diff --git a/java/src/org/broadinstitute/sting/playground/gatk/walkers/CoverageHistogram.java b/java/src/org/broadinstitute/sting/playground/gatk/walkers/CoverageHistogram.java
index 94739d151..0d9a3d2c7 100644
--- a/java/src/org/broadinstitute/sting/playground/gatk/walkers/CoverageHistogram.java
+++ b/java/src/org/broadinstitute/sting/playground/gatk/walkers/CoverageHistogram.java
@@ -7,6 +7,7 @@ import org.broadinstitute.sting.gatk.refdata.RefMetaDataTracker;
 import org.broadinstitute.sting.gatk.walkers.LocusWalker;
 import org.broadinstitute.sting.gatk.walkers.By;
 import org.broadinstitute.sting.gatk.walkers.DataSource;
+import org.broadinstitute.sting.utils.ExpandingArrayList;
 
 import java.util.*;
 
@@ -19,29 +20,37 @@ public class CoverageHistogram extends LocusWalker<Integer,Integer>
 	//@Argument(fullName="start", shortName="start", required=false, doc="start") public Integer START = 0;
 
 	// Private state.
-	long[] coverage_hist;
-	int max_depth;
 
-	long sum_coverage;
-	long num_sites;
+	//long[] coverage_hist;
+    ExpandingArrayList<Integer> coverage_hist = new ExpandingArrayList<Integer>();
+	int max_depth = 0;
+
+	long sum_coverage = 0;
+	long num_sites = 0;
 
 	/////////
 	// Walker Interface Functions 
     public void initialize() 
 	{
-		coverage_hist = new long[1000000];
-        Arrays.fill(coverage_hist, 0);
-		max_depth = 0;
-
-		sum_coverage = 0;
-		num_sites = 0;
+		//coverage_hist = new long[1000000];
+        //Arrays.fill(coverage_hist, 0);
 	}
 
+    private void incCov(int depth) {
+        int c = coverage_hist.expandingGet(depth, 0);
+        coverage_hist.set(depth, c + 1);
+    }
+
+    private int getCov(int depth) {
+        return coverage_hist.get(depth);
+    }
+
+
     public Integer map(RefMetaDataTracker tracker, ReferenceContext ref, AlignmentContext context)
 	{
 		if (ref.getBase() == 'N') { return null; }
 		int depth = context.getReads().size();
-		coverage_hist[depth] += 1;
+		incCov(depth);
 		if (depth > max_depth) { max_depth = depth; }
 
 		sum_coverage += depth;
@@ -53,40 +62,40 @@ public class CoverageHistogram extends LocusWalker<Integer,Integer>
     public void onTraversalDone(Integer sum) 
 	{
         double mean_coverage = (double)sum_coverage / (double)num_sites;
-        double mean_good_coverage = (double)sum_coverage / ((double)(num_sites - coverage_hist[0]));
+        double mean_good_coverage = (double)sum_coverage / ((double)(num_sites - getCov(0)));
 		out.printf("# all_sites                  : mean:%f num_sites:%d%n", mean_coverage, num_sites);
-        out.printf("# sites with at least 1 read : mean:%f num_sites:%d%n", mean_good_coverage, num_sites - coverage_hist[0]);
+        out.printf("# sites with at least 1 read : mean:%f num_sites:%d%n", mean_good_coverage, num_sites - getCov(0));
 
         // Code for calculting std devs adapted from Michael Melgar's python script
 
         // Find the maximum extent of 'good' data
         // First, find the mode
-        long maxValue = coverage_hist[1]; // ignore doc=0
+        long maxValue = getCov(1); // ignore doc=0
         int mode = 1;
         for (int i = 2; i <= max_depth; i++) {
-            if (coverage_hist[i] > maxValue) {
-                maxValue = coverage_hist[i];
+            if (getCov(i) > maxValue) {
+                maxValue = getCov(i);
                 mode = i;
             }
         }
         // now, procede to find end of good Gaussian fit
         long dist = (long)Math.pow(10, 9);
-        while ( Math.abs(coverage_hist[mode] - coverage_hist[1]) < dist )
-            dist = Math.abs(coverage_hist[mode++] - coverage_hist[1]);
+        while ( Math.abs(getCov(mode) - getCov(1)) < dist )
+            dist = Math.abs(getCov(mode++) - getCov(1));
         int maxGoodDepth = mode + 1;
 
         // calculate the mean of the good region
         long totalGoodSites = 0, totalGoodDepth = 0;
         for (int i = 1; i <= maxGoodDepth; i++) { // ignore doc=0
-            totalGoodSites += coverage_hist[i];
-            totalGoodDepth += i * coverage_hist[i];
+            totalGoodSites += getCov(i);
+            totalGoodDepth += i * getCov(i);
         }
         double meanGoodDepth = (double)totalGoodDepth / (double)totalGoodSites;
 
         // calculate the variance and standard deviation of the good region
         double var = 0.0;
         for (int i = 1; i <= maxGoodDepth; i++) {  // ignore doc=0
-            var += coverage_hist[i] * Math.pow(meanGoodDepth - (double)i, 2);
+            var += getCov(i) * Math.pow(meanGoodDepth - (double)i, 2);
         }
         double stdev = Math.sqrt(var / (double)totalGoodSites);
         out.printf("# sites within Gaussian fit  : mean:%f num_sites:%d std_dev:%f%n", meanGoodDepth, totalGoodSites, stdev);
@@ -97,9 +106,9 @@ public class CoverageHistogram extends LocusWalker<Integer,Integer>
 		out.println("\ndepth count freq(percent)");
 		for (int i = 0; i <= max_depth; i++)
 		{
-			out.printf("%d %d %f\n", i, coverage_hist[i], (100.0*coverage_hist[i]) / (double)num_sites);
+			out.printf("%d %d %f\n", i, getCov(i), (100.0*getCov(i)) / (double)num_sites);
 		}
-		return;
+
 	}
 
     public Integer reduceInit() 
diff --git a/java/src/org/broadinstitute/sting/utils/ExpandingArrayList.java b/java/src/org/broadinstitute/sting/utils/ExpandingArrayList.java
index 10f3f8c95..2c32ceafc 100755
--- a/java/src/org/broadinstitute/sting/utils/ExpandingArrayList.java
+++ b/java/src/org/broadinstitute/sting/utils/ExpandingArrayList.java
@@ -22,13 +22,22 @@ public class ExpandingArrayList<E> extends ArrayList<E> {
             return null;
     }
 
-    public E set(int index, E element) {
+    public E expandingGet(int index, E default_value) throws IndexOutOfBoundsException {
+        maybeExpand(index, default_value);
+        return super.get(index);
+    }
+
+    private void maybeExpand(int index, E value) {
         if ( index >= size() ) {
             // We need to add null items until we can safely set index to element
             for ( int i = size(); i <= index; i++ )
-                add(null);
+                add(value);
         }
+    }
 
+
+    public E set(int index, E element) {
+        maybeExpand(index, null);
         return super.set(index, element);
     }
 }
\ No newline at end of file