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POInheritance - using super.equals() in subclasses that override methods used in equals of superclass
 

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    <p>I've been testing a code and stumbled across a problem: Should you call <code>super.equals()</code> method in subclass that can override some of the methods used in <code>equals()</code> method of the super class?</p> <p>Let's consider the following code:</p> <pre><code>public abstract class Item { private int id; private float price; public Item(int id, String name, float price, String category) { this.id = id; this.name = name; this.price = price; this.category = category; } public int getID() { return id; } public float getPrice() { return price; } @Override public boolean equals(Object object){ if(object instanceof Item){ Item item = (Item) object; if( id == item.getID() &amp;&amp; price == item.getPrice()) { return true; } } return false; } } </code></pre> <p>And the subclass DiscountedItem:</p> <pre><code>public class DiscountedItem extends Item { // discount stored in % private int discount; @Override public boolean equals(Object object) { if(object instanceof DiscountedItem){ DiscountedItem item = (DiscountedItem) object; return (super.equals(item) &amp;&amp; discount == item.getDiscount() ); } return false; } public int getDiscount() { return discount; } @Override public float getPrice() { return super.getPrice()*(100 - discount); } } </code></pre> <p>I've been just re-reading <a href="http://www.angelikalanger.com/Articles/JavaSolutions/SecretsOfEquals/Equals.html" rel="nofollow">Angelika Langer's secrets of equals()</a>, where she even states:</p> <blockquote> <p>There is agreement that super.equals() should be invoked if the class has a superclass other than Object.</p> </blockquote> <p>But I think it's highly unpredictable when the subclass will override some of the methods. For instance when I compare 2 <code>DiscountedItem</code> objects using equals, the super method is called and <code>item.getPrice()</code> is dynamically dispatched to the correct method in the subclass <code>DiscountedItem</code>, whereas the other price value is accessed directly using variable.</p> <p>So, is it really up to me (as I should implement the method correctly) or is there a way around it? </p>
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    1. COYour subclass' `equals` method should be implemented in a manner that is consistent with the contract of that method. The reason to delegate to `super.equals()` is that the child class probably can't access fields of the parent that `super.equals()` may depend on. If you violate encapsulation, all bets are off. If you implement your `super.equals()` poorly by enabling a child class to break the contract of its parent, then again, all bets are off.
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    2. COOne could argue that the root problem here is embedding business logic in a bean getter, and not the more academic question asked :)
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    3. CO@Affe Oh this isn't even a real web application, it's just an academic example given in software testing class :) But I don't think this error was made on purpose..
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