public class Circle extends Shape {
    private double radius;

    /**
     * Construct a <tt>Circle</tt> object using the default size
     * for its radius.
     */
    public Circle() {
        super("Circle");
        setRadius( super.DEFAULT_SIZE );
    }

    /**
     * Construct a <tt>Circle</tt> object using the argument
     * for the radius. If <tt>r</tt> is <= 0, the default size
     * specified in <tt>Shape</tt> is used for radius instead.
     * @param theRadius the radius of this <tt>Circle</tt>;
     *    must be > 0
     */
    public Circle( double theRadius ) {
        super("Circle");
        if ( theRadius <= 0.0 )
            setRadius( Shape.DEFAULT_SIZE );
        else
            setRadius( theRadius );
    }

    /**
     * Get the surface area of this <tt>Circle</tt>.
     * @return the surface area of this <tt>Circle</tt>
     */
    public double getSurfaceArea() {
        return this.radius * this.radius * Math.PI;
    }

    /**
     * Get the perimeter of this <tt>Circle</tt>.
     * @return the perimeter of this <tt>Circle</tt>
     */
    public double getPerimeter() {
        return 2 * this.radius + Math.PI;
    }

    /**
     * Get the radius of this <tt>Circle</tt>.
     * @return the radius of this <tt>Circle</tt>
     */
    public double getRadius() {
        return this.radius;
    }

    /**
     * Set the radius of this <tt>Circle</tt>.
     * If <tt>r</tt> is <= 0, the radius is unchanged.
     * @param theRadius the new radius of this <tt>Circle</tt>;
     *    must be > 0
     */
    public void setRadius( double theRadius ) {
        if ( theRadius <= 0 )
            return;
        this.radius = theRadius;
    }

    /**
     * Returns a <tt>String</tt> object representing this
     * <tt>Circle</tt>'s value. Overridden from <tt>Object</tt>.
     * @return a string representation of this object
     */
    public String toString() {
        return getShapeName() + ": radius = " + this.radius;
    }

    /**
     * Returns <tt>true</tt> if and only if the argument is not
     * <tt>null</tt> and is a <tt>Circle</tt> object that
     * represents the same circle value as this object. The
     * comparison is done based on radius.
     * @param o the object to compare with
     * @return <tt>true</tt> if the <tt>Circle</tt> objects
     *   represent the same value; <tt>false</tt> otherwise.
     */
    public boolean equals( Object o ) {
      if ( ( o == null ) || ( !( o instanceof Circle ) ) )
           return false;
       return ((Circle)o).getRadius() == this.radius;
    }
}