allow SuperColumn.getSubColumn to return null. patch by Sandeep Tata;…

… reviewed by jbellis for #91

src/org/apache/cassandra/db/SuperColumn.java

@@ -80,7 +80,7 @@ public Collection<IColumn> getSubColumns()
     public IColumn getSubColumn(String columnName)
     {
         IColumn column = columns_.get(columnName);
-        assert column instanceof Column;
+        assert column == null || column instanceof Column;
         return column;
     }
 
@@ -169,7 +169,6 @@ public byte[] value()
     public byte[] value(String key)
     {
     	IColumn column = columns_.get(key);
-    	assert column instanceof Column;
     	if ( column != null )
     		return column.value();
     	throw new IllegalArgumentException("Value was requested for a column that does not exist.");

test/org/apache/cassandra/db/SuperColumnTest.java

@@ -0,0 +1,17 @@
+package org.apache.cassandra.db;
+
+import static org.testng.Assert.assertNotNull;
+import static org.testng.Assert.assertNull;
+
+import org.testng.annotations.Test;
+public class SuperColumnTest
+{   
+    @Test
+    public void testMissingSubcolumn() {
+    	byte[] val = "sample value".getBytes();
+    	SuperColumn sc = new SuperColumn("sc1");
+    	sc.addColumn("col1", new Column("col1",val,1L));
+    	assertNotNull(sc.getSubColumn("col1"));
+    	assertNull(sc.getSubColumn("col2"));
+    }
+}