a.rb

 
// Lazily initialized, cached hashCode 
private volatile int hashCode = 0;  // (See Item 48) 
 
public int hashCode() { 
    if (hashCode == 0) { 
        int result = 17; 
        result = 37*result + areaCode; 
        result = 37*result + exchange; 
        result = 37*result + extension; 
        hashCode = result; 
    } 
    return hashCode; 
}