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POWhat is the best way to compare two very large lists(that do not fit in memory)?
 

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  1. POWhat is the best way to compare two very large lists(that do not fit in memory)?
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    <p>I have a program that takes each item from a list and compares it against all other items in another list. Its been working fine so far but the data is getting large and is going to exceed system memory. </p> <p>I'm wondering what the best way to compare two lists that are very large(maybe 5-10 GB each list)?</p> <p>Here is a very simple example of what I'm doing(except the list is huge and the values in the for loop are actually being processed/compared).</p> <pre><code>import java.util.Collection; import java.util.HashSet; import java.util.Arrays; public class comparelists { public static void main( String [] args ) { String[] listOne = {"a","b", "c","d", "e","f", "g","h", "i","j", "k","l"}; String[] listTwo = {"one", "two", "three", "four", "five","six","seven"}; for(int listOneItem=0; listOneItem&lt;listOne.length; listOneItem++){ for (int listTwoItem=0; listTwoItem&lt;listTwo.length; listTwoItem++) { System.out.println(listOne[listOneItem] + " " + listTwo[listTwoItem]); } } } } </code></pre> <p>I realize there has to be some disk IO here since it won't fit in memory and my intial approach was to save both lists as files and save a bunch of lines from listOne then stream the entire file of listTwo and then get some more lines from listOne and so on. Is there a better way? or a Java way to access the lists like I'm doing above but its swapping to disk as needed?</p>
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    1. USuser1735075
    1. USuser1735075
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    1. COhttp://programmers.stackexchange.com/
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      1. USSam I am
    2. COAre *both* lists too large to fit into memory? What comparison do you need to perform, and what will the result be?
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      1. USJon Skeet
    3. CO@JonSkeet Yes both are too big for memory. Each item is a bunch of data and is processed in the program. Its not simple text comparison or anything.
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