• Mutable Objects:
  • Have attributes that can be changed after they are initialized
• Immutable Objects:
  • Have attributes that can not be changed after they are initialized

• Related to Information Hiding:
  • Attributes can be declared public or private
• Related to Assignment:
  • Attributes and variables can be declared final or not

• Classes of Mutable Objects:
  • Attributes must not be final
  • Attributes can be public or private (depending on the methods included in the class)
• Classes of Immutable Objects:
  • Attributes must be either final or private (or both)

public class Point
{
    public double x, y;
}
  

public class Point
{
    public final double x, y;

    public Point(double x, double y)
    {
        this.x = x;
        this.y = y;
    }
}
  

public class Point
{
    private double x, y;

    public Point(double x, double y)
    {
        this.x = x;
        this.y = y;
    }

    public double getX()
    {
        return x;
    }

    public double getY()
    {
        return y;
    }
}
  

public class Point
{
    private double x, y;

    public Point(double x, double y)
    {
        this.x = x;
        this.y = y;
    }

    public double getX()
    {
        return x;
    }

    public double getY()
    {
        return y;
    }

    public void setX(double x)
    {
        this.x = x;
    }

    public void setY(double y)
    {
        this.y = y;
    }
}
  

• Convenience:
  • For the class of mutable objects, the implementation with public attributes seems much more convenient
• Safety:
  • In some situations, public attributes are not safe because they can't take on all possible values

Another Class of Mutable Objects with Public Attributes

public class Rectangle
{
    public double x, y;
    public double width, height;
}
  

This implementation isn't safe if the width and the height can't be negative.

Another Class of Mutable Objects with Private Attributes

public class Rectangle
{
    private double x, y;
    private double width, height;

    public Rectangle(double x, double y, double width, double height)
    {
        setX(x);
        setY(y);
        setWidth(width);
        setHeight(height);
    }

    public void setHeight(double height)
    {
        if (height < 0) {

            y           = y + height;
            this.height = -height;
        }
    }

    public void setWidth(double width)
    {
        if (width < 0) {

            x           = x + width;
            this.width  = -width;
        }
    }

    public void setX(double x)
    {
        this.x = x;
    }


    public void setY(double y)
    {
        this.y = y;
    }
}
  

• A Reminder:
  • Attributes can be either primitive types or reference types
• A Common Confusion:
  • The immutability of classes that have attributes that are reference types

Another Rectangle Class

public class Rectangle
{
    public final double[] corner;
    public final double[] size;

    public Rectangle(double[] corner, double[] size)
    {
        this.corner = corner;
        this.size   = size;
    }
}
  

A Use of this Rectangle Class

    double[]  c = { 0.0,  0.0};
    double[]  s = {10.0, 20.0};
    Rectangle r;

    r    = new Rectangle(c, s);
    s[0] = 50.0;
  

• But the Attributes are final?
  • Correct, but nothing is being assigned to the attributes
• I'll Make the Attributes private!
  • That won't solve the problem

Still Another Rectangle Class

public class Rectangle
{
    private final Point corner;
    private final Point size;

    public Rectangle(Point corner, Point size)
    {
        this.corner = corner;
        this.size   = size;
    }
}
  

A Use of this Rectangle Class

    Point     c = new Point( 0.0,  0.0);
    Point     s = new Point(10.0, 20.0);
    Rectangle r = new Rectangle(c, s);

    s.setX(50.0);
  

• What Causes the Behavior Above?
  • The use of aliases
• Can Anything be Done?
  • Make a deep copy

Still Another Rectangle Class

public class Rectangle
{
    private final Point corner;
    private final Point size;

    public Rectangle(Point corner, Point size)
    {
        this.corner = new Point(corner.getX(), corner.getY());
        this.size   = new Point(size.getX(), size.getY());
    }
}