private <IN1, IN2, OUT> List<OUT> executeBinaryOperator(DualInputOperator<?, ?, ?, ?> operator, int superStep) throws Exception {
Operator<?> inputOp1 = operator.getFirstInput();
Operator<?> inputOp2 = operator.getSecondInput();
if (inputOp1 == null) {
throw new InvalidProgramException("The binary operation " + operator.getName() + " has no first input.");
}
if (inputOp2 == null) {
throw new InvalidProgramException("The binary operation " + operator.getName() + " has no second input.");
}
// compute inputs
@SuppressWarnings("unchecked")
List<IN1> inputData1 = (List<IN1>) execute(inputOp1, superStep);