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 {@link #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); }
@author Stephen Colebourne
@author Gary Gregory
@author Pete Gieser
@since 1.0
@version $Id: HashCodeBuilder.java 161243 2005-04-14 04:30:28Z ggregory $
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). toHashCode(); } } If required, the superclass hashCode() can be added using {@link #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); }
@author Apache Software Foundation
@author Gary Gregory
@author Pete Gieser
@since 1.0
@version $Id: HashCodeBuilder.java 960834 2010-07-06 07:36:38Z bayard $
Alternatively, there is a method that uses reflection to determine the fields to test. Because these fields are usually private, the method, reflectionHashCode, uses Field.setAccessible to change the visibility of the fields. This will fail under a security manager, unless the appropriate permissions are set. It is also slower than testing explicitly.
A typical invocation for this method would look like:
public boolean hashCode(Object o) { return HashCodeBuilder.reflectionHashCode(this); }
@author Stephen Colebourne
@version $Id: HashCodeBuilder.java 517121 2007-03-12 07:45:49Z ate $
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