Question: I don't want to calculate a file's checksum, just to know if a given string is a valid checksum

Answer: 
There is no such thing as an MD5 or SHA-1 string, at least not one that is standardized. All you can test for is the size of the byte array: 16 for MD5 (a hash with an output size of 128 bits) or 20 bytes for SHA-1 (a hash with an output size of 160 bits) encoded using hexadecimal encoding or base 64 encoding.

If you use `md5sum` then generally the checksum is shown as hexadecimal encoding, using *only* lowercase characters (followed by the file name or `-` for standard input). Hexadecimals are generally preferred, but hashes may also contain separator character or use a different encoding such as base 64 or base 64 URL (etc. etc.).

The byte size you are testing for might however belong to an entirely different hash such as [RIPEMD](https://en.wikipedia.org/wiki/RIPEMD) that may also have the same output size. Or it may be another value with the same amount of bytes.
MD5 verifier:

```
public boolean isValidMD5(String s) {
return s.matches("[a-fA-F0-9]{32}");}

```

And remove "-" of the string value.