Applying pull up method refactoring to move B.m to class A, makes A abstract.
Making a class abstract may be not safe in some domains, such as, library (API) developments.  Even, in other domains, the developer may not want to make a class abstract. I think at least NetBeans should report a message to alloy the user choosing between make a class abstract or give up the transformation.

Before Refactoring:

public class A {}
abstract class B extends A {
  public abstract int m(int a);
}


Resulting Program:

public abstract class A {
  public abstract int m(int a); 
}

abstract class B extends A {}