Java Doc for HashCodeBuilder.java in  » Portal » jetspeed-2.1.3 » org » apache » jetspeed » util » Java Source Code / Java DocumentationJava Source Code and Java Documentation

This class enables a good hashcode to be built for any class. It follows the rules laid out in the book Effective Java, by Joshua Bloch. Writing a good hashCode is actually quite difficult. This class aims to simplify the process.

All relevant fields from the object should be included in the hashCode. 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();
 }
 }
 

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);
 }
 

It uses Field.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manger, if the permissions are not set up. 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. This constructor uses two hard coded choices for the constants needed to build a hash code.
Parameters:
  object - the object to create a hash code for int hash code
throws:
  IllegalArgumentException - if the object is null

It uses Field.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manger, if the permissions are not set up. 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. This constructor uses two hard coded choices for the constants needed to build a hash code.
Parameters:
  object - the object to create a hash code for
Parameters:
  testTransients - whether to include transient fields int hash code
throws:
  IllegalArgumentException - if the object is null

It uses Field.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manger, if the permissions are not set up. 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.

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
Parameters:
  multiplierNonZeroOddNumber - a non-zero, odd number used as the multiplier
Parameters:
  object - the object to create a hash code for int hash code
throws:
  IllegalArgumentException - if the object is null
throws:
  IllegalArgumentException - if the number is zero or even

It uses Field.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manger, if the permissions are not set up. 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.

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 -
Parameters:
  multiplierNonZeroOddNumber -
Parameters:
  object - the object to create a hash code for
Parameters:
  testTransients - whether to include transient fields int hash code
throws:
  IllegalArgumentException - if the object is null
throws:
  IllegalArgumentException - if the number is zero or even