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copied!<p>Okay, there's been a lot of discussion and not a lot of code :)</p> <p>Here's a quick benchmark. It's got the normal caveats when it comes to this kind of thing - testing memory has oddities due to JITting etc, but with suitably large numbers it's useful anyway. It has two types, each with 80 members - LotsOfBytes has 80 bytes, LotsOfInts has 80 ints. We build lots of them, make sure they're not GC'd, and check memory usage:</p> <pre><code>class LotsOfBytes { byte a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af; byte b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf; byte c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf; byte d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df; byte e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef; } class LotsOfInts { int a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af; int b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf; int c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf; int d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df; int e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef; } public class Test { private static final int SIZE = 1000000; public static void main(String[] args) throws Exception { LotsOfBytes[] first = new LotsOfBytes[SIZE]; LotsOfInts[] second = new LotsOfInts[SIZE]; System.gc(); long startMem = getMemory(); for (int i=0; i < SIZE; i++) { first[i] = new LotsOfBytes(); } System.gc(); long endMem = getMemory(); System.out.println ("Size for LotsOfBytes: " + (endMem-startMem)); System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE))); System.gc(); startMem = getMemory(); for (int i=0; i < SIZE; i++) { second[i] = new LotsOfInts(); } System.gc(); endMem = getMemory(); System.out.println ("Size for LotsOfInts: " + (endMem-startMem)); System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE))); // Make sure nothing gets collected long total = 0; for (int i=0; i < SIZE; i++) { total += first[i].a0 + second[i].a0; } System.out.println(total); } private static long getMemory() { Runtime runtime = Runtime.getRuntime(); return runtime.totalMemory() - runtime.freeMemory(); } } </code></pre> <p>Output on my box:</p> <pre><code>Size for LotsOfBytes: 88811688 Average size: 88.811688 Size for LotsOfInts: 327076360 Average size: 327.07636 0 </code></pre> <p>So obviously there's some overhead - 8 bytes by the looks of it, although somehow only 7 for LotsOfInts (? like I said, there are oddities here) - but the point is that the byte fields appear to be packed in for LotsOfBytes such that it takes (after overhead removal) only a quarter as much memory as LotsOfInts.</p>

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