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    <p>Sounds like you need a sparse matrix. Others have already suggested good 3rd party implementations that may suite your needs...</p> <p>Depending on your applications, you could get away without a third-party matrix library by just using a Map as a backing-store for your matrix data. Kind of...</p> <pre><code>public class SparseMatrix&lt;T&gt; { private T defaultValue; private int m; private int n; private Map&lt;Integer, T&gt; data = new TreeMap&lt;Integer, T&gt;(); /// create a new matrix with m rows and n columns public SparseMatrix(int m, int n, T defaultValue) { this.m = m; this.n = n; this.defaultValue = defaultValue; } /// set value at [i,j] (row, col) public void setValueAt(int i, int j, T value) { if (i &gt;= m || j &gt;= n || i &lt; 0 || j &lt; 0) throw new IllegalArgumentException( "index (" + i + ", " +j +") out of bounds"); data.put(i * n + j, value); } /// retrieve value at [i,j] (row, col) public T getValueAt(int i, int j) { if (i &gt;= m || j &gt;= n || i &lt; 0 || j &lt; 0) throw new IllegalArgumentException( "index (" + i + ", " +j +") out of bounds"); T value = data.get(i * n + j); return value != null ? value : defaultValue; } } </code></pre> <p>A simple test-case illustrating the SparseMatrix' use would be:</p> <pre><code>public class SparseMatrixTest extends TestCase { public void testMatrix() { SparseMatrix&lt;Float&gt; matrix = new SparseMatrix&lt;Float&gt;(100000, 100000, 0.0F); matrix.setValueAt(1000, 1001, 42.0F); assertTrue(matrix.getValueAt(1000,1001) == 42.0); assertTrue(matrix.getValueAt(1001,1000) == 0.0); } } </code></pre> <p>This is not the most efficient way of doing it because every non-default entry in the matrix is stored as an Object. Depending on the number of actual values you are expecting, the simplicity of this approach might trump integrating a 3rd-party solution (and possibly dealing with its License - again, depending on your situation).</p> <p>Adding matrix-operations like multiplication to the above SparseMatrix implementation should be straight-forward (and is left as an exercise for the reader ;-)</p>
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