RealDeanZhao/lazily-initialized-hash-code.md

## lazily-initialized-hash-code.md

      
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              lazily-initialized-hash-code.md
            
          
If a class is immutable and the cost of computing the hash code is significant, you might consider caching the hash code in the object rather than recalculating it each time it is requested. If you believe that most objects of this type will be used as hash keys, then you should calculate the hash code when the instance is created. Otherwise, you might choose to lazily initialize it the first time hashCode is invoked (Item 71).

// Lazily initialized, cached hashCode
private volatile int hashCode;  // (See Item 71)
@Override public int hashCode() {
       int result = hashCode;
       if (result == 0) {
           result = 17;
           result = 31 * result + areaCode;
           result = 31 * result + prefix;
           result = 31 * result + lineNumber;
           hashCode = result;
       }
       return result;
}