bug fixed & optimzed Histogram memory used

src/test/java/com/alibaba/druid/stat/HistogramMemoryTest.java

@@ -0,0 +1,26 @@
+package com.alibaba.druid.stat;
+
+import java.lang.management.ManagementFactory;
+import java.text.NumberFormat;
+
+import com.alibaba.druid.util.Histogram;
+
+import junit.framework.TestCase;
+
+public class HistogramMemoryTest extends TestCase {
+
+    public void test_0() throws Exception {
+        long memoryStart = ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed();
+        
+        final int COUNT = 1024 * 1;
+        Object[] items = new Object[COUNT];
+        for (int i = 0; i < COUNT; ++i) {
+            // items[i] = new JdbcSqlStat("");
+            items[i] = Histogram.makeHistogram(20);
+        }
+        
+        long memoryEnd = ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed();
+        
+        System.out.println("memory used : " + NumberFormat.getInstance().format(memoryEnd - memoryStart));
+    }
+}