Why is regression testing critical when validating wca4z-generated Java against the original COBOL?

Think of it like working hands-on with IBM mainframe systems — the answer here is 'Regression testing ensures optimizations in the Optimize phase did not break the COBOL source before transformation'. Running the existing COBOL regression test suite against the generated Java module is the most direct way to detect behavioral regressions, since the same inputs and expected outputs apply to both imp. On real mainframe exams, this concept comes up repeatedly so make sure you have it locked in.