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    <p><strong>Edit: Even the compiler doing this change (that I didn't know), using String concatenations is SLOWER then using StringBuilder.</strong> (Wrong => <em>No, the compiler does not use StringBuilder/StringBuffer</em>). You can test it just creating a simple benchmark. The + operator ir "overloaded" in Java for Strings, but the language does not support operator overload.</p> <p>Try something like this to test the StringBuilder/StringBuffer thing:</p> <pre><code>String str = ""; StringBuffer sbf = new StringBuffer(); StringBuilder sb = new StringBuilder(); int nTests = 100; int nConcats = 1000; long initialTime = 0L; long afterStrTime = 0L; long afterSbfTime = 0L; long afterSbTime = 0L; for ( int i = 0; i &lt; nTests; i++ ) { initialTime = System.currentTimeMillis(); str = ""; sbf = new StringBuffer(); sb = new StringBuilder(); for ( int j = 0; j &lt; nConcats; j++ ) { str += "foo"; // or str = str + "foo" } afterStrTime = System.currentTimeMillis(); for ( int j = 0; j &lt; nConcats; j++ ) { sbf.append( "foo" ); } afterSbfTime = System.currentTimeMillis(); for ( int j = 0; j &lt; nConcats; j++ ) { sb.append( "foo" ); } afterSbTime = System.currentTimeMillis(); } System.out.printf( "%d milliseconds to perform %d concatenations on String\n", afterStrTime - initialTime, nConcats ); System.out.printf( "%d milliseconds to perform %d concatenations on StringBuilder\n", afterSbfTime - afterStrTime, nConcats ); System.out.printf( "%d milliseconds to perform %d concatenations on StringBuffer\n", afterSbTime - afterSbfTime, nConcats ); </code></pre> <p>You will see that StringBuilder can be faster than StringBuffer, since it is not synchronized (StringBuffer is), and both are faster than String.</p>
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    1. POConcatenating Strings in Java
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    1. COYou can test it by compiling the code given in the question and then running `javap -c` on it, which *clearly* shows it using `StringBuilder`.
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      1. USJon Skeet
    2. CO@JonSkeet, ok, but can you explain why in my code above the String concatenations is not fast as StringBuffer one? If the compiler changes to StringBuffer, the execution should be equaly fast don't you agree?
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    3. COBecause it instantiates one StringBuilder for each iteration of the loop, and you only instantiate it once.
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