Constructor parameter changes (#26)

- Allowed the name of the constructor argument to be derived from the
java name. This requires passing the '-parameters' flag to javac,
otherwise the argument names get stripped out and become arg0, arg1,
etc.
- Updated the documentation to reflect this change.

Author: tfaulkes

README.md

@@ -294,7 +294,10 @@ public class ConstructedClass {
 }
 ```
 
-As it stands, this class cannot be used with the AeroMapper because there is no valid constructor to invoke when an object needs to be created. There is a constructor but it does not contain enough information to map the record on the database to the parameters of the constructor. (Remember that at runtime method and argument names are typically lost and become "arg1", "arg2" and so on). We can use this constructor, but we need to provide this missing information with annotations:
+As it stands, this class cannot be used with the AeroMapper because there is no valid constructor to invoke when an object needs to be created. There is a constructor but it does not contain enough information to map the record on the database to the parameters of the constructor. (Remember that at runtime method and argument names are typically lost and become "arg1", "arg2" and so on). We can use this constructor in one of two ways:
+
+1. We specify '-parameters' to javac, which will prevent it stripping out the names to the constructor
+2. We can to provide this missing information with annotations:
 
 ```java
 @AerospikeRecord(namespace = "test", set = "testSet")
@@ -318,7 +321,7 @@ public class ConstructedClass {
 }
 ```
 
-Now there is enough information to be able to construct an instance of this class from a database record. Note that the names of the @ParamFrom annotation are the bin names, not the underlying field names. So if you have a field declared as
+Now there is enough information to be able to construct an instance of this class from a database record. Note that the names of the @ParamFrom annotation (or the argument names if using -parameters) are the bin names, not the underlying field names. So if you have a field declared as
 
 ```java
 @AerospikeBin(name = "shrtNm")

src/main/java/com/aerospike/mapper/tools/ClassCacheEntry.java

@@ -348,20 +348,28 @@ private void findConstructor() {
 			current = current.superClazz;
 		}
 		int count = 0;
+		
+		// Parameters can be either specified by their name (which requires the use of the javac -parameters flag), or through an @ParamFrom annotation.  
 		for (Parameter thisParam : params) {
 			count++;
+			boolean isFromAnnotation = false;
+			String binName = thisParam.getName();
 			ParamFrom parameterDetails = thisParam.getAnnotation(ParamFrom.class);
-			if (parameterDetails == null) {
-				throw new AerospikeException("Class " + clazz.getSimpleName() + " has a preferred constructor of " + desiredConstructor.toString()+ ". However, parameter " + count + 
-						" is not marked with a @ParamFrom annotation, and hence cannot be determined how to map to this from the bins in the record.");
+			
+			if (parameterDetails != null) {
+				binName = parameterDetails.value();
+				isFromAnnotation = true;
 			}
-			String binName = parameterDetails.value();
 
 			// Validate that we have such a value
 			if (!allValues.containsKey(binName)) {
 				String valueList = String.join(",", values.keySet());
-				throw new AerospikeException("Class " + clazz.getSimpleName() + " has a preferred constructor of " + desiredConstructor.toString()+ ". However, parameter " + count + 
-						" is mapped to bin \"" + binName + "\" which is not one of the values on the class, which are: " + valueList);
+				String message = String.format("Class %s has a preferred constructor of %s. However, parameter %d is mapped to bin \"%s\" %s which is not one of the values on the class, which are: %s%s", 
+						clazz.getSimpleName(), desiredConstructor.toString(), count, binName,
+						isFromAnnotation ? "via the @ParamFrom annotation" : "via the argument name",
+						valueList,
+						(!isFromAnnotation && binName.startsWith("arg")) ? ". Did you forget to specify '-parameters' to javac when building?" : "");
+				throw new AerospikeException(message);
 			}
 			Class<?> type = thisParam.getType();
 			if (!type.isAssignableFrom(allValues.get(binName).getType())) {