diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
index d31e004b9c348..634750dca2158 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
@@ -435,37 +435,57 @@ case class Cast(child: Expression, dataType: DataType) extends UnaryExpression w
     if (evaluated == null) null else cast(evaluated)
   }
 
-  override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = this match {
+  override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
+    // TODO(cg): Add support for more data types.
+    (child.dataType, dataType) match {
 
-    case Cast(child @ BinaryType(), StringType) =>
-      castOrNull (ctx, ev, c =>
-        s"new ${ctx.stringType}().set($c)")
+      case (BinaryType, StringType) =>
+        defineCodeGen (ctx, ev, c =>
+          s"new ${ctx.stringType}().set($c)")
 
-    case Cast(child @ DateType(), StringType) =>
-      castOrNull(ctx, ev, c =>
-        s"""new ${ctx.stringType}().set(
+      case (DateType, StringType) =>
+        defineCodeGen(ctx, ev, c =>
+          s"""new ${ctx.stringType}().set(
                 org.apache.spark.sql.catalyst.util.DateUtils.toString($c))""")
 
-    case Cast(child @ BooleanType(), dt: NumericType) if !dt.isInstanceOf[DecimalType] =>
-      castOrNull(ctx, ev, c => s"(${ctx.primitiveType(dt)})($c?1:0)")
+      case (BooleanType, dt: NumericType) if !dt.isInstanceOf[DecimalType] =>
+        defineCodeGen(ctx, ev, c => s"(${ctx.primitiveType(dt)})($c ? 1 : 0)")
 
-    case Cast(child @ DecimalType(), IntegerType) =>
-      castOrNull(ctx, ev, c => s"($c).toInt()")
+      case (_: NumericType, dt: NumericType) if !dt.isInstanceOf[DecimalType] =>
+        defineCodeGen(ctx, ev, c => s"(${ctx.primitiveType(dt)})($c)")
 
-    case Cast(child @ DecimalType(), dt: NumericType) if !dt.isInstanceOf[DecimalType] =>
-      castOrNull(ctx, ev, c => s"($c).to${ctx.boxedType(dt)}()")
+      case (_: DecimalType, ByteType) =>
+        defineCodeGen(ctx, ev, c => s"($c).toByte()")
 
-    case Cast(child @ NumericType(), dt: NumericType) if !dt.isInstanceOf[DecimalType] =>
-      castOrNull(ctx, ev, c => s"(${ctx.primitiveType(dt)})($c)")
+      case (_: DecimalType, ShortType) =>
+        defineCodeGen(ctx, ev, c => s"($c).toShort()")
 
-    // Special handling required for timestamps in hive test cases since the toString function
-    // does not match the expected output.
-    case Cast(e, StringType) if e.dataType != TimestampType =>
-      castOrNull(ctx, ev, c =>
-        s"new ${ctx.stringType}().set(String.valueOf($c))")
+      case (_: DecimalType, IntegerType) =>
+        defineCodeGen(ctx, ev, c => s"($c).toInt()")
 
-    case other =>
-      super.genCode(ctx, ev)
+      case (_: DecimalType, LongType) =>
+        defineCodeGen(ctx, ev, c => s"($c).toLong()")
+
+      case (_: DecimalType, FloatType) =>
+        defineCodeGen(ctx, ev, c => s"($c).toFloat()")
+
+      case (_: DecimalType, DoubleType) =>
+        defineCodeGen(ctx, ev, c => s"($c).toDouble()")
+
+      case (_: DecimalType, dt: NumericType) if !dt.isInstanceOf[DecimalType] =>
+        defineCodeGen(ctx, ev, c => s"($c).to${ctx.boxedType(dt)}()")
+
+      // Special handling required for timestamps in hive test cases since the toString function
+      // does not match the expected output.
+      case (TimestampType, StringType) =>
+        super.genCode(ctx, ev)
+
+      case (_, StringType) =>
+        defineCodeGen(ctx, ev, c => s"new ${ctx.stringType}().set(String.valueOf($c))")
+
+      case other =>
+        super.genCode(ctx, ev)
+    }
   }
 }
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
index 1f1a2fc9694af..db085c8c277ea 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
@@ -69,7 +69,9 @@ abstract class Expression extends TreeNode[Expression] {
   }
 
