PARQUET-387: Improve NPE message when avro arrays contain null.

Previously, the NPE had no error message but the Avro support accepts
schemas that have nullable array elements.

Author: Ryan Blue <blue@apache.org>

Closes #291 from rdblue/PARQUET-387-fix-npe-message and squashes the following commits:

39d3c83 [Ryan Blue] PARQUET-387: Update test case to verify help message.
d6b6bd8 [Ryan Blue] PARQUET-387: Improve NPE message when avro arrays contain null.

Author: rdblue

parquet-avro/src/main/java/org/apache/parquet/avro/AvroWriteSupport.java

@@ -452,8 +452,24 @@ public void writeCollection(GroupType schema, Schema avroSchema,
                                 Collection<?> array) {
       if (array.size() > 0) {
         recordConsumer.startField(OLD_LIST_REPEATED_NAME, 0);
-        for (Object elt : array) {
-          writeValue(schema.getType(0), avroSchema.getElementType(), elt);
+        try {
+          for (Object elt : array) {
+            writeValue(schema.getType(0), avroSchema.getElementType(), elt);
+          }
+        } catch (NullPointerException e) {
+          // find the null element and throw a better error message
+          int i = 0;
+          for (Object elt : array) {
+            if (elt == null) {
+              throw new NullPointerException(
+                  "Array contains a null element at " + i + "\n" +
+                  "Set parquet.avro.write-old-list-structure=false to turn " +
+                  "on support for arrays with null elements.");
+            }
+            i += 1;
+          }
+          // no element was null, throw the original exception
+          throw e;
         }
         recordConsumer.endField(OLD_LIST_REPEATED_NAME, 0);
       }
@@ -464,8 +480,22 @@ protected void writeObjectArray(GroupType type, Schema schema,
                                     Object[] array) {
       if (array.length > 0) {
         recordConsumer.startField(OLD_LIST_REPEATED_NAME, 0);
-        for (Object element : array) {
-          writeValue(type.getType(0), schema.getElementType(), element);
+        try {
+          for (Object element : array) {
+            writeValue(type.getType(0), schema.getElementType(), element);
+          }
+        } catch (NullPointerException e) {
+          // find the null element and throw a better error message
+          for (int i = 0; i < array.length; i += 1) {
+            if (array[i] == null) {
+              throw new NullPointerException(
+                  "Array contains a null element at " + i + "\n" +
+                  "Set parquet.avro.write-old-list-structure=false to turn " +
+                  "on support for arrays with null elements.");
+            }
+          }
+          // no element was null, throw the original exception
+          throw e;
         }
         recordConsumer.endField(OLD_LIST_REPEATED_NAME, 0);
       }

parquet-avro/src/test/java/org/apache/parquet/avro/TestReadWriteOldListBehavior.java

@@ -43,6 +43,7 @@
 import org.apache.parquet.io.api.Binary;
 import org.apache.parquet.io.api.RecordConsumer;
 import org.apache.parquet.schema.MessageTypeParser;
+import org.junit.Assert;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.Parameterized;
@@ -347,7 +348,8 @@ public void testArrayWithNullValues() throws Exception {
       writer.write(record);
       fail("Should not succeed writing an array with null values");
     } catch (Exception e) {
-      // expected
+      Assert.assertTrue("Error message should provide context and help",
+          e.getMessage().contains("parquet.avro.write-old-list-structure"));
     } finally {
       writer.close();
     }