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
    *

jvm-packages/xgboost4j/src/test/java/ml/dmlc/xgboost4j/java/DMatrixTest.java

@@ -223,4 +223,33 @@ public class DMatrixTest {
     TestCase.assertTrue(dmat0.rowNum() == 10);
     TestCase.assertTrue(dmat0.getLabel().length == 10);
   }
+
+  @Test
+  public void testSetAndGetGroup() throws XGBoostError {
+    //create DMatrix from 10*5 dense matrix
+    int nrow = 10;
+    int ncol = 5;
+    float[] data0 = new float[nrow * ncol];
+    //put random nums
+    Random random = new Random();
+    for (int i = 0; i < nrow * ncol; i++) {
+      data0[i] = random.nextFloat();
+    }
+
+    //create label
+    float[] label0 = new float[nrow];
+    for (int i = 0; i < nrow; i++) {
+      label0[i] = random.nextFloat();
+    }
+
+    //create two groups
+    int[] groups = new int[]{5, 5};
+
+    DMatrix dmat0 = new DMatrix(data0, nrow, ncol, -0.1f);
+    dmat0.setLabel(label0);
+    dmat0.setGroup(groups);
+
+    //check
+    TestCase.assertTrue(Arrays.equals(new int[]{0, 5, 10}, dmat0.getGroup()));
+  }
 }