   /**
-   * Returns Java source code for this expression.
+   * Returns Java source code that can be compiled to evaluate this expression.
+   * The default behavior is to call the eval method of the expression. Concrete expression
+   * implementations should override this to do actual code generation.
    *
    * @param ctx a [[CodeGenContext]]
    * @param ev an [[GeneratedExpressionCode]] with unique terms.
@@ -82,10 +84,10 @@ abstract class Expression extends TreeNode[Expression] {
           /* expression: ${this} */
           Object ${ev.objectTerm} = expressions[${ctx.references.size - 1}].eval(i);
           boolean ${ev.nullTerm} = ${ev.objectTerm} == null;
-          ${ctx.primitiveType(e.dataType)} ${ev.primitiveTerm} =
-            ${ctx.defaultValue(e.dataType)};
-          if (!${ev.nullTerm}) ${ev.primitiveTerm} =
-            (${ctx.boxedType(e.dataType)})${ev.objectTerm};
+          ${ctx.primitiveType(e.dataType)} ${ev.primitiveTerm} = ${ctx.defaultValue(e.dataType)};
+          if (!${ev.nullTerm}) {
+            ${ev.primitiveTerm} = (${ctx.boxedType(e.dataType)})${ev.objectTerm};
+          }
     """
   }
 
@@ -155,17 +157,17 @@ abstract class BinaryExpression extends Expression with trees.BinaryNode[Express
 
   override def toString: String = s"($left $symbol $right)"
 
-
   /**
    * Short hand for generating binary evaluation code, which depends on two sub-evaluations of
    * the same type.  If either of the sub-expressions is null, the result of this computation
    * is assumed to be null.
    *
-   * @param f a function from two primitive term names to a tree that evaluates them.
+   * @param f accepts two variable names and returns Java code to compute the output.
    */
-  def evaluate(ctx: CodeGenContext,
-               ev: GeneratedExpressionCode,
-               f: (String, String) => String): String = {
+  protected def defineCodeGen(
+      ctx: CodeGenContext,
+      ev: GeneratedExpressionCode,
+      f: (String, String) => String): String = {
     // TODO: Right now some timestamp tests fail if we enforce this...
     if (left.dataType != right.dataType) {
       // log.warn(s"${left.dataType} != ${right.dataType}")
@@ -197,9 +199,22 @@ abstract class LeafExpression extends Expression with trees.LeafNode[Expression]
 
 abstract class UnaryExpression extends Expression with trees.UnaryNode[Expression] {
   self: Product =>
-  def castOrNull(ctx: CodeGenContext,
-                 ev: GeneratedExpressionCode,
-                 f: String => String): String = {
+
+  /**
+   * Called by unary expressions to generate a code block that returns null if its parent returns
+   * null, and if not not null, use `f` to generate the expression.
+   *
+   * As an example, the following does a boolean inversion (i.e. NOT).
+   * {{{
+   *   defineCodeGen(ctx, ev, c => s"!($c)")
+   * }}}
+   *
+   * @param f function that accepts a variable name and returns Java code to compute the output.
+   */
+  protected def defineCodeGen(
+      ctx: CodeGenContext,
+      ev: GeneratedExpressionCode,
+      f: String => String): String = {
     val eval = child.gen(ctx)
     eval.code + s"""
       boolean ${ev.nullTerm} = ${eval.nullTerm};
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala
index aad8479dafe41..a049f8878ed32 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala
@@ -87,6 +87,7 @@ case class Abs(child: Expression) extends UnaryArithmetic {
 abstract class BinaryArithmetic extends BinaryExpression {
   self: Product =>
 
+  /** Name of the function for this expression on a [[Decimal]] type. */
   def decimalMethod: String = ""
 
   override def dataType: DataType = left.dataType
@@ -119,9 +120,9 @@ abstract class BinaryArithmetic extends BinaryExpression {
 
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
     if (left.dataType.isInstanceOf[DecimalType]) {
-      evaluate(ctx, ev, { case (eval1, eval2) => s"$eval1.$decimalMethod($eval2)" } )
+      defineCodeGen(ctx, ev, (eval1, eval2) => s"$eval1.$decimalMethod($eval2)")
     } else {
-      evaluate(ctx, ev, { case (eval1, eval2) => s"$eval1 $symbol $eval2" } )
+      defineCodeGen(ctx, ev, (eval1, eval2) => s"$eval1 $symbol $eval2")
     }
   }
 
