[SPARK-47341][SQL] Fix inaccurate documentation of RuntimeConfig#get

connector/connect/client/jvm/src/main/scala/org/apache/spark/sql/internal/ConnectRuntimeConfig.scala

@@ -38,7 +38,7 @@ class ConnectRuntimeConfig private[sql] (client: SparkConnectClient)
   }
 
   /** @inheritdoc */
-  @throws[NoSuchElementException]("if the key is not set")
+  @throws[NoSuchElementException]("if the key is not set and there is no default value")
   def get(key: String): String = getOption(key).getOrElse {
     throw new NoSuchElementException(key)
   }

sql/api/src/main/scala/org/apache/spark/sql/RuntimeConfig.scala

@@ -54,17 +54,20 @@ abstract class RuntimeConfig {
   }
 
   /**
-   * Returns the value of Spark runtime configuration property for the given key.
+   * Returns the value of Spark runtime configuration property for the given key. If the key is
+   * not set yet, return `defaultValue` in its [[ConfigEntry]].
    *
    * @throws java.util.NoSuchElementException
    *   if the key is not set and does not have a default value
    * @since 2.0.0
    */
-  @throws[NoSuchElementException]("if the key is not set")
+  @throws[NoSuchElementException]("if the key is not set and there is no default value")
   def get(key: String): String
 
   /**
-   * Returns the value of Spark runtime configuration property for the given key.
+   * Returns the value of Spark runtime configuration property for the given key. If the key is
+   * not set yet, return the given `default`. This is useful when `defaultValue` of its
+   * [[ConfigEntry]] is not the desired one.
    *
    * @since 2.0.0
    */
@@ -78,7 +81,9 @@ abstract class RuntimeConfig {
   def getAll: Map[String, String]
 
   /**
-   * Returns the value of Spark runtime configuration property for the given key.
+   * Returns the value of Spark runtime configuration property for the given key. If the key is
+   * not set yet, return `defaultValue` of its [[ConfigEntry]]. If there is no `defaultValue`,
+   * return `None`.
    *
    * @since 2.0.0
    */

sql/core/src/main/scala/org/apache/spark/sql/internal/RuntimeConfigImpl.scala

@@ -42,7 +42,7 @@ class RuntimeConfigImpl private[sql](val sqlConf: SQLConf = new SQLConf) extends
   }
 
   /** @inheritdoc */
-  @throws[NoSuchElementException]("if the key is not set")
+  @throws[NoSuchElementException]("if the key is not set and there is no default value")
   def get(key: String): String = {
     sqlConf.getConfString(key)
   }

sql/core/src/test/scala/org/apache/spark/sql/RuntimeConfigSuite.scala

@@ -19,7 +19,7 @@ package org.apache.spark.sql
 
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.internal.config
-import org.apache.spark.sql.internal.RuntimeConfigImpl
+import org.apache.spark.sql.internal.{RuntimeConfigImpl, SQLConf}
 import org.apache.spark.sql.internal.SQLConf.CHECKPOINT_LOCATION
 import org.apache.spark.sql.internal.StaticSQLConf.GLOBAL_TEMP_DATABASE
 
@@ -81,4 +81,24 @@ class RuntimeConfigSuite extends SparkFunSuite {
     }
     assert(ex.getMessage.contains("Spark config"))
   }
+
+  test("set and get a config with defaultValue") {
+    val conf = newConf()
+    val key = SQLConf.SESSION_LOCAL_TIMEZONE.key
+    // By default, the value when getting an unset config entry is its defaultValue.
+    assert(conf.get(key) == SQLConf.SESSION_LOCAL_TIMEZONE.defaultValue.get)
+    assert(conf.getOption(key).contains(SQLConf.SESSION_LOCAL_TIMEZONE.defaultValue.get))
+    // Get the unset config entry with a different default value, which should return the given
+    // default parameter.
+    assert(conf.get(key, "Europe/Amsterdam") == "Europe/Amsterdam")
+
+    // Set a config entry.
+    conf.set(key, "Europe/Berlin")
+    // Get the set config entry.
+    assert(conf.get(key) == "Europe/Berlin")
+    // Unset the config entry.
+    conf.unset(key)
+    // Get the unset config entry, which should return its defaultValue again.
+    assert(conf.get(key) == SQLConf.SESSION_LOCAL_TIMEZONE.defaultValue.get)
+  }
 }