public class MComplex {
   double re; protected double im;

   public MComplex (double re, double im) { this.re = re; this.im = im; }

   public double getReal()      { return re; }
   public double getImaginary() { return im; }

   public void setReal(double re)      { this.re = re; }
   public void setImaginary(double im) { this.im = im; }

   public void add (MComplex c) { re += c.re; im += c.im; }

   public void subtract (MComplex c) { re -= c.re; im -= c.im; }

   public void multiply (MComplex c) {
      double r = re * c.re - im * c.im;
      double i = re * c.im + im * c.re;
      re = r; im = i;
   }

   public void divide (MComplex c) {
      double den = c.re * c.re + c.im * c.im;
      double r = (re * c.re - im * c.im) / den;
      double i = (re * c.im + im * c.re) / den;
      re = r; im = i;
   }

   @Override public boolean equals (Object o) {
     if (o == this)               return true;
     if (!(o instanceof MComplex)) return false;
     MComplex c = (MComplex) o;

     // See Bloch page 43 to find out why to use compare() instead of ==
     return Double.compare(re, c.re) == 0 &&
            Double.compare(im, c.im) == 0;
   }

   @Override public int hashCode () {
      int result = 17 + hashDouble(re);
      result = 31 * result + hashDouble(im);
      return result;
   }

   private int hashDouble (double val) {
      long longBits = Double.doubleToLongBits(val);
      return (int) (longBits ^ (longBits >>>32));
   }

   @Override public String toString() { return "(" + re + " + " + im + "i)"; }
}

Before we get to immutability, consider the method contracts. Where do the various contracts "come from", and is there anything in the (missing) JavaDoc that might require a bit of research?

Apply each of Bloch's 5 rules for making a class immutable:

1. Don't provide any methods that modify the object's state. How do you handle the mutators?
   
   
   
   
2. Ensure that no methods can be overridden.
   
   Why is this a problem? Show me!
   
   Fix the problem:
   
   
   • Change the class declaration, or
     
     
   • Change the method declarations, or
     
     
   • Change the constructor visibility.
   
   
   
   
   
3. Make all fields final.
   
   
   
   
4. Make all fields private.
   
   Is there a significant difference in visibility between re and im?
   
   
   
   
5. Ensure exclusive access to any mutable components.