DATAMONGO-1843 - Polishing.

Convert anonymous classes to lambdas. Typo fixes. Migrate test to AssertJ.

Original pull request: #526.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ArrayOperators.java

@@ -32,6 +32,7 @@ import org.springframework.util.Assert;
  * Gateway to {@literal array} aggregation operations.
  *
  * @author Christoph Strobl
+ * @author Mark Paluch
  * @since 1.0
  */
 public class ArrayOperators {
@@ -224,16 +225,11 @@ public class ArrayOperators {
 		 * @param expression must not be {@literal null}.
 		 * @return
 		 */
-		public ArrayOperatorFactory.ReduceInitialValueBuilder reduce(final AggregationExpression expression) {
-			return new ArrayOperatorFactory.ReduceInitialValueBuilder() {
+		public ArrayOperatorFactory.ReduceInitialValueBuilder reduce(AggregationExpression expression) {
 
-				@Override
-				public Reduce startingWith(Object initialValue) {
-					return (usesFieldRef() ? Reduce.arrayOf(fieldReference)
+			return initialValue -> (usesFieldRef() ? Reduce.arrayOf(fieldReference)
 					: Reduce.arrayOf(ArrayOperatorFactory.this.expression)).withInitialValue(initialValue).reduce(expression);
 		}
-			};
-		}
 
 		/**
 		 * Start creating new {@link AggregationExpression} that applies an {@link AggregationExpression} to each element in
@@ -242,17 +238,11 @@ public class ArrayOperators {
 		 * @param expressions
 		 * @return
 		 */
-		public ArrayOperatorFactory.ReduceInitialValueBuilder reduce(final PropertyExpression... expressions) {
-
-			return new ArrayOperatorFactory.ReduceInitialValueBuilder() {
+		public ArrayOperatorFactory.ReduceInitialValueBuilder reduce(PropertyExpression... expressions) {
 
-				@Override
-				public Reduce startingWith(Object initialValue) {
-					return (usesFieldRef() ? Reduce.arrayOf(fieldReference) : Reduce.arrayOf(expression))
+			return initialValue -> (usesFieldRef() ? Reduce.arrayOf(fieldReference) : Reduce.arrayOf(expression))
 					.withInitialValue(initialValue).reduce(expressions);
 		}
-			};
-		}
 
 		/**
 		 * Creates new {@link AggregationExpression} that transposes an array of input arrays so that the first element of