how to peek at a single char in a non-buffered reader in java

Question

I'm writing a program that takes a Reader and parses input from that reader. I cannot use a BufferedReader, but I would like to implement a peek method that looks at the current char in the reader, without actually calling read() on that character. Is there an easier/better way to do this than converting it to a character array or a string and looking at the character? I would love to use the mark() method but unfortunately that only works on a buffered reader.

Answer 1

The natural solution is to use PushBackReader which provides unread methods that can be used to implement peek. (You can work out the details!)

But it is not entirely clear if this is allowed. It depends whether you are forbidden from using BufferedReader or "a buffered Reader". In the latter case, PushBackReader is arguably a buffered Reader.

If you have to implement this without using an existing buffered Reader class, then how you should approach this depends on how much lookahead is needed:

• If you need just one character lookahead (e.g. just peek) then you can implement this using an int to represent the lookahead and a boolean to say if it is currently valid.

• If you need multiple characters lookahead, you need an array (or list), and so on.

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For the record, the mark() and reset() methods are actually in the Reader API. The problem is that not all Reader classes are able to implement these methods ... due to limitations of the underlying streams ... at the operating system level.

Answer 2

You can write your own class with peek method based on java.io.PushBackReader.