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    <p>Any type which doesn't give you any means to change the data within it is immutable - it's as simple as that. Yes, all the primitive wrapper types are immutable<sup>1</sup>, as is <code>String</code>. <a href="http://download.oracle.com/javase/6/docs/api/java/util/UUID.html" rel="noreferrer">UUID</a>, <a href="http://download.oracle.com/javase/6/docs/api/java/net/URL.html" rel="noreferrer">URL</a> and <a href="http://download.oracle.com/javase/6/docs/api/java/net/URI.html" rel="noreferrer"><code>URI</code></a> are other examples.</p> <p>Although <code>Calendar</code> and <code>Date</code> in the built-in Java API are mutable, many of the types within <a href="http://joda-time.sf.net" rel="noreferrer">Joda Time</a> are immutable - and to my mind, this is <em>one</em> reason why Joda Time is easier to work with. If an object is immutable, you can keep a reference to it somewhere else in your code and not have to worry about whether or not some other piece of code is going to make changes - it's easier to <em>reason</em> about your code.</p> <hr> <p><sup>1</sup> by which I mean <code>java.lang.Integer</code> etc. As noted elsewhere, the <code>Atomic*</code> classes are mutable, and indeed <em>have</em> to be in order to serve their purpose. There's a difference in my mind between "the standard set of primitive wrapper classes" and "the set of classes which wrap primitive values".</p> <p>You can write your own mutable wrapper class very easily:</p> <pre><code>public class MutableInteger { private int value; public MutableInteger(int value) { this.value = value; } public int getValue() { return value; } public void setValue(int value) { this.value = value; } } </code></pre> <p>So as you can see, there's nothing <em>inherently</em> immutable about wrapper classes - it's just that the standard ones were <em>designed</em> to be immutable, by virtue of not providing any way to change the wrapped value.</p> <p>Note that this allows for the same object to be used repeatedly when boxing, for common values:</p> <pre><code>Integer x = 100; Integer y = 100; // x and y are actually guaranteed to refer to the same object Integer a = 1000; Integer b = 1000; // a and b *could* refer to the same object, but probably won't </code></pre>
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    1. CORight about Joda Time. This is also one of the reasons why it's being considered as the base for a new time API in the JDK.
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    2. CO@arjan: Actually JSR 310 is somewhat different from Joda Time, but is being led by the same person. And yes, immutability is one aspect of it :)
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    3. COYou're right, it's somewhat different indeed, but the way I understood it it's not completely different. That's why I mentioned it's the *base* of a new implementation ;)
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