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    <p>You are confusing object mutability/immutability with copying of reference values.</p> <p>In these diagrams, <code>[var/index]</code> is a reference variable, and <code>{{an Object}}</code> is an object.</p> <pre><code>String abc[]={"abc"}; String def[]={}; [abc] ------&gt; {{a String[1]}} [0] --------------&gt; {{a String "abc"}} [def] ------&gt; {{a String[0]}} </code></pre> <p>Now you make <code>def</code> reference variable points to the same object as <code>abc</code> reference variable:</p> <pre><code>def=abc; [abc] ------&gt; {{a String[1]}} / [0] --------------&gt; {{a String "abc"}} / [def] ---/ {{a String[0]}} </code></pre> <p>At this point, the array of length zero is unreferenced, and should be garbage-collectable. We can narrow our discussion to the array of length one. Note that a <code>String[]</code> is an array of references. With this next line, you changed what the only element in the length one array points to.</p> <pre><code>def[0]=def[0]+"changed"; [abc] ------&gt; {{a String[1]}} / [0] ---------\ {{a String "abc"}} / \ [def] ---/ \--&gt; {{a String "abcchanged"}} </code></pre> <p>Note that <code>{{a String "abc"}}</code> itself was not mutated. <code>[abc]</code> and <code>[def]</code> now points to the same <code>{{a String[1]}}</code>, which is mutable (i.e. you can make the elements of the array, which are references to <code>String</code> objects, to point to anything).</p> <hr> <blockquote> <p>in order to prevent <code>abc</code> from changed when i changed <code>def</code>, i will have to do <code>def = abc.clone()</code>;</p> </blockquote> <p>Actually, that's not quite accurate. Let's see what happens if you <code>clone()</code> an array of references to a mutable type <code>StringBuilder</code>.</p> <pre><code> StringBuilder[] abc = new StringBuilder[] { new StringBuilder("Hello") }; StringBuilder[] def = abc.clone(); def[0].append(" world!"); System.out.println(abc[0]); // prints "Hello world!" </code></pre> <p>I won't make the diagrams for you this time, but you can easily draw it out on paper. What's happening here is that even though <code>clone()</code> makes a second <code>{{a StringBuilder[1]}}</code> object with its own element (i.e. <code>def != abc</code>), that element is pointing to the same <code>{{a StringBuilder}}</code> object (i.e. <code>def[0] == abc[0]</code>).</p> <hr> <p>In short:</p> <ul> <li>Immutability means that objects of a certain type can not change in any meaningful way to outside observers <ul> <li><code>Integer</code>, <code>String</code>, etc are immutable</li> <li>Generally all value types should be</li> </ul></li> <li>Array objects are mutable <ul> <li>It may be an array of references to immutable types, but the array itself is mutable <ul> <li>Meaning you can set those references to anything you want</li> <li>Also true for array of primitives</li> </ul></li> <li>An immutable array will not be practical</li> </ul></li> <li>References to objects can be shared <ul> <li>If the object is mutable, mutation will be seen through all these references</li> </ul></li> </ul> <p>If you want more in-depth understanding of the issues, I recommend the following:</p> <ul> <li><a href="https://stackoverflow.com/questions/40480/is-java-pass-by-reference">Is Java pass by reference?</a> -- NO!</li> <li><a href="https://stackoverflow.com/questions/1110826/are-java-function-parameters-always-passed-by-value">Are Java function parameters always passed-by-value?</a> -- YES!</li> </ul>
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    1. CO@polygenelubricants thank you for depth explanation. awesome
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