Error in executing spark examples:

Using Spark's default log4j profile: org/apache/spark/log4j-defaults.properties
18/05/25 11:09:45 INFO SparkContext: Running Spark version 2.1.1
18/05/25 11:09:45 WARN NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
18/05/25 11:09:45 ERROR SparkContext: Error initializing SparkContext.
org.apache.spark.SparkException: A master URL must be set in your configuration
	at org.apache.spark.SparkContext.<init>(SparkContext.scala:379)
	at org.apache.spark.SparkContext$.getOrCreate(SparkContext.scala:2320)
	at org.apache.spark.sql.SparkSession$Builder$$anonfun$6.apply(SparkSession.scala:868)
	at org.apache.spark.sql.SparkSession$Builder$$anonfun$6.apply(SparkSession.scala:860)
	at scala.Option.getOrElse(Option.scala:121)
	at org.apache.spark.sql.SparkSession$Builder.getOrCreate(SparkSession.scala:860)
	at org.apache.spark.examples.graphx.AggregateMessagesExample$.main(AggregateMessagesExample.scala:42)
	at org.apache.spark.examples.graphx.AggregateMessagesExample.main(AggregateMessagesExample.scala)
18/05/25 11:09:45 INFO SparkContext: Successfully stopped SparkContext
Exception in thread "main" org.apache.spark.SparkException: A master URL must be set in your configuration
	at org.apache.spark.SparkContext.<init>(SparkContext.scala:379)
	at org.apache.spark.SparkContext$.getOrCreate(SparkContext.scala:2320)
	at org.apache.spark.sql.SparkSession$Builder$$anonfun$6.apply(SparkSession.scala:868)
	at org.apache.spark.sql.SparkSession$Builder$$anonfun$6.apply(SparkSession.scala:860)
	at scala.Option.getOrElse(Option.scala:121)
	at org.apache.spark.sql.SparkSession$Builder.getOrCreate(SparkSession.scala:860)
	at org.apache.spark.examples.graphx.AggregateMessagesExample$.main(AggregateMessagesExample.scala:42)
	at org.apache.spark.examples.graphx.AggregateMessagesExample.main(AggregateMessagesExample.scala)

Process finished with exit code 1

Solution: add parameters to the boot VM

-Dspark.master=local