@@ -205,6 +206,9 @@ case class Divide(left: Expression, right: Expression) extends BinaryArithmetic
     }
   }
 
+  /**
+   * Special case handling due to division by 0 => null.
+   */
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
     val eval1 = left.gen(ctx)
     val eval2 = right.gen(ctx)
@@ -221,8 +225,7 @@ case class Divide(left: Expression, right: Expression) extends BinaryArithmetic
     eval1.code + eval2.code +
       s"""
       boolean ${ev.nullTerm} = false;
-      ${ctx.primitiveType(left.dataType)} ${ev.primitiveTerm} =
-        ${ctx.defaultValue(left.dataType)};
+      ${ctx.primitiveType(left.dataType)} ${ev.primitiveTerm} = ${ctx.defaultValue(left.dataType)};
       if (${eval1.nullTerm} || ${eval2.nullTerm} || $test) {
         ${ev.nullTerm} = true;
       } else {
@@ -263,6 +266,9 @@ case class Remainder(left: Expression, right: Expression) extends BinaryArithmet
     }
   }
 
+  /**
+   * Special case handling for x % 0 ==> null.
+   */
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
     val eval1 = left.gen(ctx)
     val eval2 = right.gen(ctx)
@@ -279,8 +285,7 @@ case class Remainder(left: Expression, right: Expression) extends BinaryArithmet
     eval1.code + eval2.code +
       s"""
       boolean ${ev.nullTerm} = false;
-      ${ctx.primitiveType(left.dataType)} ${ev.primitiveTerm} =
-        ${ctx.defaultValue(left.dataType)};
+      ${ctx.primitiveType(left.dataType)} ${ev.primitiveTerm} = ${ctx.defaultValue(left.dataType)};
       if (${eval1.nullTerm} || ${eval2.nullTerm} || $test) {
         ${ev.nullTerm} = true;
       } else {
@@ -337,7 +342,7 @@ case class BitwiseOr(left: Expression, right: Expression) extends BinaryArithmet
 }
 
 /**
- * A function that calculates bitwise xor(^) of two numbers.
+ * A function that calculates bitwise xor of two numbers.
  */
 case class BitwiseXor(left: Expression, right: Expression) extends BinaryArithmetic {
   override def symbol: String = "^"
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/decimalFunctions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/decimalFunctions.scala
index 80c51cb3588ad..f1d8313b5f1dc 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/decimalFunctions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/decimalFunctions.scala
@@ -67,14 +67,13 @@ case class MakeDecimal(child: Expression, precision: Int, scale: Int) extends Un
     val eval = child.gen(ctx)
     eval.code + s"""
       boolean ${ev.nullTerm} = ${eval.nullTerm};
-      org.apache.spark.sql.types.Decimal ${ev.primitiveTerm} =
-       ${ctx.defaultValue(DecimalType())};
+      org.apache.spark.sql.types.Decimal ${ev.primitiveTerm} = ${ctx.defaultValue(DecimalType())};
 
