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There is a typo in JPDAThreadGroup's method. That probably means nobody ever used it. Obvious suggestion - write a test. This is a general request, close it with description how you approached the tests, what they test, how we can try them. So we can do real verification of this issue.
Of special interest are tests for debugger core, jpda, lookup and interoperability with rest of NetBeans (issue 41332).
Unit tests are available in debuggercore and debuggerjdpa module (main modules, test/unit directories) and they test debugger core and jpda respectively. Tests in core work with a fake debugger engine provider and test breakpoints and watches registration and debugger startup. JPDA tests launch a debugee VM with a sample class and tests various aspects of JPDA API: connectors, all sorts of breakpoints, stack information, locals info, expression evaluation and stepping. Lookup tests are part of the debugger core test suite.
The tests are there and the coverage increase is amazing.