Covariant return type causes false positive for method removal due to return type change.

[busbey]

Over in Apache HBase we had a change (509c1b63) that added a method to a supertype of a class such that the original class' method became an override via covariant return type.

In summary, before the change:

class Query {}

class Scan extends Query {
  public Scan setLoadColumnFamiliesOnDemand(boolean value);
}

and after the change:

class Query {
  public Query setLoadColumnFamiliesOnDemand(boolean value);
}

class Scan extends Query {
  public Scan setLoadColumnFamiliesOnDemand(boolean value);
}

Using japi-compliance-checker 2.4, this gets reported as a change in return type that will cause a High Severity binary compatibility problem for MethodNotFoundException and a Medium Severity source compatibility problem for downstream users.

e.g.

Problems with Methods, High Severity  1 

---

  hbase-client-1.2.6.jar, Scan.class
  package org.apache.hadoop.hbase.client
  [+] Scan.setLoadColumnFamiliesOnDemand ( boolean value )  :  Scan  1   
 

  org/apache/hadoop/hbase/client/Scan.setLoadColumnFamiliesOnDemand:(Z)Lorg/apache/hadoop/hbase/client/Scan;
           

	Change	Effect
1	Return value type has been changed from Scan to Query.	This method has been removed because the return type is part of the method signature. A client program may be interrupted by NoSuchMethodError exception.

 
Examining the class file for the updated Scan class shows that since the method now has a covariant return type two method signatures are included:

busbey$ javap -cp target/compat-check/dst/hbase-client/target/hbase-client-1.2.7.jar org.apache.hadoop.hbase.client.Scan | grep setLoadColumnFamiliesOnDemand
  public org.apache.hadoop.hbase.client.Scan setLoadColumnFamiliesOnDemand(boolean);
  public org.apache.hadoop.hbase.client.Query setLoadColumnFamiliesOnDemand(boolean);

japi-compliance-checker appears to mistakenly just pick up the second method signature and thus the compatibility issues.