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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>范圍</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 2 頁(yè)（共11 頁(yè)）</nobr></font></td>
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<i>范圍（extent）</i>是一個(gè) JDO 構(gòu)造，它用于性能，并作為 JDO 對(duì)象查詢的基礎(chǔ)。范圍表示類或類及其子類的所有實(shí)例。范圍可以被認(rèn)為是集合，但它為優(yōu)化留有空間。JDO 實(shí)現(xiàn)可以實(shí)現(xiàn)范圍，這樣只有返回的成批對(duì)象才真正可以在內(nèi)存中立即使用。這種情況下，其余對(duì)象由 JDO 實(shí)現(xiàn)按一個(gè)接著一個(gè)，或以成組的形式進(jìn)行填充，因?yàn)榈饔龅搅藷o(wú)法在內(nèi)存中獲得的對(duì)象。延遲裝入對(duì)象的能力是對(duì)集合的一個(gè)改進(jìn)，這樣可以要求 JDO 提供程序裝入成千上萬(wàn)個(gè)對(duì)象來(lái)表示表。</p>
<p>您還會(huì)在后幾頁(yè)中注意到：范圍形成了大多數(shù)對(duì)象查詢的基礎(chǔ)。范圍作為查詢處理的候選對(duì)象集合被傳遞給查詢。</p>
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