Bigger than a finch cage, because the bird is
A Java sparrow, also called a Java finch, needs at least 30 inches long by 18 inches wide by 18 inches high for a pair, and 36 inches or longer in length once you keep the small flock this species prefers. A Java sparrow is a substantial finch, noticeably larger and heavier-billed than a zebra or society finch, with a strong, direct flight it uses constantly. The pretty little cages sold for generic finches give a Java sparrow room to hop and flutter, but not to fly, and a bird that cannot fly in its own home loses fitness and confidence quickly.
Keep bar spacing at about half an inch. That gap contains a Java sparrow safely while letting the birds grip and move along the bars, and it also suits the mixed peaceful finches some keepers house alongside them. Prioritise a rectangular cage with a sliding tray and a large door panel, because catching or checking a bird in a flock is calmer for everyone when you are not reaching through a small hatch into the flight path.
Lay out the long axis for real flight
Put the main perches at the two short ends and keep the corridor between them empty, so every flight crosses the cage's full length. Java sparrows commute back and forth as a group, often all launching together, and a cage furnished with ladders, swings, and hanging toys across the middle breaks that group flight into nervous fluttering. Use natural perches of varied thickness at the ends, a flat resting shelf, and a shallow bath at floor level, since a settled flock bathes socially and enthusiastically.
Feed a finch seed mix suited to a larger bill, with leafy greens and a little egg food during moulting and breeding condition, and use covered or well-placed dishes that a flock's worth of feet cannot foul within the hour. Position the cage where the birds see household life from the side rather than from a busy walkway, because Java sparrows are social with each other but appreciate a settled viewpoint on people.
Size the cage for the flock you will actually keep
Java sparrows are flock birds through and through: a pair is the minimum, and four to six birds in a long cage look and behave like the species should, chattering, bathing, and roosting in a row. Each addition argues for length, not height, so plan the flock size before you buy the cage rather than adding birds to a cage bought for two. In a spacious planted aviary they mix well with other sturdy, peaceful finches, but crowded indoor cages bring out feeder guarding that the same birds never show with room to spare.
Pairs breed readily when given a roomy nest box, so a keeper not wanting youngsters should skip nest boxes and sleeping pouches, which also keeps the flight corridor clearer. Whatever the group size, quarantine newcomers separately before they join the flock, because one sick bird in a tight flock cage reaches every perch mate quickly.
Measure, then write it down
Judge any cage by its interior length with perches fitted, measure bar spacing at a few points rather than once, and reject tall, narrow cages however decorative the stand. Note the final length, spacing, and how many birds the cage houses in the flock's own record, because with KinsLe's Transferrable Profiles Java sparrows that move to a new keeper as a group can bring their flight-cage dimensions and flock details with them, so the new home matches the space they are used to.