I Built an AI-Powered CLI That Migrates Legacy Java Code to Java 17/21/25

I Built an AI-Powered CLI That Migrates Legacy Java Code to Java 17/21/25 If you've spent any time in enterprise Java, you know the feeling. You open a service that's been running since 2014 and you're greeted by walls of anonymous inner classes, verbose null checks, Collections.unmodifiableList wrapping a new ArrayList , and switch statements with more break keywords than actual logic. Individually each pattern takes 30 seconds to fix. But across a codebase with 300 files, it's a week of mechanical work — and that's before you factor in the code review. So I built java-migrate : a CLI tool that scans your Java files, detects legacy patterns, and sends them to Claude with a precise system prompt to get them modernised. One command, instant diff, no surprises. What it looks like in practice Here's a typical legacy file before migration: public class LegacyService { // POJO with getters/setters public static class User { private String name ; private int age ; public String getName () { return name ; } public void setName ( String name ) { this . name = name ; } public int getAge () { return age ; } public void setAge ( int age ) { this . age = age ; } } public List < User > sortUsers ( List < User > users ) { // Anonymous Comparator users . sort ( new Comparator < User >() { @Override public int compare ( User a , User b ) { return a . getName (). compareTo ( b . getName ()); } }); return Collections . unmodifiableList ( users ); } public String describeObject ( Object obj ) { // instanceof + cast if ( obj instanceof String ) { String s = ( String ) obj ; return "String of length " + s . length (); } return "Unknown" ; } public String classify ( int value ) { // switch statement String result ; switch ( value ) { case 1 : result = "one" ; break ; case 2 : result = "two" ; break ; default : result = "other" ; } return result ; } } Run java-migrate LegacyService.java --dry-run --verbose and you get this diff: - public static class User { - private String name; - privat