Modified to use enums instead of String literals.

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

 package org.gitlab4j.api;
 
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 
 import javax.ws.rs.core.GenericType;
 import javax.ws.rs.core.Response;
 
 import org.gitlab4j.api.models.Pipeline;
+import org.gitlab4j.api.models.PipelineStatus;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonValue;
 
 /**
  * This class provides an entry point to all the GitLab API pipeline calls.
  */
 public class PipelineApi extends AbstractApi {
 
+    /** Enum to use for specifying the scope when calling getPipelines(). */
+    public enum Scope {
+
+        RUNNING, PENDING, FINISHED, BRANCHES, TAGS;
+
+        private static Map<String, Scope> valuesMap = new HashMap<>(5);
+        static {
+            for (Scope scope : Scope.values())
+                valuesMap.put(scope.toValue(), scope);
+        }
+
+        @JsonCreator
+        public static Scope forValue(String value) {
+            return valuesMap.get(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 OrderBy {
+
+        ID, STATUS, REF, USER_ID;
+
+        private static Map<String, OrderBy> valuesMap = new HashMap<>(4);
+        static {
+            for (OrderBy orderBy : OrderBy.values())
+                valuesMap.put(orderBy.toValue(), orderBy);
+        }
+
+        @JsonCreator
+        public static OrderBy forValue(String value) {
+            return valuesMap.get(value);
+        }
+
+        @JsonValue
+        public String toValue() {
+            return (name().toLowerCase());
+        }
+
+        @Override
+        public String toString() {
+            return (name().toLowerCase());
+        }
+    }
+
     public PipelineApi(GitLabApi gitLabApi) {
         super(gitLabApi);
     }
@@ -37,19 +97,19 @@ public class PipelineApi extends AbstractApi {
      * GET /projects/:id/pipelines
      *
      * @param projectId the project ID to get the list of pipelines for
-     * @param scope the scope of pipelines, one of: running, pending, finished, branches, tags
-     * @param status the status of pipelines, one of: running, pending, success, failed, canceled, skipped
+     * @param scope the scope of pipelines, one of: RUNNING, PENDING, FINISHED, BRANCHES, TAGS
+     * @param status the status of pipelines, one of: RUNNING, PENDING, SUCCESS, FAILED, CANCELED, SKIPPED
      * @param ref the ref of pipelines
      * @param yamlErrors returns pipelines with invalid configurations
      * @param name the name of the user who triggered pipelines
      * @param username the username of the user who triggered pipelines
-     * @param orderBy order pipelines by id, status, ref, or user_id (default: id)
-     * @param sort sort pipelines in asc or desc order (default: desc)
+     * @param orderBy order pipelines by ID, STATUS, REF, USER_ID (default: ID)
+     * @param sort sort pipelines in "ASC" or "DESC" order (default: "DESC")
      * @return a list containing the pipelines for the specified project ID
      * @throws GitLabApiException if any exception occurs during execution
      */
-    public List<Pipeline> getPipelines(int projectId, String scope, String status, String ref, boolean yamlErrors, 
-            String name, String username, String orderBy, String sort) throws GitLabApiException {
+    public List<Pipeline> getPipelines(int projectId, Scope scope, PipelineStatus status, String ref, boolean yamlErrors, 
+            String name, String username, OrderBy orderBy, String sort) throws GitLabApiException {
         GitLabApiForm formData = new GitLabApiForm()
                 .withParam("scope", scope)
                 .withParam("status", status)
@@ -58,7 +118,7 @@ public class PipelineApi extends AbstractApi {
                 .withParam("name", name)
                 .withParam("username", username)
                 .withParam("order_by", orderBy)
-                .withParam("sort", sort);
+                .withParam("sort", (sort != null ? sort.toLowerCase() : null));
 
         Response response = get(Response.Status.OK, formData.asMap(), "projects", projectId, "pipelines");
         return (response.readEntity(new GenericType<List<Pipeline>>() {

src/main/java/org/gitlab4j/api/models/Pipeline.java

@@ -11,7 +11,7 @@ import javax.xml.bind.annotation.XmlRootElement;
 public class Pipeline {
 
     private Integer id;
-    private String status;
+    private PipelineStatus status;
     private String ref;
     private String sha;
     private String beforeSha;
@@ -34,11 +34,11 @@ public class Pipeline {
         this.id = id;
     }
 
-    public String getStatus() {
+    public PipelineStatus getStatus() {
         return status;
     }
 
-    public void setStatus(String status) {
+    public void setStatus(PipelineStatus status) {
         this.status = status;
     }