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.
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 hashcode 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).
hashCode();
}
}
If required, the superclass hashCode() can be added using 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);
}
| Constructor and Description |
|---|
HashCodeBuilder()
Uses two hard coded choices for the constants needed to build a hashCode.
|
HashCodeBuilder(int initialOddNumber,
int multiplierOddNumber)
Two randomly chosen, odd numbers must be passed in.
|
| Modifier and Type | Method and Description |
|---|---|
HashCodeBuilder |
append(Object... objectValues)
Append a hashCode for an Object.
|
HashCodeBuilder |
append(Object objectValue)
Append a hashCode for an Object.
|
HashCodeBuilder |
appendSuper(int superHashCode)
Adds the result of super.hashCode() to this builder.
|
boolean |
equals(Object obj) |
int |
hashCode()
The generated hash code
|
static int |
reflectionsHashCode(Object object,
String... excludeFields)
Uses reflection to build a valid hash code from the fields of object.
|
public HashCodeBuilder(int initialOddNumber,
int multiplierOddNumber)
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.
initialOddNumber - an odd number used as the initial valuemultiplierOddNumber - an odd number used as the multiplierIllegalArgumentException - if the number is evenpublic HashCodeBuilder()
public HashCodeBuilder append(Object objectValue)
objectValue - the Object to add to the hashCodepublic HashCodeBuilder append(Object... objectValues)
objectValues - array of Object to add to the hashCodepublic HashCodeBuilder appendSuper(int superHashCode)
superHashCode - the result of calling super.hashCode()public int hashCode()
public static int reflectionsHashCode(Object object, String... excludeFields)
Uses reflection to build a valid hash code from the fields of object.
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. If no fields are found to include in the hash code, the result of this method will be constant.
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 nullCopyright © 2017. All rights reserved.