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According to the Java API \[[API 2006|AA. JavaBibliography#API References#API 06]\] class {{java.lang.Object}} documentation: |
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This compliant solution shows how the {{hashCode()}} method can be overridden so that the same value is generated for any two instances that compare equal when {{Object.equals()}} is used. Bloch discusses the recipe to generate such a hash function in good detail \[[Bloch 2008|AA. JavaBibliography#Bloch References#Bloch 08]\]. |
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import java.util.Map;
import java.util.HashMap;
public final class CreditCard {
private final int number;
public CreditCard(int number) {
this.number = (short) number;
}
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof CreditCard)) {
return false;
}
CreditCard cc = (CreditCard)o;
return cc.number == number;
}
public int hashCode() {
int result = 7;
result = 37 * result + number;
return result;
}
public static void main(String[] args) {
Map<CreditCard, String> m = new HashMap<CreditCard, String>();
m.put(new CreditCard(100), "Java");
System.out.println(m.get(new CreditCard(100)));
}
}
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\[[API 2006|AA. Java References#APIBibliography#API 06]\] [Class Object|http://java.sun.com/javase/6/docs/api/java/lang/Object.html] \[[Bloch 2008|AA. Java References#BlochBibliography#Bloch 08]\] Item 9: Always override {{hashCode}} when you override {{equals}} \[[MITRE 2009|AA. Java References#MITREBibliography#MITRE 09]\] [CWE ID 581|http://cwe.mitre.org/data/definitions/581.html] "Object Model Violation: Just One of Equals and Hashcode Defined" |
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