Mega Code Archive
Hash Code Assist
<![CDATA[
//package org.streets.commons.assist;
import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
/**
* <p>
* Assists in implementing {@link Object#hashCode()} methods.
* </p>
*
* <p>
* This class enables a good <code>hashCode</code> method to be built for any
* class. It follows the rules laid out in the book <a
* href="http://java.sun.com/docs/books/effective/index.html">Effective Java</a>
* by Joshua Bloch. Writing a good <code>hashCode</code> method is actually
* quite difficult. This class aims to simplify the process.
* </p>
*
* <p>
* All relevant fields from the object should be included in the
* <code>hashCode</code> method. Derived fields may be excluded. In general, any
* field used in the <code>equals</code> method must be used in the
* <code>hashCode</code> method.
* </p>
*
* <p>
* To use this class write code as follows:
* </p>
*
* <pre>
* public class Person {
* String name;
* int age;
* boolean smoker;
* ...
*
* 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>
* If required, the superclass <code>hashCode()</code> can be added using
* {@link #appendSuper}.
* </p>
*
* <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>AccessibleObject.setAccessible</code> to change the visibility of the
* fields. This will fail under a security manager, unless the appropriate
* permissions are set up correctly. It is also slower than testing explicitly.
* </p>
*
* <p>
* A typical invocation for this method would look like:
* </p>
*
* <pre>
* public int hashCode() {
* return HashCodeBuilder.reflectionHashCode(this);
* }
* </pre>
*
* @author Stephen Colebourne
* @author Gary Gregory
* @author Pete Gieser
* @since 1.0
* @version $Id: HashCodeBuilder.java 564070 2007-08-09 01:58:11Z bayard $
*/
public class HashCodeAssist {
/**
* <p>
* A registry of objects used by reflection methods to detect cyclical
* object references and avoid infinite loops.
* </p>
*
* @since 2.3
*/
private static ThreadLocal<Set<Integer>> registry = new ThreadLocal<Set<Integer>>() {
protected Set<Integer> initialValue() {
// The HashSet implementation is not synchronized,
// which is just what we need here.
return new HashSet<Integer>();
}
};
/**
* <p>
* Returns <code>true</code> if the registry contains the given object. Used
* by the reflection methods to avoid infinite loops.
* </p>
*
* @param value
* The object to lookup in the registry.
* @return boolean <code>true</code> if the registry contains the given
* object.
* @since 2.3
*/
static boolean isRegistered(Object value) {
return registry.get().contains(toIdentityHashCodeInteger(value));
}
/**
* <p>
* Appends the fields and values defined by the given object of the given
* <code>Class</code>.
* </p>
*
* @param object
* the object to append details of
* @param clazz
* the class to append details of
* @param builder
* the builder to append to
* @param useTransients
* whether to use transient fields
* @param excludeFields
* Collection of String field names to exclude from use in
* calculation of hash code
*/
private static void reflectionAppend(Object object, Class<?> clazz,
HashCodeAssist builder, boolean useTransients,
String... excludeFields) {
if (isRegistered(object)) {
return;
}
try {
register(object);
Field[] fields = clazz.getDeclaredFields();
List<String> excludedFieldList = excludeFields != null ? Arrays
.asList(excludeFields) : Collections.EMPTY_LIST;
AccessibleObject.setAccessible(fields, true);
for (int i = 0; i < fields.length; i++) {
Field field = fields[i];
if (!excludedFieldList.contains(field.getName())
&& (field.getName().indexOf('$') == -1)
&& (useTransients || !Modifier.isTransient(field
.getModifiers()))
&& (!Modifier.isStatic(field.getModifiers()))) {
try {
Object fieldValue = field.get(object);
builder.append(fieldValue);
} 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");
}
}
}
} finally {
unregister(object);
}
}
/**
* <p>
* This method uses reflection to build a valid hash code.
* </p>
*
* <p>
* It uses <code>AccessibleObject.setAccessible</code> to gain access to
* private fields. This means that it will throw a security exception if run
* under a security manager, if the permissions are not set up correctly. It
* is also not as efficient as testing explicitly.
* </p>
*
* <p>
* Transient members will be not be used, as they are likely derived fields,
* and not part of the value of the <code>Object</code>.
* </p>
*
* <p>
* Static fields will not be tested. Superclass fields will be included.
* </p>
*
* <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.
