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    <p>Apart from using a database, if you just have a number of intergers you could write them to a plain file. Plain files retain order, however removing entries can be expensive.</p> <p>You can write/rewrite 1 million integers in about 0.1 seconds using a plain file.</p> <p>An efficient collecton for int primitives is TIntArrayList. Its like @JPelletier's suggestion but wraps a int[]. A million int values should take about 4 MB of memory or disk.</p> <p>EDIT: This shows that for 1 million numbers ArrayList is a bad choice. Mainly because remove(0) is O(n) rather than O(1)</p> <pre><code>import java.util.ArrayList; import java.util.LinkedList; import java.util.List; // based on http://www.cs.princeton.edu/introcs/43stack/RingBuffer.java.html public class IntRingBuffer { private final int[] a; // queue elements private int N = 0; // number of elements on queue private int first = 0; // index of first element of queue private int last = 0; // index of next available slot // cast needed since no generic array creation in Java public IntRingBuffer(int capacity) { a = new int[capacity]; } public boolean isEmpty() { return N == 0; } public int size() { return N; } public void enqueue(int item) { if (N == a.length) { throw new RuntimeException("Ring buffer overflow"); } a[last] = item; last = (last + 1) % a.length; // wrap-around N++; } // remove the least recently added item - doesn't check for underflow public int dequeue() { if (isEmpty()) { throw new RuntimeException("Ring buffer underflow"); } int item = a[first]; N--; first = (first + 1) % a.length; // wrap-around return item; } public static void main(String... args) { int size = 1000000; { long start = System.nanoTime(); IntRingBuffer list = new IntRingBuffer(size); for(int i=0;i&lt; size;i++) list.enqueue(i); for(int i=0;i&lt; size;i++) list.dequeue(); long time = System.nanoTime() - start; System.out.println(list.getClass().getSimpleName()+": Took "+time/1000/1000+" ms to add/remove "+size+" elements"); } { long start = System.nanoTime(); List&lt;Integer&gt; list = new LinkedList&lt;Integer&gt;(); for(int i=0;i&lt; size;i++) list.add(i); for(int i=0;i&lt; size;i++) list.remove(0); long time = System.nanoTime() - start; System.out.println(list.getClass().getSimpleName()+": Took "+time/1000/1000+" ms to add/remove "+size+" elements"); } { long start = System.nanoTime(); List&lt;Integer&gt; list = new ArrayList&lt;Integer&gt;(); for(int i=0;i&lt; size;i++) list.add(i); for(int i=0;i&lt; size;i++) list.remove(0); long time = System.nanoTime() - start; System.out.println(list.getClass().getSimpleName()+": Took "+time/1000/1000+" ms to add/remove "+size+" elements"); } } } </code></pre> <p>Prints</p> <pre><code>IntRingBuffer: Took 31 ms to add/remove 1000000 elements LinkedList: Took 252 ms to add/remove 1000000 elements ArrayList: Took 325832 ms to add/remove 1000000 elements </code></pre>
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    1. POWhat is the correct data structure to use in java?
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    1. USPeter Lawrey
    1. USPeter Lawrey
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    1. COWould give this several upvotes if I could because a) it's a better solution than the currently accepted one and b) it gives the timings to prove it.
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      1. USDJClayworth
 

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