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java.lang.Object | +--org.apache.commons.lang.builder.HashCodeBuilder
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 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);
}
| Constructor Summary | |
HashCodeBuilder()
Uses two hard coded choices for the constants needed to build a hashCode. |
|
HashCodeBuilder(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber)
Two randomly chosen, non-zero, odd numbers must be passed in. |
|
| Method Summary | |
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. |
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)
This method uses reflection to build a valid hash code. |
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 hashCode. |
| Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
| Constructor Detail |
public HashCodeBuilder()
Uses two hard coded choices for the constants
needed to build a hashCode.
public HashCodeBuilder(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber)
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 valuemultiplierNonZeroOddNumber - a non-zero, odd number used as the multiplierIllegalArgumentException - if the number is zero or even| Method Detail |
public 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 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.
object - the Object to create a hashCode forIllegalArgumentException - if the object is null
public static int reflectionHashCode(Object object,
Collection excludeFields)
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.
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 null
public static int reflectionHashCode(Object object,
String[] excludeFields)
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.
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 null
public 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 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.
object - the Object to create a hashCode fortestTransients - whether to include transient fieldsIllegalArgumentException - if the object is null
public static int reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
Object object)
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.
initialNonZeroOddNumber - a non-zero, odd number used as the initial valuemultiplierNonZeroOddNumber - 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 even
public static int reflectionHashCode(int initialNonZeroOddNumber,
int multiplierNonZeroOddNumber,
Object object,
boolean testTransients)
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.
initialNonZeroOddNumber - a non-zero, odd number used as the initial valuemultiplierNonZeroOddNumber - 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 even
public 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 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.
initialNonZeroOddNumber - a non-zero, odd number used as the initial valuemultiplierNonZeroOddNumber - 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 nullIllegalArgumentException - if the Object is nullIllegalArgumentException - if the number is zero or even
public 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.
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.
initialNonZeroOddNumber - a non-zero, odd number used as the initial valuemultiplierNonZeroOddNumber - 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 evenpublic HashCodeBuilder appendSuper(int superHashCode)
Adds the result of super.hashCode() to this builder.
superHashCode - the result of calling super.hashCode()public HashCodeBuilder append(Object object)
Append a hashCode for an Object.
object - the Object 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(int value)
Append a hashCode for an int.
value - the int 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(char value)
Append a hashCode for a char.
value - the char 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(double value)
Append a hashCode for a double.
value - the double 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(boolean value)
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.
value - the boolean 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(long[] array)
Append a hashCode for a long array.
array - the array 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(short[] array)
Append a hashCode for a short array.
array - the array 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(byte[] array)
Append a hashCode for a byte array.
array - the array 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[] array)
Append a hashCode for a float array.
array - the array to add to the hashCodepublic HashCodeBuilder append(boolean[] array)
Append a hashCode for a boolean array.
array - the array to add to the hashCodepublic int toHashCode()
Return the computed hashCode.
hashCode based on the fields appended
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