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Date: Sat, 24 Oct 2009 13:02:59 +0000 (UTC)
From: "Stephen Colebourne (JIRA)" <jira@apache.org>
To: issues@commons.apache.org
Subject: [jira] Updated: (LANG-467) EqualsBuilder and HashCodeBuilder treat
 java.math.BigDecimal inconsistantly and break general contract of hashCode
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     [ https://issues.apache.org/jira/browse/LANG-467?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Stephen Colebourne updated LANG-467:
------------------------------------

    Priority: Major  (was: Minor)

We can't have EqualsBuilder and HashCodeBuilder out of line.

My preferred fix would be to revert the invalid change to EqualsBuilder.

> EqualsBuilder and HashCodeBuilder treat java.math.BigDecimal inconsistantly and break general contract of hashCode
> ------------------------------------------------------------------------------------------------------------------
>
>                 Key: LANG-467
>                 URL: https://issues.apache.org/jira/browse/LANG-467
>             Project: Commons Lang
>          Issue Type: Bug
>    Affects Versions: 2.4
>            Reporter: David Jones
>             Fix For: 3.0
>
>
> A POJO with a BigDecimal field and equals() and hashCode() methods implemented using EqualsBuilder and HashCodeBuilder breaks the general contract of Object.hashCode();
> EqualsBuilder treats BigDecimal specially by comparing it using BigDecimal.compareTo() == 0 rather than BigDecimal.equals()
> Using BigDecimal.compareTo() ignores the scale of the BigDecimal()
> However the append(Object o) method of HashCodeBuilder uses BigDecimal.hashCode() in the case that o is a BigDecimal, which takes the scale into account when generating the hashCode.
> The following test case shows the problem!
> {code:title=TestApacheCommonsLangHashCodeBuilder.java|borderStyle=solid}
> // package declaration and imports not shown
> public class TestApacheCommonsLangHashCodeBuilder extends TestCase {
>     
>     public void testHashCode() {
>         MyPojo myPojo1 = new MyPojo(new String("foo"), new BigDecimal("10.2"));
>         MyPojo myPojo2 = new MyPojo(new String("foo"), new BigDecimal("10.20"));
>         
>         // equals method ignores the scale of the big decimal
>         // so this test passes
>         assertTrue(myPojo1.equals(myPojo2));
>         
>         // however in the case the equals returns true the
>         // hashCode must be the same according to the contract
>         // TEST FAILS AT THIS LINE
>         assertEquals(myPojo1.hashCode(), myPojo2.hashCode());
>     }
>     
>     private class MyPojo {
>         private String name;
>         private BigDecimal value;
>         
>         public MyPojo(String name, BigDecimal value) {
>             this.name = name;
>             this.value = value;
>         }
>         
>         public String getName() {
>             return name;
>         }
>         public BigDecimal getValue() {
>             return value;
>         }
>         /**
>          * equals method implemented using EqualsBuilder 
>          * as documented by apache commons
>          */
>         @Override public boolean equals(Object obj) {
>             if(this == obj) {
>                 return true;
>             }
>             
>             if(!(obj instanceof MyPojo)) {
>                 return false;
>             }
>             
>             MyPojo other = (MyPojo) obj;
>             return new EqualsBuilder()
>                 .append(name, other.getName())
>                 .append(value, other.getValue())
>                 .isEquals();
>         }
>         
>         /**
>          * hashCode method implemented using HashCodeBuilder
>          * as documented by apache commons
>          */
>         @Override public int hashCode() {
>             HashCodeBuilder hcb = new HashCodeBuilder(17, 31);
>             return hcb
>                 .append(name)
>                 .append(value)
>                 .toHashCode();
>         }
>     }
> }
> {code}
> Note that the only reason I haven't provided a patch is because I could not think of one which I thought was reasonable.
> *Option 1*
> Always set the scale to some value and then get the hashCode()
> Example change in HashCodeBuilder.append(Object) add the following:
> {code}
> else if (object instanceof BigDecimal) {
> 	append(((BigDecimal) object).setScale(DEFAULT_BIGDECIMAL_SCALE, RoundingMode.DOWN).hashCode());
> }
> {code}
> However what is a reasonable scale for calculating this hashCode? I cannot see a reasonanble scale to choose, that depends on the circumstance
> *Option 2*
> stripTrailingZeros() before calculating the hashCode()
> Example change in HashCodeBuilder.append(Object) add the following:
> {code}
> else if (object instanceof BigDecimal) {
> 	append(((BigDecimal) object).stripTrailingZeros().hashCode());
> }
> {code}
> The performance of this method under different circumstances is not documented.
> *Option 3*
> Document the problem and propose that the client solves the problem.
> For example change HashCodeBuilder documentation as follows
> {code}
> /*
>  * ...
>  * public class Person {
>  *   String name;
>  *   int age;
>  *   boolean smoker;
>  *   BigDecimal netWorth;
>  *   ...
>  *
>  *   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).
>  *       // take special care when using BigDecimal as scale takes 
>  *       // is included in the hashCode calculation breaking hashCode contract
>  *       // choose a scale which is reasonable for hashCode calculation
>  *       append(netWorth == null ? null : netWorth.setScale(2)).
>  *       toHashCode();
>  *   }
>  * }
>  * ...
>  */
> {code}

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