Move of method to superclass with generics METHOD_REMOVED

[mederel]

Found a narrow usecase of moving a method to a superclass adding generic return type for Builder pattern.

Before:

• MyClass.java:

public class MyClass {
  int blabla;

  public MyClass withBlabla(int blibli) {
    this.blabla = blibli;
    return this;
  }
}

After:

• MyClass.java:

public class MyClass extends ParentClass<MyClass> {
}

• ParentClass.java:

public class ParentClass<T extends ParentClass<T>> {
  int blabla;

  public T withBlabla(int blibli) {
    this.blabla = blibli;
    return (T) this;
  }
}

When using the japicmp-maven-plugin against this change we get:

There is at least one incompatibility: blabla.MyClass.withBlabla(int):METHOD_REMOVED

Plugin config is:

          <parameter>
            <overrideCompatibilityChangeParameters>
              <overrideCompatibilityChangeParameter>
                <!-- Allow the adding of new method to interfaces -->
                <compatibilityChange>METHOD_ADDED_TO_INTERFACE</compatibilityChange>
                <binaryCompatible>true</binaryCompatible>
                <sourceCompatible>true</sourceCompatible>
              </overrideCompatibilityChangeParameter>
            </overrideCompatibilityChangeParameters>
            <breakBuildOnSourceIncompatibleModifications>true</breakBuildOnSourceIncompatibleModifications>
            <breakBuildOnBinaryIncompatibleModifications>true</breakBuildOnBinaryIncompatibleModifications>
            <!-- Allow new Maven API modules -->
            <ignoreMissingOldVersion>true</ignoreMissingOldVersion>
            <!-- Report only the failures in HTML report -->
            <onlyModified>true</onlyModified>
            <skipXmlReport>true</skipXmlReport>
            <skipDiffReport>true</skipDiffReport>
          </parameter>

All that is put together in this reproducer project:

reproducer-japicmp-error.zip

[siom79]

The point here is that a method with a generic return type (like public T withBlabla(int blibli)) is translated by the compiler to public Object withBlabla(int blibli). Hence; the return type has changed, despite the fact that you have restricted T to MyClass in the super class. The restriction of MyClass is only evaluated by the compiler, at runtime it is possible to invoke the method and assign the return value to any Object reference.