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    <p>If you have something like this:</p> <pre><code>Collection&lt;String&gt; strings = ...; String all = ""; for (String s : strings) all += s; </code></pre> <p>... then it's equivalent to:</p> <pre><code>Collection&lt;String&gt; strings = ...; String all = ""; for (String s : strings) all = new StringBuilder(all).append(s).toString(); </code></pre> <p>Each loops creates a new <code>StringBuilder</code> which is essentially a copy of <code>all</code>, appends a copy of <code>s</code> to it, and then copies the result of the concatenation to a new <code>String</code>. Obviously, using a single StringBuilder saves a lot of unnecessary allocations:</p> <pre><code>Collection&lt;String&gt; strings = ...; StringBuilder sb = new StringBuilder(); for (String s : strings) sb.append(s); String all = sb.toString(); </code></pre> <p>As for <code>x += y</code> versus <code>x = x + y</code>, they compile to the same thing.</p> <pre><code>class Concat { public String concat1(String a, String b) { a += b; return a; } public String concat2(String a, String b) { a = a + b; return a; } } </code></pre> <p>Compile it with <code>javac</code>, and then disassemble it with <code>javap</code>:</p> <pre><code>$ javap -c Concat Compiled from "Concat.java" class Concat extends java.lang.Object{ Concat(); Code: 0: aload_0 1: invokespecial #1; //Method java/lang/Object."&lt;init&gt;":()V 4: return public java.lang.String concat1(java.lang.String, java.lang.String); Code: 0: new #2; //class java/lang/StringBuilder 3: dup 4: invokespecial #3; //Method java/lang/StringBuilder."&lt;init&gt;":()V 7: aload_1 8: invokevirtual #4; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 11: aload_2 12: invokevirtual #4; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 15: invokevirtual #5; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 18: astore_1 19: aload_1 20: areturn public java.lang.String concat2(java.lang.String, java.lang.String); Code: 0: new #2; //class java/lang/StringBuilder 3: dup 4: invokespecial #3; //Method java/lang/StringBuilder."&lt;init&gt;":()V 7: aload_1 8: invokevirtual #4; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 11: aload_2 12: invokevirtual #4; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 15: invokevirtual #5; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 18: astore_1 19: aload_1 20: areturn } </code></pre> <p>Personally, I'd favor <code>+=</code> because with it, you make a clearer statement of intent - "I want to add the content of <code>b</code> to <code>a</code>". Any variations in performance between the two forms are with a 100% certainty the result of something outside of your code (e.g., GC pauses, random cache misses or something like it). </p> <p>Bad compilers might also have a slightly easier time to optimize the <code>+=</code> form (which is irrelevant to you, since even if <code>javac</code> would be crappy, HotSpot sure isn't).</p>
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