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    <p>There is no polymorphism for fields in Java. There is however, inheritance. What you've effectively done is create two fields in your Rectangle class, with the same name. The names of the field are, effectively:</p> <pre><code>public class Rectangle { public int Shape.x; public int Rectangle.x; } </code></pre> <p><em>The above doesn't represent valid Java, its just an illustration of how the fields are scoped in your class</em></p> <p>Within the entire scope of the Rectangle class, the superclass field of the same name is <em>hidden</em>. So anytime you reference the simple name <code>x</code>, or the scoped name <code>this.x</code>, within the class, you are referring to the field that is defined in <code>Rectangle</code>. You can actually access the superclass field as well, with the scoped name <code>super.x</code>.</p> <p>Now, from outside of the class, the rules for which field is being accessed is slightly different. The scope will be determined by the <strong><em>compile</em></strong> time type of the class that the field is being referenced from. So in your code:</p> <pre><code>Shape s = new Shape(); Rectangle r = new Rectangle(); s = r; System.out.println(s.x); </code></pre> <p>The output is <code>0</code> because the compile time type of <code>s</code> is <code>Shape</code> (not <code>Rectangle</code>). You can observe a change in this behavior when you do this:</p> <pre><code>Shape s = new Shape(); Rectangle r = new Rectangle(); s = r; System.out.println(((Rectangle)s).x); </code></pre> <p>Presto! Your output is now <code>1</code>, because the compiler sees that you've scoped the field access to <code>Rectangle</code>.</p> <p>To condense the rules of visibility:</p> <p>You can read more about instance variable hiding in the <a href="http://docs.oracle.com/javase/specs/jls/se5.0/html/classes.html#8.3.3.2" rel="noreferrer">JLS, Section 8.3.3.2</a></p>
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    1. COThank now I understand! So do the methods use rectangles implementation as I said when using s.getArea()? I know they appear to but what is actually happening at compile time? Thanks
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    2. CO@PragmaticProgrammer - the rules for methods is different, for one thing, they actually can be overriden. Remember how I mentioned that for hidden variables, resolution is done at compile time? Well for overriden methods, resolution is done at runtime. So, if you did `Shape s = new Rectangle(); s.getArea();` the method called is for the subclass `Rectangle`, and not for the superclass `Shape`.
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