public class HashCodeBuilder extends java.lang.Object implements Builder<java.lang.Integer>
Assists in implementing Object.hashCode()
methods.
This class enables a good hashCode
method to be built for any class. It follows the rules laid out in
the book Effective Java by Joshua Bloch. Writing a
good hashCode
method is actually quite difficult. This class aims to simplify the process.
The following is the approach taken. When appending a data field, the current total is multiplied by the multiplier then a relevant value for that data type is added. For example, if the current hashCode is 17, and the multiplier is 37, then appending the integer 45 will create a hash code of 674, namely 17 * 37 + 45.
All relevant fields from the object should be included in the hashCode
method. 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 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(); } }
If required, the superclass hashCode()
can be added using appendSuper(int)
.
Alternatively, there is a method that uses reflection to determine the fields to test. Because these fields are
usually private, the method, reflectionHashCode
, uses AccessibleObject.setAccessible
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.
A typical invocation for this method would look like:
public int hashCode() { return HashCodeBuilder.reflectionHashCode(this); }
The HashCodeExclude
annotation can be used to exclude fields from being
used by the reflectionHashCode
methods.
Modifier and Type | Field and Description |
---|---|
private static int |
DEFAULT_INITIAL_VALUE
The default initial value to use in reflection hash code building.
|
private static int |
DEFAULT_MULTIPLIER_VALUE
The default multiplier value to use in reflection hash code building.
|
private int |
iConstant
Constant to use in building the hashCode.
|
private int |
iTotal
Running total of the hashCode.
|
private static java.lang.ThreadLocal<java.util.Set<IDKey>> |
REGISTRY
A registry of objects used by reflection methods to detect cyclical object references and avoid infinite loops.
|
Constructor and Description |
---|
HashCodeBuilder()
Uses two hard coded choices for the constants needed to build a
hashCode . |
HashCodeBuilder(int initialOddNumber,
int multiplierOddNumber)
Two randomly chosen, odd numbers must be passed in.
|
Modifier and Type | Method and Description |
---|---|
HashCodeBuilder |
append(boolean value)
Append a
hashCode for a boolean . |
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(java.lang.Object object)
Append a
hashCode for an Object . |
HashCodeBuilder |
append(java.lang.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. |
private void |
appendArray(java.lang.Object object)
Append a
hashCode for an array. |
HashCodeBuilder |
appendSuper(int superHashCode)
Adds the result of super.hashCode() to this builder.
|
java.lang.Integer |
build()
Returns the computed
hashCode . |
(package private) static java.util.Set<IDKey> |
getRegistry()
Returns the registry of objects being traversed by the reflection methods in the current thread.
|
int |
hashCode()
The computed
hashCode from toHashCode() is returned due to the likelihood
of bugs in mis-calling toHashCode() and the unlikeliness of it mattering what the hashCode for
HashCodeBuilder itself is. |
(package private) static boolean |
isRegistered(java.lang.Object value)
Returns
true if the registry contains the given object. |
private static void |
reflectionAppend(java.lang.Object object,
java.lang.Class<?> clazz,
HashCodeBuilder builder,
boolean useTransients,
java.lang.String[] excludeFields)
Appends the fields and values defined by the given object of the given
Class . |
static int |
reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
java.lang.Object object)
Uses reflection to build a valid hash code from the fields of
object . |
static int |
reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
java.lang.Object object,
boolean testTransients)
Uses reflection to build a valid hash code from the fields of
object . |
static <T> int |
reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
T object,
boolean testTransients,
java.lang.Class<? super T> reflectUpToClass,
java.lang.String... excludeFields)
Uses reflection to build a valid hash code from the fields of
object . |
static int |
reflectionHashCode(java.lang.Object object,
boolean testTransients)
Uses reflection to build a valid hash code from the fields of
object . |
static int |
reflectionHashCode(java.lang.Object object,
java.util.Collection<java.lang.String> excludeFields)
Uses reflection to build a valid hash code from the fields of
object . |
static int |
reflectionHashCode(java.lang.Object object,
java.lang.String... excludeFields)
Uses reflection to build a valid hash code from the fields of
object . |
private static void |
register(java.lang.Object value)
Registers the given object.
|
int |
toHashCode()
Return the computed
hashCode . |
private static void |
unregister(java.lang.Object value)
Unregisters the given object.
