fix: addresses PR comments

Author: thiagotnunes

google-cloud-spanner/src/main/java/com/google/cloud/spanner/InstanceAdminClientImpl.java

@@ -47,8 +47,6 @@ protected com.google.iam.v1.Policy toPb(Policy policy) {
  }
  }
 
- static final String NOT_BOTH_NODE_COUNT_AND_PROCESSING_UNITS =
- "Only one of nodeCount and processingUnits can be set when creating a new instance";
  private static final PathTemplate PROJECT_NAME_TEMPLATE =
  PathTemplate.create("projects/{project}");
  private final DatabaseAdminClient dbClient;
@@ -100,7 +98,7 @@ public OperationFuture<Instance, CreateInstanceMetadata> createInstance(Instance
  throws SpannerException {
  Preconditions.checkArgument(
  instance.getNodeCount() == 0 || instance.getProcessingUnits() == 0,
- NOT_BOTH_NODE_COUNT_AND_PROCESSING_UNITS);
+ "Only one of nodeCount and processingUnits can be set when creating a new instance");
  String projectName = PROJECT_NAME_TEMPLATE.instantiate("project", projectId);
  OperationFuture<com.google.spanner.admin.instance.v1.Instance, CreateInstanceMetadata>
  rawOperationFuture =

google-cloud-spanner/src/main/java/com/google/cloud/spanner/InstanceInfo.java

@@ -78,16 +78,16 @@ public abstract static class Builder {
  public abstract Builder setDisplayName(String displayName);
 
  /**
- * Sets the number of nodes for the instance. Only one of processing units or node count must be
- * set when creating a new instance.
+ * Sets the number of nodes for the instance. Exactly one of processing units or node count must
+ * be set when creating a new instance.
  */
  public abstract Builder setNodeCount(int nodeCount);
 
  /**
- * Sets the number of processing units for the instance. Only one of processing units or node
+ * Sets the number of processing units for the instance. Exactly one of processing units or node
  * count must be set when creating a new instance. Processing units must be between 1 and 999
  * (inclusive) when creating a new instance with node count = 0. Processing units from 1000 and
- * up must always be a multiple of 1000 (i.e. equal to an integer number of nodes).
+ * up must always be a multiple of 1000 (that is equal to an integer number of nodes).
  */
  public Builder setProcessingUnits(int processingUnits) {
  throw new UnsupportedOperationException("Unimplemented");

google-cloud-spanner/src/test/java/com/google/cloud/spanner/InstanceAdminClientImplTest.java

@@ -177,7 +177,8 @@ public void testCreateInstanceWithBothNodeCountAndProcessingUnits() throws Excep
  } catch (IllegalArgumentException e) {
  assertTrue(
  e.getMessage()
- .contains(InstanceAdminClientImpl.NOT_BOTH_NODE_COUNT_AND_PROCESSING_UNITS));
+ .contains(
+ "Only one of nodeCount and processingUnits can be set when creating a new instance"));
  }
  }