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 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
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);
}
append
public HashCodeBuilder append(Object object)
Append a hashCode for an Object.
object - the Object to add to the hashCode
append
public HashCodeBuilder append(Object[] array)
Append a hashCode for an Object array.
array - the array to add to the hashCode
append
public 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 hashCode
append
public HashCodeBuilder append(boolean[] array)
Append a hashCode for a boolean array.
array - the array to add to the hashCode
append
public HashCodeBuilder append(byte value)
Append a hashCode for a byte.
value - the byte to add to the hashCode
append
public HashCodeBuilder append(byte[] array)
Append a hashCode for a byte array.
array - the array to add to the hashCode
append
public HashCodeBuilder append(char value)
Append a hashCode for a char.
value - the char to add to the hashCode
append
public HashCodeBuilder append(char[] array)
Append a hashCode for a char array.
array - the array to add to the hashCode
append
public HashCodeBuilder append(double value)
Append a hashCode for a double.
value - the double to add to the hashCode
append
public HashCodeBuilder append(double[] array)
Append a hashCode for a double array.
array - the array to add to the hashCode
append
public HashCodeBuilder append(float value)
Append a hashCode for a float.
value - the float to add to the hashCode
append
public HashCodeBuilder append(float[] array)
Append a hashCode for a float array.
array - the array to add to the hashCode
append
public HashCodeBuilder append(int value)
Append a hashCode for an int.
value - the int to add to the hashCode
append
public HashCodeBuilder append(int[] array)
Append a hashCode for an int array.
array - the array to add to the hashCode
append
public HashCodeBuilder append(long value)
Append a hashCode for a long.
value - the long to add to the hashCode
append
public HashCodeBuilder append(long[] array)
Append a hashCode for a long array.
array - the array to add to the hashCode
append
public HashCodeBuilder append(short value)
Append a hashCode for a short.
value - the short to add to the hashCode
append
public HashCodeBuilder append(short[] array)
Append a hashCode for a short array.
array - the array to add to the hashCode
appendSuper
public HashCodeBuilder appendSuper(int superHashCode)
Adds the result of super.hashCode() to this builder.
superHashCode - the result of calling super.hashCode()
- this HashCodeBuilder, used to chain calls.
reflectionHashCode
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 for
reflectionHashCode
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 fields
reflectionHashCode
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 for
reflectionHashCode
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 fields
reflectionHashCode
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 null
toHashCode
public int toHashCode()
Return the computed hashCode.
hashCode based on the fields appended