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<div class="theme-default-content content__default"><h1 id="immutable-objects"><a href="#immutable-objects" class="header-anchor">#</a> Immutable Objects</h1> <h2 id="creating-an-immutable-version-of-a-type-using-defensive-copying"><a href="#creating-an-immutable-version-of-a-type-using-defensive-copying" class="header-anchor">#</a> Creating an immutable version of a type using defensive copying.</h2> <p>Some basic types and classes in Java are fundamentally mutable. For example, all array types are mutable, and so are classes like <code>java.util.Data</code>. This can be awkward in situations where an immutable type is mandated.</p> <p>One way to deal with this is to create an immutable wrapper for the mutable type. Here is a simple wrapper for an array of integers</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ImmutableIntArray</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> array<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">ImmutableIntArray</span><span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> array<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>array <span class="token operator">=</span> array<span class="token punctuation">.</span><span class="token function">clone</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">getValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">clone</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>This class works by using <strong>defensive copying</strong> to isolate the mutable state (the <code>int[]</code>) from any code that might mutate it:</p> <li>
The constructor uses `clone()` to create a distinct copy of the parameter array. If the caller of the constructor subsequent changed the parameter array, it would not affect the state of the `ImmutableIntArray`.
</li> <li>
The `getValue()` method also uses `clone()` to create the array that is returned. If the caller were to change the result array, it would not affect the state of the `ImmutableIntArray`.
</li> <p>We could also add methods to <code>ImmutableIntArray</code> to perform read-only operations on the wrapped array; e.g. get its length, get the value at a particular index, and so on.</p> <p>Note that an immutable wrapper type implemented this way is not type compatible with the original type. You cannot simply substitute the former for the latter.</p> <h2 id="the-recipe-for-an-immutable-class"><a href="#the-recipe-for-an-immutable-class" class="header-anchor">#</a> The recipe for an immutable class</h2> <p>An immutable object is an object whose state cannot be changed. An immutable class is a class whose instances are immutable by design, and implementation. The Java class which is most commonly presented as an example of immutability is <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/String.html" target="_blank" rel="noopener noreferrer">java.lang.String<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>.</p> <p>The following is a stereotypical example:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">String</span> ssn<span class="token punctuation">;</span> <span class="token comment">// (SSN == social security number)</span>
<span class="token keyword">public</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">,</span> <span class="token class-name">String</span> ssn<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>ssn <span class="token operator">=</span> ssn<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getSSN</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> ssn<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>A variation on this is to declare the constructor as <code>private</code> and provide a <code>public static</code> factory method instead.</p> <p>The <strong>standard recipe</strong> for an immutable class is as follows:</p> <ul><li>All properties must be set in the constructor(s) or factory method(s).</li> <li>There should be no setters.</li> <li>If it is necessary to include setters for interface compatibility reasons, they should either do nothing or throw an exception.</li> <li>All properties should be declared as <code>private</code> and <code>final</code>.
</li><li>For all properties that are references to mutable types:
<ul></ul></li> <li>the property should be initialized with a deep copy of the value passed via the constructor, and</li> <li>the property's getter should return a deep copy of the property value.</li></ul> <p>A couple of other things to note:</p> <ul><li>Immutability does not prevent object from being nullable; e.g. <code>null</code> can be assigned to a <code>String</code> variable.</li> <li>If an immutable classes properties are declared as <code>final</code>, instances are inherently thread-safe. This makes immutable classes a good building block for implementing multi-threaded applications.</li></ul> <h2 id="typical-design-flaws-which-prevent-a-class-from-being-immutable"><a href="#typical-design-flaws-which-prevent-a-class-from-being-immutable" class="header-anchor">#</a> Typical design flaws which prevent a class from being immutable</h2> <p><strong>Using some setters, without setting all needed properties in the constructor(s)</strong></p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token punctuation">{</span> <span class="token comment">// example of a bad immutability</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">String</span> surname<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> name<span class="token punctuation">;</span><span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getSurname</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> surname<span class="token punctuation">;</span><span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setSurname</span><span class="token punctuation">(</span><span class="token class-name">String</span> surname<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">this</span><span class="token punctuation">.</span>surname <span class="token operator">=</span> surname<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>It’s easy to show that <code>Person</code> class is not immutable:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">Person</span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Joe"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
person<span class="token punctuation">.</span><span class="token function">setSurname</span><span class="token punctuation">(</span><span class="token string">"Average"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// NOT OK, change surname field after creation</span>
</code></pre></div><p>To fix it, simply delete <code>setSurname()</code> and refactor the constructor as follows:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">,</span> <span class="token class-name">String</span> surname<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>surname <span class="token operator">=</span> surname<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><strong>Not marking instance variables as private and final</strong></p> <p>Take a look at the following class:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>The following snippet shows that the above class is not immutable:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">Person</span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Average Joe"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
person<span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token string">"Magic Mike"</span><span class="token punctuation">;</span> <span class="token comment">// not OK, new name for person after creation</span>
