Try to convert objects with numeric keys to lists

This is mostly intended to provide a way to specify lists
on the command line via Java properties, like -Dfoo.0 -Dfoo.1

Fixes #69

config/src/main/java/com/typesafe/config/impl/DefaultTransformer.java

@@ -3,6 +3,13 @@
  */
 package com.typesafe.config.impl;
 
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
 import com.typesafe.config.ConfigValueType;
 
 /**
@@ -74,6 +81,49 @@ final class DefaultTransformer {
                 // no-op STRING to STRING
                 break;
             }
+        } else if (requested == ConfigValueType.LIST && value.valueType() == ConfigValueType.OBJECT) {
+            // attempt to convert an array-like (numeric indices) object to a
+            // list. This would be used with .properties syntax for example:
+            // -Dfoo.0=bar -Dfoo.1=baz
+            // To ensure we still throw type errors for objects treated
+            // as lists in most cases, we'll refuse to convert if the object
+            // does not contain any numeric keys. This means we don't allow
+            // empty objects here though :-/
+            AbstractConfigObject o = (AbstractConfigObject) value;
+            Map<Integer, AbstractConfigValue> values = new HashMap<Integer, AbstractConfigValue>();
+            for (String key : o.keySet()) {
+                int i;
+                try {
+                    i = Integer.parseInt(key, 10);
+                    if (i < 0)
+                        continue;
+                    values.put(i, o.get(key));
+                } catch (NumberFormatException e) {
+                    continue;
+                }
+            }
+            if (!values.isEmpty()) {
+                ArrayList<Map.Entry<Integer, AbstractConfigValue>> entryList = new ArrayList<Map.Entry<Integer, AbstractConfigValue>>(
+                        values.entrySet());
+                // sort by numeric index
+                Collections.sort(entryList,
+                        new Comparator<Map.Entry<Integer, AbstractConfigValue>>() {
+                            @Override
+                            public int compare(Map.Entry<Integer, AbstractConfigValue> a,
+                                    Map.Entry<Integer, AbstractConfigValue> b) {
+                                // Integer.compare was added in 1.7 so not using
+                                // it here yet
+                                return Integer.valueOf(a.getKey()).compareTo(b.getKey());
+                            }
+                        });
+                // drop the indices (we allow gaps in the indices, for better or
+                // worse)
+                ArrayList<AbstractConfigValue> list = new ArrayList<AbstractConfigValue>();
+                for (Map.Entry<Integer, AbstractConfigValue> entry : entryList) {
+                    list.add(entry.getValue());
+                }
+                return new SimpleConfigList(value.origin(), list);
+            }
         }
 
         return value;

config/src/test/scala/com/typesafe/config/impl/PropertiesTest.scala

@@ -9,6 +9,7 @@ import java.util.Properties
 import com.typesafe.config.Config
 import com.typesafe.config.ConfigParseOptions
 import com.typesafe.config.ConfigFactory
+import com.typesafe.config.ConfigException
 
 class PropertiesTest extends TestUtils {
     @Test
@@ -92,4 +93,63 @@ class PropertiesTest extends TestUtils {
         assertEquals("foo", conf.getString("a.b"))
         assertEquals("foo", conf.getString("x.y.z"))
     }
+
+    @Test
+    def makeListWithNumericKeys() {
+        import scala.collection.JavaConverters._
+
+        val props = new Properties()
+        props.setProperty("a.0", "0")
+        props.setProperty("a.1", "1")
+        props.setProperty("a.2", "2")
+        props.setProperty("a.3", "3")
+        props.setProperty("a.4", "4")
+
+        val conf = ConfigFactory.parseProperties(props, ConfigParseOptions.defaults())
+        assertEquals(Seq(0, 1, 2, 3, 4), conf.getIntList("a").asScala.toSeq)
+    }
+
+    @Test
+    def makeListWithNumericKeysWithGaps() {
+        import scala.collection.JavaConverters._
+
+        val props = new Properties()
+        props.setProperty("a.1", "0")
+        props.setProperty("a.2", "1")
+        props.setProperty("a.4", "2")
+
+        val conf = ConfigFactory.parseProperties(props, ConfigParseOptions.defaults())
+        assertEquals(Seq(0, 1, 2), conf.getIntList("a").asScala.toSeq)
+    }
+
+    @Test
+    def makeListWithNumericKeysWithNoise() {
+        import scala.collection.JavaConverters._
+
+        val props = new Properties()
+        props.setProperty("a.-1", "-1")
+        props.setProperty("a.foo", "-2")
+        props.setProperty("a.0", "0")
+        props.setProperty("a.1", "1")
+        props.setProperty("a.2", "2")
+        props.setProperty("a.3", "3")
+        props.setProperty("a.4", "4")
+
+        val conf = ConfigFactory.parseProperties(props, ConfigParseOptions.defaults())
+        assertEquals(Seq(0, 1, 2, 3, 4), conf.getIntList("a").asScala.toSeq)
+    }
+
+    @Test
+    def noNumericKeysAsListFails() {
+        import scala.collection.JavaConverters._
+
+        val props = new Properties()
+        props.setProperty("a.bar", "0")
+
+        val conf = ConfigFactory.parseProperties(props, ConfigParseOptions.defaults())
+        val e = intercept[ConfigException.WrongType] {
+            conf.getList("a")
+        }
+        assertTrue("expected exception thrown", e.getMessage.contains("LIST"))
+    }
 }