Giant Laulau (Syzygium megacarpa) is widespread across the Pacific. These red fruits apparently taste similar to apples though I haven’t yet tried one. This one was at Hawai’i Tropical Bioreserve & Garden.
For more information about Hawai’i Tropical Bioreserve & Garden, go to htbg.com.
Graham's Island 
These look like a fruit I know as Otaheite apples, which are also known rose apples or Malay apples. As a child it was always a treat to get it when I could. The flesh is pure white with a kind of spongy texture. I thought it was called an apple because of the color rather than the taste. The taste is mild, not acidic and not very sweet. I think this name Syzygium megacarpa seems to refer to a range of fruit but the one I know is less elongated and more like this …
https://www.tasteatlas.com/water-apple
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I think yours is from the same Syzygium family, but it’s a bit different to this one. Seems like there are a lot of options in this family.
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I was most interested to see this Syzygium with such large fruit. We have an an indigenous Syzygium here in SA – Syzygium cordatum (also known as the common water berry or as the umdoni) – its fruits are edible but they are small – about the size of a marble.
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These are pretty big. I’ll have to get hold of one and see what it tastes like.
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