org.apache.commons.lang.builder
public class HashCodeBuilder extends Object
Assists in implementing {@link 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.
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 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(); } }
If required, the superclass hashCode()
can be added
using {@link #appendSuper}.
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); }
Since: 1.0
Version: $Id: HashCodeBuilder.java 161243 2005-04-14 04:30:28Z ggregory $
Constructor Summary | |
---|---|
HashCodeBuilder() Uses two hard coded choices for the constants
needed to build a | |
HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber) Two randomly chosen, non-zero, odd numbers must be passed in. |
Method Summary | |
---|---|
HashCodeBuilder | append(Object object) Append a |
HashCodeBuilder | append(long value) Append a |
HashCodeBuilder | append(int value) Append a |
HashCodeBuilder | append(short value) Append a |
HashCodeBuilder | append(char value) Append a |
HashCodeBuilder | append(byte value) Append a |
HashCodeBuilder | append(double value) Append a |
HashCodeBuilder | append(float value) Append a |
HashCodeBuilder | append(boolean value) Append a This adds |
HashCodeBuilder | append(Object[] array) Append a |
HashCodeBuilder | append(long[] array) Append a |
HashCodeBuilder | append(int[] array) Append a |
HashCodeBuilder | append(short[] array) Append a |
HashCodeBuilder | append(char[] array) Append a |
HashCodeBuilder | append(byte[] array) Append a |
HashCodeBuilder | append(double[] array) Append a |
HashCodeBuilder | append(float[] array) Append a |
HashCodeBuilder | append(boolean[] array) Append a |
HashCodeBuilder | appendSuper(int superHashCode) Adds the result of super.hashCode() to this builder. |
static int | reflectionHashCode(Object object) This method uses reflection to build a valid hash code. This constructor uses two hard coded choices for the constants needed to build a hash code. It uses |
static int | reflectionHashCode(Object object, boolean testTransients) This method uses reflection to build a valid hash code. This constructor uses two hard coded choices for the constants needed to build a hash code. It uses |
static int | reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object) This method uses reflection to build a valid hash code. It uses |
static int | reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients) This method uses reflection to build a valid hash code. It uses |
static int | reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients, Class reflectUpToClass) This method uses reflection to build a valid hash code. It uses |
int | toHashCode() Return the computed |
Uses two hard coded choices for the constants
needed to build a hashCode
.
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
Append a hashCode
for an Object
.
Parameters: object the Object to add to the hashCode
Returns: this
Append a hashCode
for a long
.
Parameters: value the long to add to the hashCode
Returns: this
Append a hashCode
for an int
.
Parameters: value the int to add to the hashCode
Returns: this
Append a hashCode
for a short
.
Parameters: value the short to add to the hashCode
Returns: this
Append a hashCode
for a char
.
Parameters: value the char to add to the hashCode
Returns: this
Append a hashCode
for a byte
.
Parameters: value the byte to add to the hashCode
Returns: this
Append a hashCode
for a double
.
Parameters: value the double to add to the hashCode
Returns: this
Append a hashCode
for a float
.
Parameters: value the float to add to the hashCode
Returns: this
Append a hashCode
for a boolean
.
This adds iConstant * 1
to the hashCode
and not a 1231
or 1237
as done in java.lang.Boolean.
This is in accordance with the Effective Java
design.
Parameters: value the boolean to add to the hashCode
Returns: this
Append a hashCode
for an Object
array.
Parameters: array the array to add to the hashCode
Returns: this
Append a hashCode
for a long
array.
Parameters: array the array to add to the hashCode
Returns: this
Append a hashCode
for an int
array.
Parameters: array the array to add to the hashCode
Returns: this
Append a hashCode
for a short
array.
Parameters: array the array to add to the hashCode
Returns: this
Append a hashCode
for a char
array.
Parameters: array the array to add to the hashCode
Returns: this
Append a hashCode
for a byte
array.
Parameters: array the array to add to the hashCode
Returns: this
Append a hashCode
for a double
array.
Parameters: array the array to add to the hashCode
Returns: this
Append a hashCode
for a float
array.
Parameters: array the array to add to the hashCode
Returns: this
Append a hashCode
for a boolean
array.
Parameters: array the array to add to the hashCode
Returns: this
Adds the result of super.hashCode() to this builder.
Parameters: superHashCode the result of calling super.hashCode()
Returns: this HashCodeBuilder, used to chain calls.
Since: 2.0
This method uses reflection to build a valid hash code.
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.
Parameters: object the Object to create a hashCode
for
Returns: int hash code
Throws: IllegalArgumentException if the object is null
This method uses reflection to build a valid hash code.
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.
Parameters: object the Object to create a hashCode
for testTransients whether to include transient fields
Returns: int hash code
Throws: IllegalArgumentException if the object is null
This method uses reflection to build a valid 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.
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 hashCode
for
Returns: int hash code
Throws: IllegalArgumentException if the Object is null
IllegalArgumentException if the number is zero or even
This method uses reflection to build a valid 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.
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 hashCode
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
This method uses reflection to build a valid 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 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.
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 hashCode
for testTransients whether to include transient fields reflectUpToClass the superclass to reflect up to (inclusive),
may be null
Returns: int hash code
Throws: IllegalArgumentException if the Object is null
IllegalArgumentException if the number is zero or even
Since: 2.0
Return the computed hashCode
.
Returns: hashCode
based on the fields appended