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420240
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2009-01-07T13:05:40.493
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2017-11-13T20:12:26.453
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From <a href="http://web.archive.org/web/20090126135350/http://leepoint.net/notes-java/data/expressions/22compareobjects.html" rel="noreferrer">JavaNotes</a>: <ul> <li><code>a.compareTo(b)</code>: Comparable interface : Compares values and returns an int which tells if the values compare less than, equal, or greater than. If your class objects have a natural order, implement the <code>Comparable<T></code> interface and define this method. All Java classes that have a natural ordering implement <code>Comparable<T></code> - Example: <code>String</code>, <a href="http://www.javatpoint.com/wrapper-class-in-java" rel="noreferrer">wrapper classes</a>, <code>BigInteger</code></li> <li><code>compare(a, b)</code>: Comparator interface : Compares values of two objects. This is implemented as part of the <code>Comparator<T></code> interface, and the typical use is to define one or more small utility classes that implement this, to pass to methods such as <code>sort()</code> or for use by sorting data structures such as <code>TreeMap</code> and <code>TreeSet</code>. You might want to create a Comparator object for the following: <ul> <li>Multiple comparisons. To provide several different ways to sort something. For example, you might want to sort a Person class by name, ID, age, height, ... You would define a Comparator for each of these to pass to the <code>sort()</code> method.</li> <li>System class To provide comparison methods for classes that you have no control over. For example, you could define a Comparator for Strings that compared them by length.</li> <li>Strategy pattern To implement a Strategy pattern, which is a situation where you want to represent an algorithm as an object that you can pass as a parameter, save in a data structure, etc.</li> </ul></li> </ul> If your class objects have one natural sorting order, you may not need compare(). <hr> Summary from <a href="http://www.digizol.com/2008/07/java-sorting-comparator-vs-comparable.html" rel="noreferrer">http://www.digizol.com/2008/07/java-sorting-comparator-vs-comparable.html</a> Comparable A comparable object is capable of comparing itself with another object. Comparator A comparator object is capable of comparing two different objects. The class is not comparing its instances, but some other class’s instances. <hr> Use case contexts: Comparable interface The equals method and <code>==</code> and <code>!=</code> operators test for equality/inequality, but do not provide a way to test for relative values. Some classes (eg, String and other classes with a natural ordering) implement the <code>Comparable<T></code> interface, which defines a <code>compareTo()</code> method. You will want to implement <code>Comparable<T></code> in your class if you want to use it with <code>Collections.sort()</code> or <code>Arrays.sort()</code> methods. Defining a Comparator object You can create Comparators to sort any arbitrary way for any class. For example, the <code>String</code> class defines the <a href="http://www.java2s.com/Code/JavaAPI/java.lang/StringCASEINSENSITIVEORDER.htm" rel="noreferrer"><code>CASE_INSENSITIVE_ORDER</code> comparator</a>. <hr> The difference between the two approaches can be linked to the notion of: Ordered Collection: When a Collection is ordered, it means you can iterate in the collection in a specific (not-random) order (a <code>Hashtable</code> is not ordered). A Collection with a natural order is not just ordered, but sorted. Defining a natural order <a href="http://www.codinghorror.com/blog/archives/001018.html" rel="noreferrer">can be difficult!</a> (as in <a href="http://web.archive.org/web/20090310030619/http://weblogs.java.net/blog/skelvin/archive/2006/01/natural_string.html" rel="noreferrer">natural String order</a>). <hr> Another difference, pointed out by <a href="https://stackoverflow.com/users/57344/haveaguess">HaveAGuess</a> in <a href="https://stackoverflow.com/questions/420223/what-is-the-difference-between-compare-and-compareto/420240?noredirect=1#comment46816131_420240">the comments</a>: <ul> <li><code>Comparable</code> is in the implementation and not visible from the interface, so when you sort you don't really know what is going to happen. </li> <li><code>Comparator</code> gives you reassurance that the ordering will be well defined. </li> </ul>
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1. CO+1 for the JavaNotes references. Didn't know about it.
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