Equals

if a class overrides equals, it must override hashCode.

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

    // Overriding equals() to compare two Complex objects
    @Override
    public boolean equals(Object o) {

        // If the object is compared with itself then return true
        if (o == this) {
            return true;
        }

        /* Check if o is an instance of Complex or not
          "null instanceof [type]" also returns false */
        if (!(o instanceof Complex)) {
            return false;
        }

        // typecast o to Complex so that we can compare data members
        Complex c = (Complex) o;

        // Compare the data members and return accordingly
        return Double.compare(re, c.re) == 0
                && Double.compare(im, c.im) == 0;
    }

      public int hashCode() {
        int hash = 7;
        hash = 31 * hash + re.hashCode();
        hash = 31 * hash + im.hashCode();
        return hash;
    }