What this means for learners
Javanese and Indonesian share 26 sounds — roughly 79% of Indonesian's inventory overlaps with Javanese. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 5 sounds found only in Indonesian represent the greatest pronunciation challenge for Javanese speakers. The adult brain tends to map unfamiliar sounds onto the closest native equivalent — a process that produces the characteristic "accent" of a second-language speaker. Learning to hear and produce these sounds as distinct requires focused ear training, not just repetition.
Conversely, Javanese has 7 sounds not used in Indonesian. Native Indonesian speakers learning Javanese will face the mirror-image challenge with these sounds.
Phoneme inventories from PHOIBLE. Data reflects one documented inventory per language; some variation exists across dialects and sources.