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POJava dynamic binding and method overriding
 

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    <p>Yesterday I had a two-hour technical phone interview (which I passed, woohoo!), but I completely muffed up the following question regarding dynamic binding in Java. And it's doubly puzzling because I used to teach this concept to undergraduates when I was a TA a few years ago, so the prospect that I gave them misinformation is a little disturbing...</p> <p>Here's the problem I was given:</p> <pre><code>/* What is the output of the following program? */ public class Test { public boolean equals( Test other ) { System.out.println( "Inside of Test.equals" ); return false; } public static void main( String [] args ) { Object t1 = new Test(); Object t2 = new Test(); Test t3 = new Test(); Object o1 = new Object(); int count = 0; System.out.println( count++ );// prints 0 t1.equals( t2 ) ; System.out.println( count++ );// prints 1 t1.equals( t3 ); System.out.println( count++ );// prints 2 t3.equals( o1 ); System.out.println( count++ );// prints 3 t3.equals(t3); System.out.println( count++ );// prints 4 t3.equals(t2); } } </code></pre> <p>I asserted that the output should have been two separate print statements from within the overridden <code>equals()</code> method: at <code>t1.equals(t3)</code> and <code>t3.equals(t3)</code>. The latter case is obvious enough, and with the former case, even though <code>t1</code> has a reference of type Object, it is instantiated as type Test, so dynamic binding should call the overridden form of the method.</p> <p>Apparently not. My interviewer encouraged me to run the program myself, and lo and behold, there was only a single output from the overridden method: at the line <code>t3.equals(t3)</code>.</p> <p>My question then is, why? As I mentioned already, even though <code>t1</code> is a reference of type Object (so static binding would invoke Object's <code>equals()</code> method), dynamic binding <em>should</em> take care of invoking the most specific version of the method based on the instantiated type of the reference. What am I missing?</p>
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