       if (!${ev.nullTerm}) {
-       ${ev.primitiveTerm} = new org.apache.spark.sql.types.Decimal();
-       ${ev.primitiveTerm} =
-         ${ev.primitiveTerm}.setOrNull(${eval.primitiveTerm}, $precision, $scale);
-       ${ev.nullTerm} = ${ev.primitiveTerm} == null;
+        ${ev.primitiveTerm} = new org.apache.spark.sql.types.Decimal();
+        ${ev.primitiveTerm} =
+          ${ev.primitiveTerm}.setOrNull(${eval.primitiveTerm}, $precision, $scale);
+        ${ev.nullTerm} = ${ev.primitiveTerm} == null;
       }
       """
   }
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/literals.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/literals.scala
index 21e21000c9437..1899c47613aae 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/literals.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/literals.scala
@@ -88,6 +88,7 @@ case class Literal protected (value: Any, dataType: DataType) extends LeafExpres
           ${ctx.primitiveType(dataType)} ${ev.primitiveTerm} = ${ctx.defaultValue(dataType)};
         """
     } else {
+      // TODO(cg): Add support for more data types.
       dataType match {
         case StringType =>
           val v = value.asInstanceOf[UTF8String]
@@ -96,12 +97,12 @@ case class Literal protected (value: Any, dataType: DataType) extends LeafExpres
             final boolean ${ev.nullTerm} = false;
             ${ctx.stringType} ${ev.primitiveTerm} = new ${ctx.stringType}().set(${arr});
            """
-        case FloatType =>
+        case FloatType =>  // This must go before NumericType
           s"""
             final boolean ${ev.nullTerm} = false;
             float ${ev.primitiveTerm} = ${value}f;
            """
-        case dt: DecimalType =>
+        case dt: DecimalType =>  // This must go before NumericType
           s"""
             final boolean ${ev.nullTerm} = false;
             ${ctx.primitiveType(dt)} ${ev.primitiveTerm} =
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/nullFunctions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/nullFunctions.scala
index 7b26bd2697195..e380eafc3fc2a 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/nullFunctions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/nullFunctions.scala
@@ -61,9 +61,9 @@ case class Coalesce(children: Seq[Expression]) extends Expression {
     children.map { e =>
       val eval = e.gen(ctx)
       s"""
-        if(${ev.nullTerm}) {
+        if (${ev.nullTerm}) {
           ${eval.code}
-          if(!${eval.nullTerm}) {
+          if (!${eval.nullTerm}) {
             ${ev.nullTerm} = false;
             ${ev.primitiveTerm} = ${eval.primitiveTerm};
           }
@@ -137,9 +137,9 @@ case class AtLeastNNonNulls(n: Int, children: Seq[Expression]) extends Predicate
     val code = children.map { e =>
       val eval = e.gen(ctx)
       s"""
-        if($nonnull < $n) {
+        if ($nonnull < $n) {
           ${eval.code}
-          if(!${eval.nullTerm}) {
+          if (!${eval.nullTerm}) {
             $nonnull += 1;
           }
         }
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala
index ad4535a09e04e..67cac26fd0d55 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala
@@ -85,7 +85,7 @@ case class Not(child: Expression) extends UnaryExpression with Predicate with Ex
   }
 
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
-    castOrNull(ctx, ev, c => s"!($c)")
+    defineCodeGen(ctx, ev, c => s"!($c)")
   }
 }
 
@@ -220,13 +220,13 @@ abstract class BinaryComparison extends BinaryExpression with Predicate {
   self: Product =>
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
     left.dataType match {
-      case dt: NumericType if ctx.isNativeType(dt) => evaluate (ctx, ev, {
+      case dt: NumericType if ctx.isNativeType(dt) => defineCodeGen (ctx, ev, {
         (c1, c3) => s"$c1 $symbol $c3"
       })
       case TimestampType =>
         // java.sql.Timestamp does not have compare()
         super.genCode(ctx, ev)
-      case other => evaluate (ctx, ev, {
+      case other => defineCodeGen (ctx, ev, {
         (c1, c2) => s"$c1.compare($c2) $symbol 0"
       })
     }
@@ -277,7 +277,7 @@ case class EqualTo(left: Expression, right: Expression) extends BinaryComparison
     else java.util.Arrays.equals(l.asInstanceOf[Array[Byte]], r.asInstanceOf[Array[Byte]])
   }
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
-    evaluate(ctx, ev, ctx.equalFunc(left.dataType))
+    defineCodeGen(ctx, ev, ctx.equalFunc(left.dataType))
   }
 }
 
@@ -392,7 +392,7 @@ case class If(predicate: Expression, trueValue: Expression, falseValue: Expressi
       boolean ${ev.nullTerm} = false;
       ${ctx.primitiveType(dataType)} ${ev.primitiveTerm} = ${ctx.defaultValue(dataType)};
       ${condEval.code}
-      if(!${condEval.nullTerm} && ${condEval.primitiveTerm}) {
+      if (!${condEval.nullTerm} && ${condEval.primitiveTerm}) {
         ${trueEval.code}
         ${ev.nullTerm} = ${trueEval.nullTerm};
         ${ev.primitiveTerm} = ${trueEval.primitiveTerm};