cobol to java: payroll processor

This repo shows a real migration from COBOL batch processing to Java. The original ran on IBM MVS mainframes in the late 1980s. It processed weekly payroll for a manufacturing company — reading fixed-width employee records, calculating gross pay with overtime, applying tax codes, and printing a report to a sequential file.

what the original does

PAYROLL.cob reads an 80-character fixed-width sequential file. Each record holds an employee ID, name, hourly rate, hours worked for the week, and a tax code (S=single, M=married, E=exempt). It calculates overtime at 1.5x for hours over 40, applies the tax rate, and writes a formatted report line. It's typical OS/VS COBOL — WORKING-STORAGE for all variables, FILE SECTION with FD entries, PERFORM loops instead of functions.

what changed in the migration

The Java version keeps the same business logic but uses modern constructs:

• BigDecimal throughout — COBOL's packed decimal arithmetic doesn't map cleanly to double, so we use BigDecimal with RoundingMode.HALF_UP to match the original's rounding behavior
• Java records for Employee and PayrollResult — immutable, no setter sprawl
• The fixed-width parser uses the same byte offsets as the COBOL FD definition, so existing data files work without conversion
• Tax rates in a Map instead of an EVALUATE block — same logic, easier to extend

What we didn't change: the calculation logic itself. Same overtime threshold, same tax rate values, same rounding. The goal was a 1:1 functional replacement, not a redesign.

running it

COBOL (requires GnuCOBOL):

cobc -x PAYROLL.cob -o payroll
./payroll

Java (requires JDK 16+):

javac PayrollProcessor.java
java PayrollProcessor employees.dat

The input file format is the same for both: 80-char fixed-width records matching the COBOL FD layout.

about codemigra

We do this for a living — COBOL, VB6, Fortran, Delphi to modern languages. codemigra.com