Add public group getter for java and scala (#4838)

* Add public group getter for java and scala

* Remove unnecessary param from javadoc

* Fix typo

* Fix another typo

* Add semicolon

* Fix javadoc return statement

* Fix missing return statement

* Add a unit test

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/DMatrix.java

@@ -204,6 +204,16 @@ public class DMatrix {
     XGBoostJNI.checkCall(XGBoostJNI.XGDMatrixSetGroup(handle, group));
   }
 
+  /**
+   * Get group sizes of DMatrix
+   *
+   * @throws XGBoostError native error
+   * @return group size as array
+   */
+  public int[] getGroup() throws XGBoostError {
+    return getIntInfo("group_ptr");
+  }
+
   private float[] getFloatInfo(String field) throws XGBoostError {
     float[][] infos = new float[1][];
     XGBoostJNI.checkCall(XGBoostJNI.XGDMatrixGetFloatInfo(handle, field, infos));

jvm-packages/xgboost4j/src/main/scala/ml/dmlc/xgboost4j/scala/DMatrix.scala

@@ -149,6 +149,14 @@ class DMatrix private[scala](private[scala] val jDMatrix: JDMatrix) {
     jDMatrix.setGroup(group)
   }
 
+  /**
+   * Get group sizes of DMatrix (used for ranking)
+   */
+  @throws(classOf[XGBoostError])
+  def getGroup(): Array[Int] = {
+    jDMatrix.getGroup()
+  }
+
   /**
    * get label values
    *