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Asked: 2026-05-26T02:24:30+00:00

I’ve got a RandomAccessFile in Java where i manage some data. Simplified: At the

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I’ve got a RandomAccessFile in Java where i manage some data. Simplified:
At the start of the file i have an index. (8 byte long value per dataset which represents the offset where the real data can be found).

So if i want to now where i can find the data of dataset no 3 for example. I read 8 Bytes at offset (2*8). (Indexing starts with 0).

A dataset itsself consists of 4 Bytes which represents the size of the dataset and then all the bytes belonging to the dataset.

So that works fine in case i always rewrite the whole file.

It’s pretty important here, that Dataset no 3 could have been written as the first entry in the file so the index is ordered but not the data itsself.

If i insert a new dataset, i always append it to the end of the file. But the number of datasets that could be i n one file is limited. If i can store 100 datasets in the file there will be always 100 entries in the index. If the offset read from the index of a dataset is 0 the dataset is new and will be appended to the file.

Bu there’s one case which is not working for me yet. If i read dataset no. 3 from the file and i add some data to it in my application and i want to update it in the file i have no idea how to do this.

If it has the same length as befor i can simply overwrite the old data. But if the new dataset has more bytes than the old one i’ll have to move all the data in the file which is behind this dataset and update the indexes for these datasets.

Any idea how to do that?
Or is there maybe a better way to manage storing these datasets in a file?

PS: Yes of course i thought of using a database but this is not applicable for my project. I really do need simple files.

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1 Answer

  1. Editorial Team

    You can’t easily insert data into the middle of a file. You’d basically have to read all the remaining data, write the “new” data and then rewrite the “old” data. Alternatively, you could potentially invalidate the old “slow” (potentially allowing it to be reused later) and then just write the whole new record to the end of the file. Your file format isn’t really clear to me to be honest, but fundamentally you need to be aware that you can’t insert (or delete) in the middle of a file.

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