Class HashCodeBuilder
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.
- Since:
- 1.0
-
Constructor Summary
ConstructorsConstructorDescriptionUses two hard coded choices for the constants needed to build ahashCode.HashCodeBuilder(int initialOddNumber, int multiplierOddNumber) Two randomly chosen, odd numbers must be passed in. -
Method Summary
Modifier and TypeMethodDescriptionappend(boolean value) Append ahashCodefor aboolean.append(boolean[] array) Append ahashCodefor abooleanarray.append(byte value) Append ahashCodefor abyte.append(byte[] array) Append ahashCodefor abytearray.append(char value) Append ahashCodefor achar.append(char[] array) Append ahashCodefor achararray.append(double value) Append ahashCodefor adouble.append(double[] array) Append ahashCodefor adoublearray.append(float value) Append ahashCodefor afloat.append(float[] array) Append ahashCodefor afloatarray.append(int value) Append ahashCodefor anint.append(int[] array) Append ahashCodefor anintarray.append(long value) Append ahashCodefor along.append(long[] array) Append ahashCodefor alongarray.append(short value) Append ahashCodefor ashort.append(short[] array) Append ahashCodefor ashortarray.Append ahashCodefor anObject.Append ahashCodefor anObjectarray.appendSuper(int superHashCode) Adds the result of super.hashCode() to this builder.build()Returns the computedhashCode.booleanImplements equals using the hash code.inthashCode()The computedhashCodefrom 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 intreflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object) Uses reflection to build a valid hash code from the fields ofobject.static intreflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients) Uses reflection to build a valid hash code from the fields ofobject.static <T> intreflectionHashCode(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 ofobject.static intreflectionHashCode(Object object, boolean testTransients) Uses reflection to build a valid hash code from the fields ofobject.static intreflectionHashCode(Object object, String... excludeFields) Uses reflection to build a valid hash code from the fields ofobject.static intreflectionHashCode(Object object, Collection<String> excludeFields) Uses reflection to build a valid hash code from the fields ofobject.intReturns the computedhashCode.
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Constructor Details
-
HashCodeBuilder
public HashCodeBuilder()Uses two hard coded choices for the constants needed to build ahashCode. -
HashCodeBuilder
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.
- Parameters:
initialOddNumber- an odd number used as the initial valuemultiplierOddNumber- an odd number used as the multiplier- Throws:
IllegalArgumentException- if the number is even
-
-
Method Details
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reflectionHashCode
public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object) Uses reflection to build a valid hash code from the fields ofobject.It uses
AccessibleObject.setAccessibleto 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. 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 ahashCodefor- Returns:
- int hash code
- Throws:
NullPointerException- if the Object isnullIllegalArgumentException- if the number is zero or even- See Also:
-
reflectionHashCode
public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients) Uses reflection to build a valid hash code from the fields ofobject.It uses
AccessibleObject.setAccessibleto 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 theObject.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. 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 ahashCodefortestTransients- whether to include transient fields- Returns:
- int hash code
- Throws:
NullPointerException- if the Object isnullIllegalArgumentException- if the number is zero or even- See Also:
-
reflectionHashCode
public 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 ofobject.It uses
AccessibleObject.setAccessibleto 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 theObject.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.
- Type Parameters:
T- the type of the object involved- Parameters:
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 ahashCodefortestTransients- whether to include transient fieldsreflectUpToClass- the superclass to reflect up to (inclusive), may benullexcludeFields- array of field names to exclude from use in calculation of hash code- Returns:
- int hash code
- Throws:
NullPointerException- if the Object isnullIllegalArgumentException- if the number is zero or even- Since:
- 2.0
- See Also:
-
reflectionHashCode
Uses reflection to build a valid hash code from the fields ofobject.This constructor uses two hard coded choices for the constants needed to build a hash code.
It uses
AccessibleObject.setAccessibleto 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 theObject.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.
- Parameters:
object- the Object to create ahashCodefortestTransients- whether to include transient fields- Returns:
- int hash code
- Throws:
NullPointerException- if the object isnull- See Also:
-
reflectionHashCode
Uses reflection to build a valid hash code from the fields ofobject.This constructor uses two hard coded choices for the constants needed to build a hash code.
It uses
AccessibleObject.setAccessibleto 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.
- Parameters:
object- the Object to create ahashCodeforexcludeFields- Collection of String field names to exclude from use in calculation of hash code- Returns:
- int hash code
- Throws:
NullPointerException- if the object isnull- See Also:
-
reflectionHashCode
Uses reflection to build a valid hash code from the fields ofobject.This constructor uses two hard coded choices for the constants needed to build a hash code.
It uses
AccessibleObject.setAccessibleto 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.
- Parameters:
object- the Object to create ahashCodeforexcludeFields- array of field names to exclude from use in calculation of hash code- Returns:
- int hash code
- Throws:
NullPointerException- if the object isnull- See Also:
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append
Append ahashCodefor aboolean.This adds
1when true, and0when false to thehashCode.This is in contrast to the standard
java.lang.Boolean.hashCodehandling, which computes ahashCodevalue of1231forjava.lang.Booleaninstances that representtrueor1237forjava.lang.Booleaninstances that representfalse.This is in accordance with the Effective Java design.
- Parameters:
value- the boolean to add to thehashCode- Returns:
- this
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append
Append ahashCodefor abooleanarray.- Parameters:
array- the array to add to thehashCode- Returns:
- this
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append
Append ahashCodefor abyte.- Parameters:
value- the byte to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor abytearray.- Parameters:
array- the array to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor achar.- Parameters:
value- the char to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor achararray.- Parameters:
array- the array to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor adouble.- Parameters:
value- the double to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor adoublearray.- Parameters:
array- the array to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor afloat.- Parameters:
value- the float to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor afloatarray.- Parameters:
array- the array to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor anint.- Parameters:
value- the int to add to thehashCode- Returns:
- this
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append
Append ahashCodefor anintarray.- Parameters:
array- the array to add to thehashCode- Returns:
- this
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append
Append ahashCodefor along.- Parameters:
value- the long to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor alongarray.- Parameters:
array- the array to add to thehashCode- Returns:
- this
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append
Append ahashCodefor anObject.- Parameters:
object- the Object to add to thehashCode- Returns:
- this
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append
Append ahashCodefor anObjectarray.- Parameters:
array- the array to add to thehashCode- Returns:
- this
-
append
Append ahashCodefor ashort.- Parameters:
value- the short to add to thehashCode- Returns:
- this
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append
Append ahashCodefor ashortarray.- Parameters:
array- the array to add to thehashCode- Returns:
- this
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appendSuper
Adds the result of super.hashCode() to this builder.- Parameters:
superHashCode- the result of callingsuper.hashCode()- Returns:
- this
- Since:
- 2.0
-
build
Returns the computedhashCode. -
equals
Implements equals using the hash code. -
hashCode
The computedhashCodefrom 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. -
toHashCode
Returns the computedhashCode.- Returns:
hashCodebased on the fields appended
-