Initial check-in.

src/main/java/org/gitlab4j/api/GitLabApiConstants.java

+package org.gitlab4j.api;
+
+import org.gitlab4j.api.utils.JacksonJsonEnumHelper;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonValue;
+
+public interface GitLabApiConstants {
+
+    /** Enum to use for ordering the results of various API calls. */
+    public enum SortOrder {
+
+        ASC, DESC;
+
+        private static JacksonJsonEnumHelper<SortOrder> enumHelper = new JacksonJsonEnumHelper<>(SortOrder.class);
+
+        @JsonCreator
+        public static SortOrder forValue(String value) {
+            return enumHelper.forValue(value);
+        }
+
+        @JsonValue
+        public String toValue() {
+            return (name().toLowerCase());
+        }
+
+        @Override
+        public String toString() {
+            return (name().toLowerCase());
+        }
+    }
+
+    /** Enum to use for ordering the results of getProjects(). */
+    public enum ProjectOrderBy {
+
+        ID, NAME, PATH, CREATED_AT, UPDATED_AT, LAST_ACTIVITY;
+        private static JacksonJsonEnumHelper<ProjectOrderBy> enumHelper = new JacksonJsonEnumHelper<>(ProjectOrderBy.class);
+
+        @JsonCreator
+        public static ProjectOrderBy forValue(String value) {
+            return enumHelper.forValue(value);
+        }
+
+        @JsonValue
+        public String toValue() {
+            return (name().toLowerCase());
+        }
+
+        @Override
+        public String toString() {
+            return (name().toLowerCase());
+        }
+    }
+
+    /** Enum to use for ordering the results of getPipelines(). */
+    public enum PipelineOrderBy {
+
+        ID, STATUS, REF, USER_ID;
+
+        private static JacksonJsonEnumHelper<PipelineOrderBy> enumHelper = new JacksonJsonEnumHelper<>(PipelineOrderBy.class);
+
+        @JsonCreator
+        public static PipelineOrderBy forValue(String value) {
+            return enumHelper.forValue(value);
+        }
+
+        @JsonValue
+        public String toValue() {
+            return (name().toLowerCase());
+        }
+
+        @Override
+        public String toString() {
+            return (name().toLowerCase());
+        }
+    }
+
+    /** Enum to use for specifying the scope when calling getPipelines(). */
+    public enum PipelineScope {
+
+        RUNNING, PENDING, FINISHED, BRANCHES, TAGS;
+
+        private static JacksonJsonEnumHelper<PipelineScope> enumHelper = new JacksonJsonEnumHelper<>(PipelineScope.class);
+
+        @JsonCreator
+        public static PipelineScope forValue(String value) {
+            return enumHelper.forValue(value);
+        }
+
+        @JsonValue
+        public String toValue() {
+            return (name().toLowerCase());
+        }
+
+        @Override
+        public String toString() {
+            return (name().toLowerCase());
+        }
+    }
+}

src/main/java/org/gitlab4j/api/utils/JacksonJsonEnumHelper.java

+package org.gitlab4j.api.utils;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+
+public class JacksonJsonEnumHelper<E extends Enum<E>> {
+
+    private Map<String, E> valuesMap;
+
+    public JacksonJsonEnumHelper(Class<E> enumType) {
+        valuesMap = new HashMap<>();
+        for (E e : enumType.getEnumConstants())
+            valuesMap.put(e.name().toLowerCase(), e);
+    }
+
+    @JsonCreator
+    public E forValue(String value) {
+        return valuesMap.get(value);
+    }
+}
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