* </p>
*
* @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 <code>hashCode</code> for
* @return int hash code
* @throws IllegalArgumentException
* if the Object is <code>null</code>
* @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, null);
}
/**
* <p>
* This method uses reflection to build a valid hash code.
* </p>
*
* <p>
* It uses <code>AccessibleObject.setAccessible</code> to gain access to
* private fields. This means that it will throw a security exception if run
* under a security manager, if the permissions are not set up correctly. It
* is also not as efficient as testing explicitly.
* </p>
*
* <p>
* If the TestTransients parameter is set to <code>true</code>, transient
* members will be tested, otherwise they are ignored, as they are likely
* derived fields, and not part of the value of the <code>Object</code>.
* </p>
*
* <p>
* Static fields will not be tested. Superclass fields will be included.
* </p>
*
* <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.
* </p>
*
* @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 <code>hashCode</code> for
* @param testTransients
* whether to include transient fields
* @return int hash code
* @throws IllegalArgumentException
* if the Object is <code>null</code>
* @throws IllegalArgumentException
* if the number is zero or even
*/
public static int reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber, Object object,
boolean testTransients) {
return reflectionHashCode(initialNonZeroOddNumber,
multiplierNonZeroOddNumber, object, testTransients, null);
}
/**
* Calls
* {@link #reflectionHashCode(int, int, Object, boolean, Class, String[])}
* with excludeFields set to <code>null</code>.
*
* @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 <code>hashCode</code> for
* @param testTransients
* whether to include transient fields
* @param reflectUpToClass
* the superclass to reflect up to (inclusive), may be
* <code>null</code>
* @return int hash code
*/
public static int reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber, Object object,
boolean testTransients, Class<?> reflectUpToClass) {
return reflectionHashCode(initialNonZeroOddNumber,
multiplierNonZeroOddNumber, object, testTransients,
reflectUpToClass);
}
/**
* <p>
* This method uses reflection to build a valid hash code.
* </p>
*
* <p>
* It uses <code>AccessibleObject.setAccessible</code> to gain access to
* private fields. This means that it will throw a security exception if run
* under a security manager, if the permissions are not set up correctly. It
* is also not as efficient as testing explicitly.
* </p>
*
* <p>
* If the TestTransients parameter is set to <code>true</code>, transient
* members will be tested, otherwise they are ignored, as they are likely
* derived fields, and not part of the value of the <code>Object</code>.
* </p>
*
* <p>
* Static fields will not be included. Superclass fields will be included up
* to and including the specified superclass. A null superclass is treated
* as java.lang.Object.
* </p>
*
* <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.
* </p>
*
* @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 <code>hashCode</code> for
* @param testTransients
* whether to include transient fields
* @param reflectUpToClass
* the superclass to reflect up to (inclusive), may be
* <code>null</code>
* @param excludeFields
* array of field names to exclude from use in calculation of
* hash code
* @return int hash code
* @throws IllegalArgumentException
* if the Object is <code>null</code>
* @throws IllegalArgumentException
* if the number is zero or even
* @since 2.0
*/
public static int reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber, Object object,
boolean testTransients, Class<?> reflectUpToClass,
String... excludeFields) {
if (object == null) {
throw new IllegalArgumentException(
"The object to build a hash code for must not be null");
}
HashCodeAssist builder = new HashCodeAssist(initialNonZeroOddNumber,
multiplierNonZeroOddNumber);
Class<?> clazz = object.getClass();
reflectionAppend(object, clazz, builder, testTransients, excludeFields);
while (clazz.getSuperclass() != null && clazz != reflectUpToClass) {
clazz = clazz.getSuperclass();
reflectionAppend(object, clazz, builder, testTransients,
excludeFields);
}
return builder.toHashCode();
}
/**
* <p>
* This method uses reflection to build a valid hash code.
* </p>
*
* <p>
* This constructor uses two hard coded choices for the constants needed to
* build a hash code.
* </p>
*
* <p>
* It uses <code>AccessibleObject.setAccessible</code> to gain access to
* private fields. This means that it will throw a security exception if run
* under a security manager, if the permissions are not set up correctly. It
* is also not as efficient as testing explicitly.
* </p>
*
* <p>
* Transient members will be not be used, as they are likely derived fields,
* and not part of the value of the <code>Object</code>.
* </p>
*
* <p>
* Static fields will not be tested. Superclass fields will be included.
