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    <p>I have nothing substantial to add to ruakh's answer, but here's a quick test function. I keep a scrap project around for writing little tests like these. Adjust the sourceSize to something representative of your data, and you can get a rough idea of the magnitude of the effect. As shown, I saw about a factor of 2 between them. </p> <pre><code>import java.util.ArrayList; import java.util.Random; public class ALTest { public static long fill(ArrayList&lt;Byte&gt; al, byte[] source) { long start = System.currentTimeMillis(); for (byte b : source) { al.add(b); } return System.currentTimeMillis()-start; } public static void main(String[] args) { int sourceSize = 1&lt;&lt;20; // 1 MB int smallIter = 50; int bigIter = 4; Random r = new Random(); byte[] source = new byte[sourceSize]; for (int i = 0;i&lt;bigIter;i++) { r.nextBytes(source); { long time = 0; for (int j = 0;j&lt;smallIter;j++) { ArrayList&lt;Byte&gt; al = new ArrayList&lt;Byte&gt;(sourceSize); time += fill(al,source); } System.out.print("With: "+time+"ms\t"); } { long time = 0; for (int j = 0;j&lt;smallIter;j++) { ArrayList&lt;Byte&gt; al = new ArrayList&lt;Byte&gt;(); time += fill(al,source); } System.out.print("Without: "+time+"ms\t"); } { long time = 0; for (int j = 0;j&lt;smallIter;j++) { ArrayList&lt;Byte&gt; al = new ArrayList&lt;Byte&gt;(); time += fill(al,source); } System.out.print("Without: "+time+"ms\t"); } { long time = 0; for (int j = 0;j&lt;smallIter;j++) { ArrayList&lt;Byte&gt; al = new ArrayList&lt;Byte&gt;(sourceSize); time += fill(al,source); } System.out.print("With: "+time+"ms"); } System.out.println(); } } } </code></pre> <p>Output:</p> <pre><code>With: 401ms Without: 799ms Without: 731ms With: 347ms With: 358ms Without: 744ms Without: 749ms With: 342ms With: 348ms Without: 719ms Without: 739ms With: 347ms With: 339ms Without: 734ms Without: 774ms With: 358ms </code></pre>
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