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    <p>Using <code>String</code> concatenation is translated into <code>StringBuilder</code> operations by the compiler.</p> <p>To see how the compiler is doing I'll take a sample class, compile it and decompile it with jad to see what's the generated bytecode.</p> <p>Original class:</p> <pre><code>public void method1() { System.out.println("The answer is: " + 42); } public void method2(int value) { System.out.println("The answer is: " + value); } public void method3(int value) { String a = "The answer is: " + value; System.out.println(a + " what is the question ?"); } </code></pre> <p>The decompiled class:</p> <pre><code>public void method1() { System.out.println("The answer is: 42"); } public void method2(int value) { System.out.println((new StringBuilder("The answer is: ")).append(value).toString()); } public void method3(int value) { String a = (new StringBuilder("The answer is: ")).append(value).toString(); System.out.println((new StringBuilder(String.valueOf(a))).append(" what is the question ?").toString()); } </code></pre> <ul> <li>On <code>method1</code> the compiler performed the operation at compile time.</li> <li>On <code>method2</code> the <code>String</code> concatenation is equivalent to manually use <code>StringBuilder</code>.</li> <li>On <code>method3</code> the <code>String</code> concatenation is definitely bad as the compiler is creating a second <code>StringBuilder</code> rather than reusing the previous one.</li> </ul> <p>So my simple rule is that concatenations are good unless you need to concatenate the result again: for instance in loops or when you need to store an intermediate result.</p>
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    1. COIn the Java Language spec, it SUGGESTS that this is what the compiler would do, leaving open the possibility that it would not. 15.18.1.2 Optimization of String Concatenation
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    2. COAny idea how long this optimization has been in effect in SUN JVM's and if it exists in IBM JVM's?
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    3. COI think it is probably here in Sun's JVM since version 1.4 and I don't know if it exists on IBM's JVM. You can check with a decompiler such as jad to be sure.
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