package org.obda.query.domain;


public abstract class ComparisonOperatorPredicate implements BinaryPredicate, OperatorPredicate {

	private int arity = 2;
}

package org.obda.query.domain;


public abstract class ComparisonOperatorPredicate implements OperatorPredicate {

	protected int arity = 2;
}

Remove the unused interface of BinaryPredicate. I also noticed that the variable arity has an access modifier of being "private" which has no function at all. I modified it into "protected" to make this variable accessible to its children.