|
private static final int DEFAULT_INITIAL_VALUE
private static final int DEFAULT_MULTIPLIER_VALUE
private static final java.lang.ThreadLocal<java.util.Set<IDKey>> REGISTRY
A registry of objects used by reflection methods to detect cyclical object references and avoid infinite loops.
private final int iConstant
private int iTotal
public HashCodeBuilder()
Uses two hard coded choices for the constants needed to build a hashCode
.
public HashCodeBuilder(int initialOddNumber, int multiplierOddNumber)
Two randomly chosen, 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.
initialOddNumber
- an odd number used as the initial valuemultiplierOddNumber
- an odd number used as the multiplierjava.lang.IllegalArgumentException
- if the number is evenstatic java.util.Set<IDKey> getRegistry()
Returns the registry of objects being traversed by the reflection methods in the current thread.
static boolean isRegistered(java.lang.Object value)
Returns true
if the registry contains the given object. Used by the reflection methods to avoid
infinite loops.
value
- The object to lookup in the registry.true
if the registry contains the given object.private static void reflectionAppend(java.lang.Object object, java.lang.Class<?> clazz, HashCodeBuilder builder, boolean useTransients, java.lang.String[] excludeFields)
Appends the fields and values defined by the given object of the given Class
.
object
- the object to append details ofclazz
- the class to append details ofbuilder
- the builder to append touseTransients
- whether to use transient fieldsexcludeFields
- Collection of String field names to exclude from use in calculation of hash codepublic static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, java.lang.Object object)
Uses reflection to build a valid hash code from the fields of object
.
It uses AccessibleObject.setAccessible
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.
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. Superclass fields will be included.
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.
initialNonZeroOddNumber
- a non-zero, odd number used as the initial value. This will be the returned
value if no fields are found to include in the hash codemultiplierNonZeroOddNumber
- a non-zero, odd number used as the multiplierobject
- the Object to create a hashCode
forjava.lang.IllegalArgumentException
- if the Object is null
java.lang.IllegalArgumentException
- if the number is zero or evenHashCodeExclude
public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, java.lang.Object object, boolean testTransients)
Uses reflection to build a valid hash code from the fields of object
.
It uses AccessibleObject.setAccessible
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.
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. Superclass fields will be included.
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.
initialNonZeroOddNumber
- a non-zero, odd number used as the initial value. This will be the returned
value if no fields are found to include in the hash codemultiplierNonZeroOddNumber
- a non-zero, odd number used as the multiplierobject
- the Object to create a hashCode
fortestTransients
- whether to include transient fieldsjava.lang.IllegalArgumentException
- if the Object is null
java.lang.IllegalArgumentException
- if the number is zero or evenHashCodeExclude
public static <T> int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, T object, boolean testTransients, java.lang.Class<? super T> reflectUpToClass, java.lang.String... excludeFields)
Uses reflection to build a valid hash code from the fields of object
.
It uses AccessibleObject.setAccessible
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.
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 included. Superclass fields will be included up to and including the specified superclass. A null superclass is treated as java.lang.Object.
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.
T
- the type of the object involvedinitialNonZeroOddNumber
- a non-zero, odd number used as the initial value. This will be the returned
value if no fields are found to include in the hash codemultiplierNonZeroOddNumber
- a non-zero, odd number used as the multiplierobject
- the Object to create a hashCode
fortestTransients
- whether to include transient fieldsreflectUpToClass
- the superclass to reflect up to (inclusive), may be null
excludeFields
- array of field names to exclude from use in calculation of hash codejava.lang.IllegalArgumentException
- if the Object is null
java.lang.IllegalArgumentException
- if the number is zero or evenHashCodeExclude
public static int reflectionHashCode(java.lang.Object object, boolean testTransients)
Uses reflection to build a valid hash code from the fields of object
.
This constructor uses two hard coded choices for the constants needed to build a hash code.
It uses AccessibleObject.setAccessible
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.
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. Superclass fields will be included. If no fields are found to include in the hash code, the result of this method will be constant.
object
- the Object to create a hashCode
fortestTransients
- whether to include transient fieldsjava.lang.IllegalArgumentException
- if the object is null
HashCodeExclude
public static int reflectionHashCode(java.lang.Object object, java.util.Collection<java.lang.String> excludeFields)
Uses reflection to build a valid hash code from the fields of object
.
