public class HashCodeBuilder extends Object implements Builder<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.
| 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(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. |
HashCodeBuilder |
appendSuper(int superHashCode)
Adds the result of super.hashCode() to this builder.
|
Integer |
build()
Returns the computed
hashCode. |
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. |
static int |
reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
Object object)
Uses reflection to build a valid hash code from the fields of
object. |
static int |
reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
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,
Class<? super T> reflectUpToClass,
String... excludeFields)
Uses reflection to build a valid hash code from the fields of
object. |
static int |
reflectionHashCode(Object object,
boolean testTransients)
Uses reflection to build a valid hash code from the fields of
object. |
static int |
reflectionHashCode(Object object,
Collection<String> excludeFields)
Uses reflection to build a valid hash code from the fields of
object. |
static int |
reflectionHashCode(Object object,
String... excludeFields)
Uses reflection to build a valid hash code from the fields of
object. |
int |
toHashCode()
Return the computed
hashCode. |
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 multiplierIllegalArgumentException - if the number is evenpublic static int reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
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 forIllegalArgumentException - if the Object is nullIllegalArgumentException - if the number is zero or evenHashCodeExcludepublic static int reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
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 fieldsIllegalArgumentException - if the Object is nullIllegalArgumentException - if the number is zero or evenHashCodeExcludepublic static <T> int reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
T object,
boolean testTransients,
Class<? super T> reflectUpToClass,
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 nullexcludeFields - array of field names to exclude from use in calculation of hash codeIllegalArgumentException - if the Object is nullIllegalArgumentException - if the number is zero or evenHashCodeExcludepublic static int reflectionHashCode(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 fieldsIllegalArgumentException - if the object is nullHashCodeExcludepublic static int reflectionHashCode(Object object, Collection<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 codeIllegalArgumentException - if the object is nullHashCodeExcludepublic static int reflectionHashCode(Object object, 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 codeIllegalArgumentException - if the object is nullHashCodeExcludepublic 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 hashCodepublic HashCodeBuilder append(boolean[] array)
Append a hashCode for a boolean array.
array - the array to add to the hashCodepublic HashCodeBuilder append(byte value)
Append a hashCode for a byte.
value - the byte to add to the hashCodepublic HashCodeBuilder append(byte[] array)
Append a hashCode for a byte array.
array - the array to add to the hashCodepublic HashCodeBuilder append(char value)
Append a hashCode for a char.
value - the char to add to the hashCodepublic HashCodeBuilder append(char[] array)
Append a hashCode for a char array.
array - the array to add to the hashCodepublic HashCodeBuilder append(double value)
Append a hashCode for a double.
value - the double to add to the hashCodepublic HashCodeBuilder append(double[] array)
Append a hashCode for a double array.
array - the array to add to the hashCodepublic HashCodeBuilder append(float value)
Append a hashCode for a float.
value - the float to add to the hashCodepublic HashCodeBuilder append(float[] array)
Append a hashCode for a float array.
array - the array to add to the hashCodepublic HashCodeBuilder append(int value)
Append a hashCode for an int.
value - the int to add to the hashCodepublic HashCodeBuilder append(int[] array)
Append a hashCode for an int array.
array - the array to add to the hashCodepublic HashCodeBuilder append(long value)
Append a hashCode for a long.
value - the long to add to the hashCodepublic HashCodeBuilder append(long[] array)
Append a hashCode for a long array.
array - the array to add to the hashCodepublic HashCodeBuilder append(Object object)
Append a hashCode for an Object.
object - the Object to add to the hashCodepublic HashCodeBuilder append(Object[] array)
Append a hashCode for an Object array.
array - the array to add to the hashCodepublic HashCodeBuilder append(short value)
Append a hashCode for a short.
value - the short to add to the hashCodepublic HashCodeBuilder append(short[] array)
Append a hashCode for a short array.
array - the array to add to the hashCodepublic 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 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.
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