public final class Complex {
  ...
  public static final Complex ZERO = new Complex(0);
  public Complex add(Complex that) {
    return new Complex(this.re + that.re,
		       this.im + that.im);
  }
  public boolean equals(Object x) {
    return x instanceof Complex && re == ...
  }
  public int hashCode() {
    long bits = Double.doubleToLongBits(re) ^ ...
    return (int)(bits ^ (bits >> 32));
  }
  ...