DATAMONGO-1542 - Polishing.

Added some static entry points for better readability.

Original Pull Request: #421

Author: christophstrobl

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

@@ -6438,12 +6438,52 @@ private void assertNotBuilder(Object toCheck, String name) {
 		/**
 		 * Get a builder that allows fluent creation of {@link Cond}.
 		 *
-		 * @return a new {@link ConditionalExpressionBuilder}.
+		 * @return never {@literal null}.
 		 */
-		public static ConditionalExpressionBuilder newBuilder() {
+		public static WhenBuilder newBuilder() {
 			return ConditionalExpressionBuilder.newBuilder();
 		}
 
+		/**
+		 * Start creating new {@link Cond} by providing the boolean expression used in {@code if}.
+		 *
+		 * @param booleanExpression must not be {@literal null}.
+		 * @return never {@literal null}.
+		 */
+		public static ThenBuilder when(DBObject booleanExpression) {
+			return ConditionalExpressionBuilder.newBuilder().when(booleanExpression);
+		}
+
+		/**
+		 * Start creating new {@link Cond} by providing the {@link AggregationExpression} used in {@code if}.
+		 *
+		 * @param expression expression that yields in a boolean result, must not be {@literal null}.
+		 * @return never {@literal null}.
+		 */
+		public static ThenBuilder when(AggregationExpression expression) {
+			return ConditionalExpressionBuilder.newBuilder().when(expression);
+		}
+
+		/**
+		 * Start creating new {@link Cond} by providing the field reference used in {@code if}.
+		 *
+		 * @param booleanField name of a field holding a boolean value, must not be {@literal null}.
+		 * @return never {@literal null}.
+		 */
+		public static ThenBuilder when(String booleanField) {
+			return ConditionalExpressionBuilder.newBuilder().when(booleanField);
+		}
+
+		/**
+		 * Start creating new {@link Cond} by providing the {@link CriteriaDefinition} used in {@code if}.
+		 *
+		 * @param criteria criteria to evaluate, must not be {@literal null}.
+		 * @return the {@link ThenBuilder}
+		 */
+		public static ThenBuilder when(CriteriaDefinition criteria) {
+			return ConditionalExpressionBuilder.newBuilder().when(criteria);
+		}
+
 		/**
 		 * @author Mark Paluch
 		 */

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperationUnitTests.java

@@ -1706,10 +1706,6 @@ public void shouldRenderMapAggregationExpressionOnExpression() {
 				"{ $project:{ adjustedGrades:{ $map: { input: { $size : [\"foo\"]}, as: \"grade\",in: { $add: [ \"$$grade\", 2 ] }}}}}")));
 	}
 
-	private static DBObject exctractOperation(String field, DBObject fromProjectClause) {
-		return (DBObject) fromProjectClause.get(field);
-	}
-
 	/**
 	 * @see DATAMONGO-861, DATAMONGO-1542
 	 */
@@ -1748,4 +1744,8 @@ public void fieldReplacementIfNullShouldRenderCorrectly() {
 
 		assertThat(agg, is(JSON.parse("{ $project: { result: { $ifNull: [ \"$optional\", \"$never-null\" ] } } }")));
 	}
+
+	private static DBObject exctractOperation(String field, DBObject fromProjectClause) {
+		return (DBObject) fromProjectClause.get(field);
+	}
 }