org.apache.commons.lang.builder
public class HashCodeBuilder extends Object
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.
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 HashCodeBuilder.
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 564070 2007-08-09 01:58:11Z bayard $
| 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(boolean value)
Append a |
| HashCodeBuilder | append(boolean[] array)
Append a |
| HashCodeBuilder | append(byte value)
Append a |
| HashCodeBuilder | append(byte[] array)
Append a |
| HashCodeBuilder | append(char value)
Append a |
| HashCodeBuilder | append(char[] array)
Append a |
| HashCodeBuilder | append(double value)
Append a |
| HashCodeBuilder | append(double[] array)
Append a |
| HashCodeBuilder | append(float value)
Append a |
| HashCodeBuilder | append(float[] array)
Append a |
| HashCodeBuilder | append(int value)
Append a |
| HashCodeBuilder | append(int[] array)
Append a |
| HashCodeBuilder | append(long value)
Append a |
| HashCodeBuilder | append(long[] array)
Append a |
| HashCodeBuilder | append(Object object)
Append a |
| HashCodeBuilder | append(Object[] array)
Append a |
| HashCodeBuilder | append(short value)
Append a |
| HashCodeBuilder | append(short[] array)
Append a |
| HashCodeBuilder | appendSuper(int superHashCode) Adds the result of super.hashCode() to this builder. |
| static int | reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object) This method uses reflection to build a valid hash code. |
| static int | reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients) This method uses reflection to build a valid hash code. |
| static int | reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients, Class reflectUpToClass)
Calls (int, int, Object, boolean, Class, String[]) with excludeFields set to
null.
|
| static int | reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients, Class reflectUpToClass, String[] excludeFields) This method uses reflection to build a valid hash code. |
| static int | reflectionHashCode(Object object) This method uses reflection to build a valid hash code. |
| static int | reflectionHashCode(Object object, boolean testTransients) This method uses reflection to build a valid hash code. |
| static int | reflectionHashCode(Object object, Collection excludeFields) This method uses reflection to build a valid hash code. |
| static int | reflectionHashCode(Object object, String[] excludeFields) This method uses reflection to build a valid hash code. |
| 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 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 a boolean array.
Parameters: array
the array 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 byte array.
Parameters: array
the array 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 char array.
Parameters: array
the array 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 double array.
Parameters: array
the array 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 float array.
Parameters: array
the array 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 an int array.
Parameters: array
the array 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 a long array.
Parameters: array
the array to add to the hashCode
Returns: this
Append a hashCode for an Object.
Parameters: object
the Object 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 short.
Parameters: value
the short 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
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.
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
(int, int, Object, boolean, Class, String[]) with excludeFields set to
null.
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
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 excludeFields
array of field names to exclude from use in calculation of hash code
Returns: int hash code
Throws: IllegalArgumentException
if the Object is null IllegalArgumentException
if the number is zero or even
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.
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 excludeFields
Collection of String field names to exclude from use in calculation of hash code
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.
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 excludeFields
array of field names to exclude from use in calculation of hash code
Returns: int hash code
Throws: IllegalArgumentException
if the object is null
Return the computed hashCode.
Returns: hashCode based on the fields appended