</code></pre></div><p>To fix it, simply mark name property as <code>private</code> and <code>final</code>.</p> <p><strong>Exposing a mutable object of the class in a getter</strong></p> <p>Take a look at the following class:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span></span><span class="token class-name">List</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span></span><span class="token class-name">ArrayList</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">Names</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> names<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">Names</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> names<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>names <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span>names<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> <span class="token function">getNames</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> names<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> names<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p><code>Names</code> class seems immutable at the first sight, but it is not as the following code shows:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> namesList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
namesList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Average Joe"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Names</span> names <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Names</span><span class="token punctuation">(</span>namesList<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>names<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1, only containing "Average Joe"</span>
namesList <span class="token operator">=</span> names<span class="token punctuation">.</span><span class="token function">getNames</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
namesList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Magic Mike"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>names<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 2, NOT OK, now names also contains "Magic Mike"</span>
</code></pre></div><p>This happened because a change to the reference List returned by <code>getNames()</code> can modify the actual list of <code>Names</code>.</p> <p>To fix this, simply avoid returning references that reference class's mutable objects <strong>either</strong> by making defensive copies, as follows:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> <span class="token function">getNames</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>names<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// copies elements</span>
<span class="token punctuation">}</span>
</code></pre></div><p><strong>or</strong> by designing getters in way that only other <strong>immutable objects</strong> and <strong>primitives</strong> are returned, as follows:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getName</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> names<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> names<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><strong>Injecting constructor with object(s) that can be modified outside the immutable class</strong></p> <p>This is a variation of the previous flaw. Take a look at the following class:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span></span><span class="token class-name">List</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">NewNames</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> names<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">Names</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> names<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>names <span class="token operator">=</span> names<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getName</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> names<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> names<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>As <code>Names</code> class before, also <code>NewNames</code> class seems immutable at the first sight, but it is not, in fact the following snippet proves the contrary:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> namesList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
namesList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Average Joe"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">NewNames</span> names <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">NewNames</span><span class="token punctuation">(</span>namesList<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>names<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1, only containing "Average Joe"</span>
namesList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Magic Mike"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>names<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 2, NOT OK, now names also contains "Magic Mike"</span>
</code></pre></div><p>To fix this, as in the previous flaw, simply make defensive copies of the object without assigning it directly to the immutable class, i.e. constructor can be changed as follows:</p> <div class="language- extra-class"><pre class="language-text"><code>
public Names(List<String> names) {
this.names = new ArrayList<String>(names);
}
</code></pre></div><p><strong>Letting the methods of the class being overridden</strong></p> <p>Take a look at the following class:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> name<span class="token punctuation">;</span><span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p><code>Person</code> class seems immutable at the first sight, but suppose a new subclass of <code>Person</code> is defined:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">MutablePerson</span> <span class="token keyword">extends</span> <span class="token class-name">Person</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token class-name">String</span> newName<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">MutablePerson</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> newName<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setName</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">{</span>
newName <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>now <code>Person</code> (im)mutability can be exploited through polymorphism by using the new subclass:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">Person</span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MutablePerson</span><span class="token punctuation">(</span><span class="token string">"Average Joe"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>person<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> prints <span class="token class-name">Average</span> <span class="token class-name">Joe</span>
person<span class="token punctuation">.</span><span class="token function">setName</span><span class="token punctuation">(</span><span class="token string">"Magic Mike"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// NOT OK, person has now a new name!</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>person<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// prints Magic Mike </span>
</code></pre></div><p>To fix this, <strong>either</strong> mark the class as <code>final</code> so it cannot be extended <strong>or</strong> declare all of its constructor(s) as <code>private</code>.</p> <h4 id="remarks"><a href="#remarks" class="header-anchor">#</a> Remarks</h4> <p>Immutable objects have fixed state (no setters), so all state must be known at object creation time.</p> <p>Although not technically required, it is best practice to make all fields <code>final</code>. This will make the immutable class thread-safe (cf. Java Concurrency in Practice, 3.4.1).</p> <p>The examples show several patterns that can assist with achieving this.</p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/java/immutable-objects.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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