This constructor uses two hard coded choices for the constants needed to build a hash code.
It uses AccessibleObject.setAccessible
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.
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. Superclass fields will be included. If no fields are found to include in the hash code, the result of this method will be constant.
object
- the Object to create a hashCode
forexcludeFields
- Collection of String field names to exclude from use in calculation of hash codejava.lang.IllegalArgumentException
- if the object is null
HashCodeExclude
public static int reflectionHashCode(java.lang.Object object, java.lang.String... excludeFields)
Uses reflection to build a valid hash code from the fields of object
.
This constructor uses two hard coded choices for the constants needed to build a hash code.
It uses AccessibleObject.setAccessible
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.
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. Superclass fields will be included. If no fields are found to include in the hash code, the result of this method will be constant.
object
- the Object to create a hashCode
forexcludeFields
- array of field names to exclude from use in calculation of hash codejava.lang.IllegalArgumentException
- if the object is null
HashCodeExclude
private static void register(java.lang.Object value)
Registers the given object. Used by the reflection methods to avoid infinite loops.
value
- The object to register.private static void unregister(java.lang.Object value)
Unregisters the given object.
Used by the reflection methods to avoid infinite loops.
value
- The object to unregister.public HashCodeBuilder append(boolean value)
Append a hashCode
for a boolean
.
This adds 1
when true, and 0
when false to the hashCode
.
This is in contrast to the standard java.lang.Boolean.hashCode
handling, which computes
a hashCode
value of 1231
for java.lang.Boolean
instances
that represent true
or 1237
for java.lang.Boolean
instances
that represent false
.
This is in accordance with the Effective Java design.
value
- the boolean to add to the hashCode
public HashCodeBuilder append(boolean[] array)
Append a hashCode
for a boolean
array.
array
- the array to add to the hashCode
public HashCodeBuilder append(byte value)
Append a hashCode
for a byte
.
value
- the byte to add to the hashCode
public HashCodeBuilder append(byte[] array)
Append a hashCode
for a byte
array.
array
- the array to add to the hashCode
public HashCodeBuilder append(char value)
Append a hashCode
for a char
.
value
- the char to add to the hashCode
public HashCodeBuilder append(char[] array)
Append a hashCode
for a char
array.
array
- the array to add to the hashCode
public HashCodeBuilder append(double value)
Append a hashCode
for a double
.
value
- the double to add to the hashCode
public HashCodeBuilder append(double[] array)
Append a hashCode
for a double
array.
array
- the array to add to the hashCode
public HashCodeBuilder append(float value)
Append a hashCode
for a float
.
value
- the float to add to the hashCode
public HashCodeBuilder append(float[] array)
Append a hashCode
for a float
array.
array
- the array to add to the hashCode
public HashCodeBuilder append(int value)
Append a hashCode
for an int
.
value
- the int to add to the hashCode
public HashCodeBuilder append(int[] array)
Append a hashCode
for an int
array.
array
- the array to add to the hashCode
public HashCodeBuilder append(long value)
Append a hashCode
for a long
.
value
- the long to add to the hashCode
public HashCodeBuilder append(long[] array)
Append a hashCode
for a long
array.
array
- the array to add to the hashCode
public HashCodeBuilder append(java.lang.Object object)
Append a hashCode
for an Object
.
object
- the Object to add to the hashCode
private void appendArray(java.lang.Object object)
Append a hashCode
for an array.
object
- the array to add to the hashCode
public HashCodeBuilder append(java.lang.Object[] array)
Append a hashCode
for an Object
array.
array
- the array to add to the hashCode
public HashCodeBuilder append(short value)
Append a hashCode
for a short
.
value
- the short to add to the hashCode
public HashCodeBuilder append(short[] array)
Append a hashCode
for a short
array.
array
- the array to add to the hashCode
public HashCodeBuilder appendSuper(int superHashCode)
Adds the result of super.hashCode() to this builder.
superHashCode
- the result of calling super.hashCode()
public int toHashCode()
Return the computed hashCode
.
hashCode
based on the fields appendedpublic java.lang.Integer build()
hashCode
.public int hashCode()
The computed hashCode
from toHashCode() is returned due to the likelihood
of bugs in mis-calling toHashCode() and the unlikeliness of it mattering what the hashCode for
HashCodeBuilder itself is.
hashCode
in class java.lang.Object
hashCode
based on the fields appended