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copied!<blockquote> <p>what are best practices to implement equals, semantically, not technically.</p> </blockquote> <p>In Java the <code>equals</code> method really should be considered to be <a href="http://today.java.net/pub/a/today/2006/07/27/defining-object-identity.html" rel="nofollow noreferrer">"identity equals"</a> because of how it integrates with <code>Collection</code> and <code>Map</code> implementations. Consider the following:</p> <pre><code> public class Foo() { int id; String stuff; } Foo foo1 = new Foo(10, "stuff"); fooSet.add(foo1); ... Foo foo2 = new Foo(10, "other stuff"); fooSet.add(foo2); </code></pre> <p>If <code>Foo</code> identity is the <code>id</code> field then the 2nd <code>fooSet.add</code> should <em>not</em> add another element to the <code>Set</code> but should replace the first one since they have the same <code>id</code>. If you define <code>Foo.equals</code> (and hashCode) method to include <em>both</em> the <code>id</code> and the <code>stuff</code> fields then this will be broken and the <code>Set</code> may contain 2 references to the object with the same id field.</p> <p>If you are not storing your objects in a <code>Collection</code> (or <code>Map</code>) then you don't have to define the <code>equals</code> method this way, however it is considered by many to be bad form. If in the future you <em>do</em> store it in a <code>Collection</code> then things will be break.</p> <p>If I <em>need</em> to test for equality of all fields, I tend to write another method. Something like <code>equalsAllFields(Object obj)</code> or some such.</p> <p>Then you would do something like:</p> <pre><code>assertTrue(obj1.equalsAllFields(obj2)); </code></pre> <p>In addition, a proper practice is to <em>not</em> define <code>equals</code> methods which take into account mutable fields. The problem also gets difficult when we start talking about class hierarchies. If a child object defines <code>equals</code> as a combination of its local fields <em>and</em> the base class <code>equals</code> then its symmetry has been violated:</p> <pre><code> Point p = new Point(1, 2); // ColoredPoint extends Point ColoredPoint c = new ColoredPoint(1, 2, Color.RED); // this is true because both points are at the location 1, 2 assertTrue(p.equals(c)); // however, this would return false because the Point p does not have a color assertFalse(c.equals(p)); </code></pre> <p>Some more reading I would highly recommend is the "Pitfall #3: Defining equals in terms of mutable fields" section in this great page:</p> <blockquote> <p><a href="http://www.artima.com/lejava/articles/equality.html" rel="nofollow noreferrer">How to Write an Equality Method in Java</a> </p> </blockquote> <p>Some additional links:</p> <ul> <li><a href="http://tutorials.jenkov.com/java-collections/hashcode-equals.html" rel="nofollow noreferrer">Implementing hashCode() and equals()</a></li> <li><a href="http://gracelang.org/applications/2011/02/07/values-equals-and-hashcodes/" rel="nofollow noreferrer">Graceful Blog - Values, Equals, and Hashcodes</a></li> </ul> <p>Oh, and just for posterity, regardless of what fields you choose to compare to determine equality, you need to use the same fields in the <code>hashCode</code> calculation. <code>equals</code> and <code>hashCode</code> must be symmetric. If two objects are equals, they <em>must</em> have the same hash-code. The opposite is not necessarily true.</p>

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