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POWhy is the comparator used instead of the equals() in Collections?
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  1. POWhy is the comparator used instead of the equals() in Collections?
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    copied!<p>There is a java bean Car that might contains two values: model and price.</p> <p>Now suppose I override equals() and hashcode() checking only for model in that way:</p> <pre><code>public boolean equals(Object o) { return this.model.equals(o.model); } public int hashCode() { return model.hashCode(); } </code></pre> <p>This permit me to check if an arraylist already contain an item Car of the same model (and doesn't matter the price), in that way:</p> <pre><code>List&lt;Car&gt; car = new ArrayList&lt;Car&gt;(); car.add(new Car("carA",100f)); car.add(new Car("carB",101f)); car.add(new Car("carC",110f)); System.out.println(a.contains(new Car("carB",111f))); </code></pre> <p>It returns TRUE. That's fine, because the car already exist!</p> <p>But now I decide that the ArrayList is not good, because I want to maintain the items ordered, so I substitute it with a TreeSet in this way:</p> <pre><code>Set&lt;Car&gt; car = new TreeSet&lt;Car&gt;(new Comparator&lt;Car&gt;() { @Override public int compare(Car car1, Car car2) { int compPrice = - Float.compare(car1.getPrice(), car2.getPrice()); if (compPrice &gt; 0 || compPrice &lt; 0) return compPrice; else return car1.getModel().compareTo(car2.getModel()); </code></pre> <p>}});</p> <pre><code>car.add(new Car("carA",100f)); car.add(new Car("carB",101f)); car.add(new Car("carC",110f)); System.out.println(a.contains(new Car("carB",111f))); </code></pre> <p>But now there is a problem, it return FALSE... why?</p> <p>It seems that when I invoke contains() using an arrayList the method equals() is invoked. But it seems that when I invoke contains() using a TreeSet with a comparator, the comparator is used instead.</p> <p>Why does that happen?</p>
 

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