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

Answer: 
Any 160-bit sequence is a possible SHA1 hash. Any 128-bit sequence is a possible MD5 hash.

If you're looking at the hex string representations of them, then a sha1 will look like 40 hexadecimal digits, and an md5 will look like 32 hexadecimal digits.
MD5 verifier:

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

```

And remove "-" of the string value.