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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 m = new HashMap();
    m.put(new CreditCard(100), "Java");
    System.out.println(m.get(new CreditCard(100)));
  }
}