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POWhen "" == s is false but "".equals( s ) is true
 

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    <p><strong>EDIT</strong> Thanks for the prompt responses. Please see what the real question is. I have made it bold this time.</p> <p>I do understand the difference between == and .equals. So, that's not my question (I actually added some context for that)</p> <hr /> <p>I'm performing the validation below for empty strings:</p> <pre><code>if( "" == value ) { // is empty string } </code></pre> <p>In the <strong>past</strong> when fetching values from the db or deserializing objects from another node, this test <strong>failed</strong>, because the two string instances were indeed different object references, albeit they contained the same data.</p> <p>So the fix for those situations was</p> <pre><code>if( "".equals( value ) ) { // which returns true for all the empty strings } </code></pre> <p>I'm fine with that. That's clearly understood. </p> <p>Today this happened once again, but it puzzled me because this time the application is a very small <strong>standalone application</strong> that doesn't use <strong>network at all</strong>, so no new string is fetched from the database nor deserizalized from another node.</p> <p>So the question is:</p> <p><br></p> <h1>Under which <strong>OTHER</strong> circumstances:</h1> <pre><code>"" == value // yields false </code></pre> <p>and </p> <pre><code>"".equals( value ) // yields true </code></pre> <p>For a local standalone application? </p> <p>I'm pretty sure <strong>new String()</strong> is not being used in the code.</p> <p>And the only way a string reference could be "" is because it is being assigned "" directly in the code (or that's what I thought) like in:</p> <pre><code>String a = ""; String b = a; assert "" == b ; // this is true </code></pre> <p>Somehow (after reading the code more I have a clue) two different empty string object references were created, I would like to know <strong>how</strong></p> <p>More in the line of jjnguys answer:</p> <p>Byte!</p> <p><strong>EDIT: Conclusion</strong></p> <p>I've found the reason. </p> <p>After jjnguy suggestion I was able to look with different eyes to the code.</p> <p>The guilty method: StringBuilder.toString()</p> <blockquote> <p><em>A new String object is allocated and initialized to contain the character sequence currently represented by this object.</em></p> </blockquote> <p>Doh!... </p> <pre><code> StringBuilder b = new StringBuilder("h"); b.deleteCharAt( 0 ); System.out.println( "" == b.toString() ); // prints false </code></pre> <p>Mystery solved. </p> <p>The code uses StringBuilder to deal with an ever growing string. It turns out that at some point somebody did:</p> <pre><code> public void someAction( String string ) { if( "" == string ) { return; } deleteBankAccount( string ); } </code></pre> <p>and use </p> <pre><code> someAction( myBuilder.toString() ); // bug introduced. </code></pre> <p>p.s. Have I read too much CodingHorror lately? Or why do I feel the need to add some funny animal pictures here?</p>
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    1. COif (value != null && value.length == 0) { ... }, and you should use equals because == for objects compares references, i.e., are they the same object. Whilst at compile time Java finds identical strings and makes them share the same reference (Strings are immutable), at run time it is easy to create empty strings that have different references.
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    2. COAs my answer suggests... forget what you "thought". If you mean to check if something has the same value as the empty string, then use equals. You are trying to depend on the compiler interning your strings, which Josh Bloch points out is not a tool for programmers :-). See below.
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    3. COSeveral answers now show simple String manipulations that yield different String objects of zero length. Networks, and goodness, have nothing to do with those answers.
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