uses reflection to build a valid hash code.
* <p>
* 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.
* <p>
* 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.
*
* @param initialNonZeroOddNumber a non-zero, odd number used as the initial value
* @param multiplierNonZeroOddNumber a non-zero, odd number used as the multiplier
* @param object the object to create a hash code for
* @return int hash code
* @throws IllegalArgumentException if the object is null
* @throws IllegalArgumentException if the number is zero or even
*/
public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object)
{
return reflectionHashCode(initialNonZeroOddNumber, multiplierNonZeroOddNumber, object, false);
}
/**
* This method uses reflection to build a valid hash code.
* <p>
* 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.
* <p>
* 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.
*
* @param initialNonZeroOddNumber
* @param multiplierNonZeroOddNumber
* @param object the object to create a hash code for
* @param testTransients whether to include transient fields
* @return int hash code
* @throws IllegalArgumentException if the object is null
* @throws IllegalArgumentException if the number is zero or even
*/
public static int reflectionHashCode(
int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
Object object,
boolean testTransients)
{
if (object == null)
{
throw new IllegalArgumentException("The object to build a hash code for must not be null");
}
HashCodeBuilder hashCodeBuilder = new HashCodeBuilder(initialNonZeroOddNumber, multiplierNonZeroOddNumber);
Field[] fields = object.getClass().getDeclaredFields();
Field.setAccessible(fields, true);
for (int i = 0; i < fields.length; ++i)
{
Field f = fields[i];
if (testTransients || !Modifier.isTransient(f.getModifiers()))
{
if (!Modifier.isStatic(f.getModifiers()))
{
try
{
hashCodeBuilder.append(f.get(object));
}
catch (IllegalAccessException e)
{
//this can't happen. Would get a Security exception instead
//throw a runtime exception in case the impossible happens.
throw new InternalError("Unexpected IllegalAccessException");
}
}
}
}
return hashCodeBuilder.toHashCode();
}
//-------------------------------------------------------------------------
/**
* Append a hashCode for an Object.
*
* @param object the object to add to the hashCode
* @return this
*/
public HashCodeBuilder append(Object object)
{
if (object == null)
{
iTotal = iTotal * iConstant;
}
else
{
if (object.getClass().isArray() == false)
{
//the simple case, not an array, just the element
iTotal = iTotal * iConstant + object.hashCode();
}
else
{
//'Switch' on type of array, to dispatch to the correct handler
// This handles multi dimensional arrays
if (object instanceof long[])
{
append((long[]) object);
}
else if (object instanceof int[])
{
append((int[]) object);
}
else if (object instanceof short[])
{
append((short[]) object);
}
else if (object instanceof char[])
{
append((char[]) object);
}
else if (object instanceof byte[])
{
append((byte[]) object);
}
else if (object instanceof double[])
{
append((double[]) object);
}
else if (object instanceof float[])
{
append((float[]) object);
}
else if (object instanceof boolean[])
{
append((boolean[]) object);
}
else
{
// Not an array of primitives
append((Object[]) object);
}
}
}
return this;
}
/**
* Append a hashCode for a long.
*
* @param value the long to add to the hashCode
* @return this
*/
public HashCodeBuilder append(long value)
{
iTotal = iTotal * iConstant + ((int) (value ^ (value >> 32)));
return this;
}
/**
* Append a hashCode for an int.
*
* @param value the int to add to the hashCode
* @return this
*/
public HashCodeBuilder append(int value)
{
iTotal = iTotal * iConstant + value;
return this;
}
/**
* Append a hashCode for a short.
*
* @param value the short to add to the hashCode
* @return this
*/
public HashCodeBuilder append(short value)
{
iTotal = iTotal * iConstant + value;
return this;
}
/**
* Append a hashCode for a char.
*
* @param value the char to add to the hashCode
* @return this
*/
public HashCodeBuilder append(char value)
{
iTotal = iTotal * iConstant + value;
return this;
}
/**
* Append a hashCode for a byte.