* </p>
*
* @param object
* the Object to create a <code>hashCode</code> for
* @return int hash code
* @throws IllegalArgumentException
* if the object is <code>null</code>
*/
public static int reflectionHashCode(Object object) {
return reflectionHashCode(17, 37, object, false, null);
}
/**
* <p>
* This method uses reflection to build a valid hash code.
* </p>
*
* <p>
* This constructor uses two hard coded choices for the constants needed to
* build a hash code.
* </p>
*
* <p>
* It uses <code>AccessibleObject.setAccessible</code> to gain access to
* private fields. This means that it will throw a security exception if run
* under a security manager, if the permissions are not set up correctly. It
* is also not as efficient as testing explicitly.
* </p>
*
* <P>
* If the TestTransients parameter is set to <code>true</code>, transient
* members will be tested, otherwise they are ignored, as they are likely
* derived fields, and not part of the value of the <code>Object</code>.
* </p>
*
* <p>
* Static fields will not be tested. Superclass fields will be included.
* </p>
*
* @param object
* the Object to create a <code>hashCode</code> for
* @param testTransients
* whether to include transient fields
* @return int hash code
* @throws IllegalArgumentException
* if the object is <code>null</code>
*/
public static int reflectionHashCode(Object object, boolean testTransients) {
return reflectionHashCode(17, 37, object, testTransients, null);
}
// -------------------------------------------------------------------------
/**
* <p>
* This method uses reflection to build a valid hash code.
* </p>
*
* <p>
* This constructor uses two hard coded choices for the constants needed to
* build a hash code.
* </p>
*
* <p>
* It uses <code>AccessibleObject.setAccessible</code> to gain access to
* private fields. This means that it will throw a security exception if run
* under a security manager, if the permissions are not set up correctly. It
* is also not as efficient as testing explicitly.
* </p>
*
* <p>
* Transient members will be not be used, as they are likely derived fields,
* and not part of the value of the <code>Object</code>.
* </p>
*
* <p>
* Static fields will not be tested. Superclass fields will be included.
* </p>
*
* @param object
* the Object to create a <code>hashCode</code> for
* @param excludeFields
* array of field names to exclude from use in calculation of
* hash code
* @return int hash code
* @throws IllegalArgumentException
* if the object is <code>null</code>
*/
public static int reflectionHashCode(Object object, String... excludeFields) {
return reflectionHashCode(17, 37, object, false, null, excludeFields);
}
/**
* <p>
* Registers the given object. Used by the reflection methods to avoid
* infinite loops.
* </p>
*
* @param value
* The object to register.
*/
static void register(Object value) {
registry.get().add(toIdentityHashCodeInteger(value));
}
/**
* Returns an Integer for the given object's default hash code.
*
* @see System#identityHashCode(Object)
* @param value
* object for which the hashCode is to be calculated
* @return Default int hash code
*/
private static Integer toIdentityHashCodeInteger(Object value) {
return new Integer(System.identityHashCode(value));
}
/**
* <p>
* Unregisters the given object.
* </p>
*
* <p>
* Used by the reflection methods to avoid infinite loops.
*
* @param value
* The object to unregister.
* @since 2.3
*/
static void unregister(Object value) {
registry.get().remove(toIdentityHashCodeInteger(value));
}
/**
* Constant to use in building the hashCode.
*/
private final int iConstant;
/**
* Running total of the hashCode.
*/
private int iTotal = 0;
/**
* <p>
* Uses two hard coded choices for the constants needed to build a
* <code>hashCode</code>.
* </p>
*/
public HashCodeAssist() {
iConstant = 37;
iTotal = 17;
}
/**
* <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.
* </p>
*
* <p>
* Prime numbers are preferred, especially for the multiplier.
* </p>
*
* @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 HashCodeAssist(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber) {
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;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>boolean</code>.
* </p>
* <p>
* This adds <code>iConstant * 1</code> to the <code>hashCode</code> and not
* a <code>1231</code> or <code>1237</code> as done in java.lang.Boolean.
* This is in accordance with the <quote>Effective Java</quote> design.
* </p>
*
* @param value
* the boolean to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(boolean value) {
iTotal = iTotal * iConstant + (value ? 0 : 1);
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>boolean</code> array.
* </p>
*
* @param array
* the array to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(boolean[] array) {
if (array == null) {
iTotal = iTotal * iConstant;
} else {
for (int i = 0; i < array.length; i++) {
append(array[i]);
}
}
return this;
}
// -------------------------------------------------------------------------
/**
* <p>
* Append a <code>hashCode</code> for a <code>byte</code>.
