Accept r-value references in convert_table_for_return(): (#10131)

`cudf::jni::convert_table_for_return()` is usually used on tables returned from a libcudf API call.
It currently requires an l-value reference for its table argument. This necessitates parking the result of libcudf call in an avoidable temp variable.
This commit adds the option to use an r-value reference. This allows table expressions to be used directly, reducing clutter.

Note:
  1. The previous signature is retained, because not all call sites can use the r-value 
     interface cleanly. (E.g. when the libcudf call is complex.)
  2. The third argument (vector<unique_ptr<column>>) has been converted from l-ref to r-ref,
     so that an empty default can be introduced.

This commit also includes minor code cleanup in the periphery of calls to `convert_table_for_return()`.

Authors:
  - MithunR (https://github.com/mythrocks)

Approvers:
  - Jason Lowe (https://github.com/jlowe)

URL: #10131

java/src/main/native/include/jni_utils.hpp

@@ -395,6 +395,9 @@ template <typename T> class native_jpointerArray {
 
   T **data() { return reinterpret_cast<T **>(wrapped.data()); }
 
+  T *const *begin() const { return data(); }
+  T *const *end() const { return data() + size(); }
+
   const jlongArray get_jArray() const { return wrapped.get_jArray(); }
 
   jlongArray get_jArray() { return wrapped.get_jArray(); }

java/src/main/native/src/ColumnViewJni.cpp

@@ -561,18 +561,17 @@ JNIEXPORT jlong JNICALL Java_ai_rapids_cudf_ColumnView_listSortRows(JNIEnv *env,
 
 JNIEXPORT jlongArray JNICALL Java_ai_rapids_cudf_ColumnView_stringSplit(JNIEnv *env, jclass,
                                                                         jlong column_view,
-                                                                        jlong delimiter,
+                                                                        jlong delimiter_ptr,
                                                                         jint max_split) {
   JNI_NULL_CHECK(env, column_view, "column is null", 0);
-  JNI_NULL_CHECK(env, delimiter, "string scalar delimiter is null", 0);
+  JNI_NULL_CHECK(env, delimiter_ptr, "string scalar delimiter is null", 0);
   try {
     cudf::jni::auto_set_device(env);
-    cudf::column_view *cv = reinterpret_cast<cudf::column_view *>(column_view);
-    cudf::strings_column_view scv(*cv);
-    cudf::string_scalar *ss_scalar = reinterpret_cast<cudf::string_scalar *>(delimiter);
+    cudf::strings_column_view const scv{*reinterpret_cast<cudf::column_view *>(column_view)};
+    auto delimiter = reinterpret_cast<cudf::string_scalar *>(delimiter_ptr);
 
-    std::unique_ptr<cudf::table> table_result = cudf::strings::split(scv, *ss_scalar, max_split);
-    return cudf::jni::convert_table_for_return(env, table_result);
+    return cudf::jni::convert_table_for_return(env,
+                                               cudf::strings::split(scv, *delimiter, max_split));
   }
   CATCH_STD(env, 0);
 }
@@ -1410,13 +1409,12 @@ JNIEXPORT jlongArray JNICALL Java_ai_rapids_cudf_ColumnView_extractRe(JNIEnv *en
 
   try {
     cudf::jni::auto_set_device(env);
-    cudf::column_view *column_view = reinterpret_cast<cudf::column_view *>(j_view_handle);
-    cudf::strings_column_view strings_column(*column_view);
+    cudf::strings_column_view const strings_column{
+        *reinterpret_cast<cudf::column_view *>(j_view_handle)};
     cudf::jni::native_jstring pattern(env, patternObj);
 
-    std::unique_ptr<cudf::table> table_result =
-        cudf::strings::extract(strings_column, pattern.get());
-    return cudf::jni::convert_table_for_return(env, table_result);
+    return cudf::jni::convert_table_for_return(
+        env, cudf::strings::extract(strings_column, pattern.get()));
   }
   CATCH_STD(env, 0);
 }