*
* @param value the byte to add to the hashCode
* @return this
*/
public HashCodeBuilder append(byte value)
{
iTotal = iTotal * iConstant + value;
return this;
}
/**
* Append a hashCode for a double.
*
* @param value the double to add to the hashCode
* @return this
*/
public HashCodeBuilder append(double value)
{
return append(Double.doubleToLongBits(value));
}
/**
* Append a hashCode for a float.
*
* @param value the float to add to the hashCode
* @return this
*/
public HashCodeBuilder append(float value)
{
iTotal = iTotal * iConstant + Float.floatToIntBits(value);
return this;
}
/**
* Append a hashCode for a long.
*
* @param value the long to add to the hashCode
* @return this
*/
public HashCodeBuilder append(boolean value)
{
iTotal = iTotal * iConstant + (value ? 0 : 1);
return this;
}
/**
* Append a hashCode for an Object array.
*
* @param array the array to add to the hashCode
* @return this
*/
public HashCodeBuilder append(Object[] array)
{
if (array == null)
{
iTotal = iTotal * iConstant;
}
else
{
for (int i = 0; i < array.length; i++)
{
append(array[i]);
}
}
return this;
}
/**
* Append a hashCode for a long array.
*
* @param array the array to add to the hashCode
* @return this
*/
public HashCodeBuilder append(long[] array)
{
if (array == null)
{
iTotal = iTotal * iConstant;
}
else
{
for (int i = 0; i < array.length; i++)
{
append(array[i]);
}
}
return this;
}
/**
* Append a hashCode for an int array.
*
* @param array the array to add to the hashCode
* @return this
*/
public HashCodeBuilder append(int[] array)
{
if (array == null)
{
iTotal = iTotal * iConstant;
}
else
{
for (int i = 0; i < array.length; i++)
{
append(array[i]);
}
}
return this;
}
/**
* Append a hashCode for a short array.
*
* @param array the array to add to the hashCode
* @return this
*/
public HashCodeBuilder append(short[] array)
{
if (array == null)
{
iTotal = iTotal * iConstant;
}
else
{
for (int i = 0; i < array.length; i++)
{
append(array[i]);
}
}
return this;
}
/**
* Append a hashCode for a char array.
*
* @param array the array to add to the hashCode
* @return this
*/
public HashCodeBuilder append(char[] array)
{
if (array == null)
{
iTotal = iTotal * iConstant;
}
else
{
for (int i = 0; i < array.length; i++)
{
append(array[i]);
}
}
return this;
}
/**
* Append a hashCode for a byte array.
*
* @param array the array to add to the hashCode
* @return this
*/
public HashCodeBuilder append(byte[] array)
{
if (array == null)
{
iTotal = iTotal * iConstant;
}
else
{
for (int i = 0; i < array.length; i++)
{
append(array[i]);
}
}
return this;
}
/**
* Append a hashCode for a double array.
*
* @param array the array to add to the hashCode
* @return this
*/
public HashCodeBuilder append(double[] array)
{
if (array == null)
{
iTotal = iTotal * iConstant;
}
else
{
for (int i = 0; i < array.length; i++)
{
append(array[i]);
}
}
return this;
}
/**
* Append a hashCode for a float array.
*
* @param array the array to add to the hashCode
* @return this
*/
public HashCodeBuilder append(float[] array)
{
if (array == null)
{
iTotal = iTotal * iConstant;
}
else
{
for (int i = 0; i < array.length; i++)
{
append(array[i]);
}
}
return this;
}
/**
* Append a hashCode for a boolean array.
*
* @param array the array to add to the hashCode
* @return this
*/
public HashCodeBuilder append(boolean[] array)
{
if (array == null)
{
iTotal = iTotal * iConstant;
}
else
{
for (int i = 0; i < array.length; i++)
{
append(array[i]);
}
}
return this;
}
/**
* Return the computed hashCode
*
* @return int hashCode based on the fields appended
*/
public int toHashCode()
{
return iTotal;
}
}
Related examples in the same category