* </p>
*
* @param value
* the byte to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(byte value) {
iTotal = iTotal * iConstant + value;
return this;
}
// -------------------------------------------------------------------------
/**
* <p>
* Append a <code>hashCode</code> for a <code>byte</code> array.
* </p>
*
* @param array
* the array to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(byte[] array) {
if (array == null) {
iTotal = iTotal * iConstant;
} else {
for (int i = 0; i < array.length; i++) {
append(array[i]);
}
}
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>char</code>.
* </p>
*
* @param value
* the char to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(char value) {
iTotal = iTotal * iConstant + value;
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>char</code> array.
* </p>
*
* @param array
* the array to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(char[] array) {
if (array == null) {
iTotal = iTotal * iConstant;
} else {
for (int i = 0; i < array.length; i++) {
append(array[i]);
}
}
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>double</code>.
* </p>
*
* @param value
* the double to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(double value) {
return append(Double.doubleToLongBits(value));
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>double</code> array.
* </p>
*
* @param array
* the array to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(double[] array) {
if (array == null) {
iTotal = iTotal * iConstant;
} else {
for (int i = 0; i < array.length; i++) {
append(array[i]);
}
}
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>float</code>.
* </p>
*
* @param value
* the float to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(float value) {
iTotal = iTotal * iConstant + Float.floatToIntBits(value);
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>float</code> array.
* </p>
*
* @param array
* the array to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(float[] array) {
if (array == null) {
iTotal = iTotal * iConstant;
} else {
for (int i = 0; i < array.length; i++) {
append(array[i]);
}
}
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for an <code>int</code>.
* </p>
*
* @param value
* the int to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(int value) {
iTotal = iTotal * iConstant + value;
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for an <code>int</code> array.
* </p>
*
* @param array
* the array to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(int[] array) {
if (array == null) {
iTotal = iTotal * iConstant;
} else {
for (int i = 0; i < array.length; i++) {
append(array[i]);
}
}
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>long</code>.
* </p>
* <p>
*
* @param value
* the long to add to the <code>hashCode</code>
* @return this
*/
// NOTE: This method uses >> and not >>> as Effective Java and
// Long.hashCode do. Ideally we should switch to >>> at
// some stage. There are backwards compat issues, so
// that will have to wait for the time being. cf LANG-342.
public HashCodeAssist append(long value) {
iTotal = iTotal * iConstant + ((int) (value ^ (value >> 32)));
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>long</code> array.
* </p>
*
* @param array
* the array to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(long[] array) {
if (array == null) {
iTotal = iTotal * iConstant;
} else {
for (int i = 0; i < array.length; i++) {
append(array[i]);
}
}
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for an <code>Object</code>.
* </p>
*
* @param object
* the Object to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(Object object) {
if (object == null) {
iTotal = iTotal * iConstant;
} 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 if (object instanceof Object[]) {
// Not an array of primitives
append((Object[]) object);
} else {
iTotal = iTotal * iConstant + object.hashCode();
}
}
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for an <code>Object</code> array.
* </p>
*
* @param array
* the array to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(Object[] array) {
if (array == null) {
iTotal = iTotal * iConstant;
} else {
for (int i = 0; i < array.length; i++) {
append(array[i]);
}
}
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>short</code>.
* </p>
*
* @param value
* the short to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(short value) {
iTotal = iTotal * iConstant + value;
return this;
}
/**
* <p>
* Append a <code>hashCode</code> for a <code>short</code> array.
* </p>
*
* @param array
* the array to add to the <code>hashCode</code>
* @return this
*/
public HashCodeAssist append(short[] array) {
if (array == null) {
iTotal = iTotal * iConstant;
} else {
for (int i = 0; i < array.length; i++) {
append(array[i]);
}
}
return this;
}
/**
* <p>
* Adds the result of super.hashCode() to this builder.
* </p>
*
* @param superHashCode
* the result of calling <code>super.hashCode()</code>
* @return this HashCodeBuilder, used to chain calls.
* @since 2.0
*/
public HashCodeAssist appendSuper(int superHashCode) {
iTotal = iTotal * iConstant + superHashCode;
return this;
}
/**
* <p>
* Return the computed <code>hashCode</code>.
* </p>
*
* @return <code>hashCode</code> based on the fields appended
*/
public int toHashCode() {
return iTotal;
}
}
]]>